Network Working Group                               E. Hammer-Lahav, Ed.
Internet-Draft                                                    Yahoo!
Obsoletes: 5849 (if approved)                                D. Recordon
Intended status: Standards Track                                Facebook
Expires: November 20, 2011 January 9, 2012                                        D. Hardt
                                                               Microsoft
                                                            May 19,
                                                            July 8, 2011

                  The OAuth 2.0 Authorization Protocol
                         draft-ietf-oauth-v2-16
                         draft-ietf-oauth-v2-17

Abstract

   The OAuth 2.0 authorization protocol enables a third-party
   application to obtain limited access to an HTTP service, either on
   behalf of an end-user a resource owner by orchestrating an approval interaction
   between the end-user resource owner and the HTTP service, or by allowing the third-
   party
   third-party application to obtain access on its own behalf.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 20, 2011. January 9, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
     1.1.   Roles . . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
     1.2.   Protocol Flow . . . . . . . . . . . . . . . . . . . . . .  5  6
     1.3.   Access Token  . . . . . . . . . . . . . . . . . . . . . .  6  7
     1.4.   Authorization Grant . . . . . . . . . . . . . . . . . . .  7
     1.5.   Refresh Token .  8
       1.4.1.  Authorization Code . . . . . . . . . . . . . . . . . .  8
       1.4.2.  Implicit . . .  9
     1.6.   Document Structure . . . . . . . . . . . . . . . . . . . 11
     1.7.   Notational Conventions .  8
       1.4.3.  Resource Owner Password Credentials  . . . . . . . . .  9
       1.4.4.  Client Credentials . . . . . . . 11
   2.  Protocol Endpoints . . . . . . . . . . .  9
       1.4.5.  Extensions . . . . . . . . . . . 11
     2.1.   Authorization Endpoint . . . . . . . . . . .  9
     1.5.   Refresh Token . . . . . . 12
     2.2.   Token Endpoint . . . . . . . . . . . . . . . .  9
     1.6.   Notational Conventions  . . . . . 13
   3.  Client Authentication . . . . . . . . . . . . 11
   2.  Client Registration  . . . . . . . . 14
     3.1.   Client Password Authentication . . . . . . . . . . . . . 15
     3.2.   Other 11
     2.1.   Client Authentication Methods . . Types  . . . . . . . . . 16
   4.  Obtaining Authorization . . . . . . . . . . . . . 12
     2.2.   Registration Requirements . . . . . . 16
     4.1.   Authorization Code . . . . . . . . . . 12
     2.3.   Client Identifier . . . . . . . . . 16
     4.2.   Implicit Grant . . . . . . . . . . . 12
     2.4.   Client Authentication . . . . . . . . . . 22
     4.3.   Resource Owner Password Credentials . . . . . . . . 12
       2.4.1.  Client Password  . . . 28
     4.4.   Client Credentials . . . . . . . . . . . . . . . . 13
       2.4.2.  Other Authentication Methods . . . 30
     4.5.   Extensions . . . . . . . . . . 14
     2.5.   Unregistered Clients  . . . . . . . . . . . . . 32
   5.  Issuing an Access Token . . . . . 14
   3.  Protocol Endpoints . . . . . . . . . . . . . . 33
     5.1.   Successful Response . . . . . . . . 14
     3.1.   Authorization Endpoint  . . . . . . . . . . . 33
     5.2.   Error Response . . . . . . 14
       3.1.1.  Response Type  . . . . . . . . . . . . . . . 34
   6.  Refreshing an Access Token . . . . . 15
       3.1.2.  Redirection URI  . . . . . . . . . . . . . 36
   7.  Accessing Protected Resources . . . . . . 16
     3.2.   Token Endpoint  . . . . . . . . . . 37
     7.1.   Access Token Types . . . . . . . . . . . 18
       3.2.1.  Client Authentication  . . . . . . . . 38
   8.  Extensibility . . . . . . . . 18
   4.  Obtaining Authorization  . . . . . . . . . . . . . . . . 38
     8.1.   Defining Access Token Types . . . 19
     4.1.   Authorization Code  . . . . . . . . . . . . 38
     8.2.   Defining New Endpoint Parameters . . . . . . . 19
       4.1.1.  Authorization Request  . . . . . 39
     8.3.   Defining New Authorization Grant Types . . . . . . . . . 39
     8.4.   Defining Additional Error Codes . . 21
       4.1.2.  Authorization Response . . . . . . . . . . . 39
   9.  Native Applications . . . . . 22
       4.1.3.  Access Token Request . . . . . . . . . . . . . . . . 40
   10. Security Considerations . 24
       4.1.4.  Access Token Response  . . . . . . . . . . . . . . . . 25
     4.2.   Implicit Grant  . . 41
     10.1.  Client Authentication . . . . . . . . . . . . . . . . . . 42
     10.2.  Client Impersonation . 25
       4.2.1.  Authorization Request  . . . . . . . . . . . . . . . . . 42
     10.3. 28
       4.2.2.  Access Token Credentials Response  . . . . . . . . . . . . . . . . 43
     10.4.  Refresh Tokens 29
     4.3.   Resource Owner Password Credentials . . . . . . . . . . . 32
       4.3.1.  Authorization Request and Response . . . . . . . . . . 43
     10.5. 33
       4.3.2.  Access Token Request Confidentiality . . . . . . . . . . . . . . . . . 44
     10.6.  Endpoints Authenticity . 33
       4.3.3.  Access Token Response  . . . . . . . . . . . . . . . . 44
     10.7. 34
     4.4.   Client Credentials Guessing Attacks  . . . . . . . . . . . . . . 44
     10.8.  Phishing Attacks . . . . . 34
       4.4.1.  Authorization Request and Response . . . . . . . . . . 35
       4.4.2.  Access Token Request . . . . . 44
     10.9.  Authorization Codes . . . . . . . . . . . . 35
       4.4.3.  Access Token Response  . . . . . . . . 45
     10.10. Session Fixation . . . . . . . . 36
     4.5.   Extensions  . . . . . . . . . . . . 45
     10.11. Redirection URI Validation . . . . . . . . . . . 36
   5.  Issuing an Access Token  . . . . . . 46
     10.12. Resource Owner Password Credentials . . . . . . . . . . . 46
     10.13. XSRF/CSRF Prevention . . 37
     5.1.   Successful Response . . . . . . . . . . . . . . . . 46
   11. IANA Considerations . . . 37
     5.2.   Error Response  . . . . . . . . . . . . . . . . . . 46
     11.1.  The OAuth . . . 39
   6.  Refreshing an Access Token Type Registry . . . . . . . . . . 46
     11.2.  The OAuth Parameters Registry . . . . . . . . 40
   7.  Accessing Protected Resources  . . . . . . 48
     11.3.  The OAuth Extensions Error Registry . . . . . . . . . . 41
     7.1.   Access Token Types  . 51
   12. Acknowledgements . . . . . . . . . . . . . . . . . . 42
   8.  Extensibility  . . . . . 52
   Appendix A.  Editor's Notes . . . . . . . . . . . . . . . . . . . 53
   13. References 43
     8.1.   Defining Access Token Types . . . . . . . . . . . . . . . 43
     8.2.   Defining New Endpoint Parameters  . . . . . . . . . . . 53
     13.1.  Normative References . 43
     8.3.   Defining New Authorization Grant Types  . . . . . . . . . 44
     8.4.   Defining New Authorization Endpoint Response Types  . . . 44
     8.5.   Defining Additional Error Codes . . . . . 53
     13.2.  Informative References . . . . . . . . 44
   9.  Native Applications  . . . . . . . . . 54
   Authors' Addresses . . . . . . . . . . . . 45
   10. Security Considerations  . . . . . . . . . . . . 55

1.  Introduction

   In the traditional client-server authentication model, the client
   accesses a protected resource on the server by authenticating with
   the server using the resource owner's credentials.  In order to
   provide third-party applications access to protected resources, the
   resource owner shares its credentials with the third-party.  This . . . . . . . 46
     10.1.  Client Authentication . . . . . . . . . . . . . . . . . . 47
     10.2.  Client Impersonation  . . . . . . . . . . . . . . . . . . 47
     10.3.  Access Token Credentials  . . . . . . . . . . . . . . . . 48
     10.4.  Refresh Tokens  . . . . . . . . . . . . . . . . . . . . . 48
     10.5.  Request Confidentiality . . . . . . . . . . . . . . . . . 49
     10.6.  Endpoints Authenticity  . . . . . . . . . . . . . . . . . 49
     10.7.  Credentials Guessing Attacks  . . . . . . . . . . . . . . 49
     10.8.  Phishing Attacks  . . . . . . . . . . . . . . . . . . . . 49
     10.9.  Authorization Codes . . . . . . . . . . . . . . . . . . . 50
     10.10. Authorization Code Leakage  . . . . . . . . . . . . . . . 50
     10.11. Redirection URI Validation  . . . . . . . . . . . . . . . 51
     10.12. Resource Owner Password Credentials . . . . . . . . . . . 51
     10.13. Cross-Site Request Forgery  . . . . . . . . . . . . . . . 51
     10.14. Clickjacking  . . . . . . . . . . . . . . . . . . . . . . 52
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 52
     11.1.  The OAuth Access Token Type Registry  . . . . . . . . . . 52
       11.1.1. Registration Template  . . . . . . . . . . . . . . . . 53
     11.2.  The OAuth Parameters Registry . . . . . . . . . . . . . . 53
       11.2.1. Registration Template  . . . . . . . . . . . . . . . . 54
       11.2.2. Initial Registry Contents  . . . . . . . . . . . . . . 54
     11.3.  The OAuth Authorization Endpoint Response Type
            Registry  . . . . . . . . . . . . . . . . . . . . . . . . 56
       11.3.1. Registration Template  . . . . . . . . . . . . . . . . 57
       11.3.2. Initial Registry Contents  . . . . . . . . . . . . . . 57
     11.4.  The OAuth Extensions Error Registry . . . . . . . . . . . 57
       11.4.1. Registration Template  . . . . . . . . . . . . . . . . 58
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 59
   Appendix A.  Editor's Notes  . . . . . . . . . . . . . . . . . . . 59
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 60
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 60
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 61

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 62

1.  Introduction

   In the traditional client-server authentication model, the client
   accesses a protected resource on the server by authenticating with
   the server using the resource owner's credentials.  In order to
   provide third-party applications access to protected resources, the
   resource owner shares its credentials with the third-party.  This
   creates several problems and limitations:

   o  Third-party applications are required to store the resource- resource
      owner's credentials for future use, typically a password in clear-
      text.
   o  Servers are required to support password authentication, despite
      the security weaknesses created by passwords.
   o  Third-party applications gain overly broad access to the resource- resource
      owner's protected resources, leaving resource owners without any
      ability to restrict duration or access to a limited subset of
      resources.
   o  Resource owners cannot revoke access to an individual third-party
      without revoking access to all third-parties, and must do so by
      changing their password.
   o  Compromise of any third-party application results in compromise of
      the end-user's password and all of the data protected by that
      password.

   OAuth addresses these issues by introducing an authorization layer
   and separating the role of the client from that of the resource
   owner.  In OAuth, the client requests access to resources controlled
   by the resource owner and hosted by the resource server, and is
   issued a different set of credentials than those of the resource
   owner.

   Instead of using the resource owner's credentials to access protected
   resources, the client obtains an access token - a string denoting a
   specific scope, duration, and other access attributes.  Access tokens
   are issued to third-party clients by an authorization server with the
   approval of the resource owner.  The client uses the access token to
   access the protected resources hosted by the resource server.

   For example, a web an end-user (resource owner) can grant a printing
   service (client) access to her protected photos stored at a photo
   sharing service (resource server), without sharing her username and
   password with the printing service.  Instead, she authenticates
   directly with a server trusted by the photo sharing service
   (authorization server) which issues the printing service delegation-
   specific credentials (access token).

   This specification is designed for use with HTTP [RFC2616].  The use
   of OAuth with any transport protocol other than HTTP is undefined.

1.1.  Roles

   OAuth includes four roles working together to grant and provide
   access to protected resources - access restricted resources which
   require authentication to access: requiring
   authentication:

   resource owner
      An entity capable of granting access to a protected resource.
      When resource (e.g.
      end-user).
   resource server
      The server hosting the protected resources, capable of accepting
      and responding to protected resource requests using access tokens.
   client
      An application making protected resource requests on behalf of the
      resource owner and with its authorization.
   authorization server
      The server issuing access tokens to the client after successfully
      authenticating the resource owner and obtaining authorization.

   The interaction between the authorization server and resource server
   is beyond the scope of this specification.  The authorization server
   may be the same server as the resource server or a separate entity.
   A single authorization server may issue access tokens accepted by
   multiple resource servers.

1.2.  Protocol Flow

     +--------+                               +---------------+
     |        |--(A)- Authorization Request ->|   Resource    |
     |        |                               |     Owner     |
     |        |<-(B)-- Authorization Grant ---|               |
     |        |                               +---------------+
     |        |
     |        |                               +---------------+
     |        |--(C)-- Authorization Grant -->| Authorization |
     | Client |                               |     Server    |
     |        |<-(D)----- Access Token -------|               |
     |        |                               +---------------+
     |        |
     |        |                               +---------------+
     |        |--(E)----- Access Token ------>|    Resource   |
     |        |                               |     Server    |
     |        |<-(F)--- Protected Resource ---|               |
     +--------+                               +---------------+
                     Figure 1: Abstract Protocol Flow

   The abstract flow illustrated in Figure 1 describes the interaction
   between the four roles and includes the following steps:

   (A)  The client requests authorization from the resource owner.  The
        authorization request can be made directly to the resource owner is a person, it is referred to
        (as shown), or preferably indirectly via an intermediary such as
        an end-
      user. authorization server.
   (B)  The client receives an authorization grant which represents the
        authorization provided by the resource server owner.  The server hosting authorization
        grant type depends on the protected resources, capable of accepting method used by the client and responding
        supported by the authorization server to protected resource obtain it.
   (C)  The client requests using an access tokens. token by authenticating with the
        authorization server and presenting the authorization grant.
   (D)  The authorization server authenticates the client
      An application making and validates
        the authorization grant, and if valid issues an access token.
   (E)  The client requests the protected resource requests on behalf of from the resource owner and with its authorization.
   authorization
        server and authenticates by presenting the access token.
   (F)  The resource server issuing validates the access token, and if valid,
        serves the request.

1.3.  Access Token

   Access tokens are credentials used to access protected resources.  An
   access token is a string representing an authorization issued to the client after successfully
      authenticating
   client.  The string is usually opaque to the client.  Tokens
   represent specific scopes and durations of access, granted by the
   resource owner owner, and obtaining authorization. enforced by the resource server and authorization
   server.

   The interaction between token may denote an identifier used to retrieve the authorization server
   information, or self-contain the authorization information in a
   verifiable manner (i.e. a token string consisting of some data and resource server
   is a
   signature).  Additional authentication credentials, which are beyond
   the scope of this specification.  The authorization server specification, may be required in order for the same server as the resource server or
   client to use a token.

   The access token provides an abstraction layer, replacing different
   authorization constructs (e.g. username and password) with a separate entity.
   A single authorization server may issue access tokens accepted
   token understood by
   multiple the resource servers.

1.2.  Protocol Flow

   When interacting with server.  This abstraction enables
   issuing access tokens more restrictive than the authorization server, grant
   used to obtain them, as well as removing the client identifies
   itself using resource server's need
   to understand a set wide range of client credentials which include a client
   identifier and other authentication attributes.  The means through
   which methods.

   Access tokens can have different formats, structures, and methods of
   utilization (e.g. cryptographic properties) based on the client obtains its credentials resource
   server security requirements.  Access token attributes and the
   methods used to access protected resources are beyond the scope of
   this
   specification, but typically involve registration with the
   authorization server.

     +--------+                               +---------------+
     |        |--(A)- Authorization Request ->|   Resource    |
     |        |                               |     Owner     |
     |        |<-(B)-- Authorization Grant ---|               |
     |        |                               +---------------+
     |        |
     |        | specification and are defined by companion specifications.

1.4.  Authorization Grant &  +---------------+
     |        |--(C)--- Client Credentials -->| Authorization |
     | Client |                               |     Server    |
     |        |<-(D)----- Access Token -------|               |
     |        |                               +---------------+
     |        |
     |        |                               +---------------+
     |        |--(E)----- Access Token ------>|    Resource   |
     |        |                               |     Server    |
     |        |<-(F)--- Protected Resource ---|               |
     +--------+                               +---------------+

                     Figure 1: Abstract Protocol Flow

   The abstract flow illustrated in Figure 1 describes

   An authorization grant is a general term used to describe the interaction
   between
   intermediate credentials representing the resource owner
   authorization (to access its protected resources), and serves as an
   abstraction layer.  An authorization grant is used by the client to
   obtain an access token.

   This specification defines four roles grant types: authorization code,
   implicit, resource owner password credentials, and includes the following steps:

   (A)  The client requests
   credentials, as well as an extensibility mechanism for defining
   additional types.

1.4.1.  Authorization Code

   The authorization from code is obtained by using an authorization server
   as an intermediary between the client and resource owner.  The  Instead of
   requesting authorization request can be made directly to from the resource owner, the client
   directs the resource owner
        (as shown), or preferably indirectly via to an intermediary such authorization server (via its user-
   agent as
        an defined in [RFC2616]), which in turn directs the resource
   owner back to the client with the authorization server.
   (B)  The code.

   Before directing the resource owner back to the client receives an with the
   authorization grant which represents code, the authorization provided by server authenticates the
   resource owner. owner and obtains authorization.  Because the resource owner
   only authenticates with the authorization server, the resource
   owner's credentials are never shared with the client.

   The authorization
        grant type depends on code provides a few important security benefits
   such as the method used by ability to authenticate the client and
        supported by issuing the authorization server access
   token directly to obtain it.
   (C)  The the client requests without potentially exposing it to
   others, including the resource owner.

1.4.2.  Implicit

   The authorization grant is implicit when an access token by authenticating with is issued to
   the
        authorization server using its client credentials (prearranged
        between directly as the client and authorization server) and presenting result of the resource owner
   authorization, without using intermediate credentials (such as an
   authorization grant.
   (D)  The code).

   When issuing an implicit grant, the authorization server validates does not
   authenticate the client credentials and the authorization grant, and if valid issues an client identity is verified via the
   redirection URI used to deliver the access token.
   (E) token to the client.  The client requests
   access token may be exposed to the protected resource from owner or other
   applications with access to the resource
        server owner's user-agent.

   Implicit grants improve the responsiveness and authenticates by presenting efficiency of some
   clients (such as a client implemented as an in-browser application)
   since it reduces the number of round trips required to obtain an
   access token.
   (F)  The resource server validates  However, this convenience should be weighted against
   the access token, and if valid,
        serves security implications of using implicit grants, especially when
   the request.

1.3.  Access Token

   An access token authorization code grant type is available.

1.4.3.  Resource Owner Password Credentials

   The resource owner password credentials (e.g. a string representing username and
   password) can be used directly as an authorization issued grant to
   the client. obtain an
   access token.  The string credentials should only be used when there is usually opaque to the client.  Tokens
   represent specific scopes and durations a
   high degree of access, granted by trust between the resource owner, owner and enforced by the resource server client (e.g.
   its computer operating system or a highly privileged application),
   and when other authorization
   server.

   The token may denote grant types are not available (such as
   an identifier used authorization code).

   Even though this grant type requires direct client access to retrieve the authorization
   information, or self-contain
   resource owner credentials, the authorization information in a
   verifiable manner (i.e. resource owner credentials are used
   for a token string consisting of some data single request and a
   signature).  Additional are exchanged for an access token.  Unlike
   the HTTP Basic authentication credentials may be required scheme defined in
   order [RFC2617], this grant
   type (when combined with a refresh token) eliminates the need for the
   client to use a token. store the resource owner credentials for future use.

1.4.4.  Client Credentials

   The access token provides client credentials (or other forms of client authentication) can
   be used as an abstraction layer, replacing different authorization constructs (e.g. username and password) with a single
   token understood by the resource server.  This abstraction enables
   issuing access tokens more restrictive than grant when the authorization grant
   used scope is
   limited to obtain them, as well as removing the resource server's need
   to understand a wide range of authentication methods.

   Access tokens can have different formats, structures, and methods of
   utilization (e.g. cryptographic properties) based on protected resources under the resource
   server security requirements.  Access token attributes and control of the
   methods used client,
   or to access protected resources are beyond previously arranged with the scope of
   this specification and authorization
   server.  Client credentials are defined by companion specifications.

1.4.  Authorization Grant

   An used as an authorization grant
   typically when the client is acting on its own behalf (the client is
   also the resource owner).

1.4.5.  Extensions

   Additional grant types may be defined to provide a general term bridge between
   OAuth and other protocols.

1.5.  Refresh Token

   Refresh tokens are credentials used to describe obtain access tokens.  Refresh
   tokens are issued to the
   intermediate credentials representing client by the resource owner authorization (to server and are
   used to obtain a new access its protected resources), token when the current access token
   becomes invalid or expires, or to obtain additional access tokens
   with identical or narrower scope (access tokens may have a shorter
   lifetime and serves as an
   abstraction layer.  An authorization grant fewer permissions than authorized by the resource
   owner).  Issuing a refresh token is used by the client to
   obtain optional and is included when
   issuing an access token.

   This specification defines four grant types: authorization code,
   implicit, resource owner password credentials, and client
   credentials, as well as an extensibility mechanism for defining
   additional types.

1.4.1.  Authorization Code

   The authorization code

   A refresh token is obtained by using an authorization server
   as an intermediary between a string representing the client and resource owner.  Instead of
   requesting authorization directly from the resource owner, granted to
   the client
   directs by the resource owner owner.  The string is usually opaque to
   the client.  The token denotes an authorization server (via its user-
   agent as defined in [RFC2616]), which in turn directs identifier used to retrieve the resource
   owner back
   authorization information.  Unlike access tokens, refresh tokens are
   intended for use only with authorization servers and are never sent
   to resource servers.

  +--------+                                           +---------------+
  |        |--(A)------- Authorization Grant --------->|               |
  |        |                                           |               |
  |        |<-(B)----------- Access Token -------------|               |
  |        |               & Refresh Token             |               |
  |        |                                           |               |
  |        |                            +----------+   |               |
  |        |--(C)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(D)- Protected Resource --| Resource |   | Authorization |
  | Client |                            |  Server  |   |     Server    |
  |        |--(E)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(F)- Invalid Token Error -|          |   |               |
  |        |                            +----------+   |               |
  |        |                                           |               |
  |        |--(G)----------- Refresh Token ----------->|               |
  |        |                                           |               |
  |        |<-(H)----------- Access Token -------------|               |
  +--------+           & Optional Refresh Token        +---------------+

               Figure 2: Refreshing an Expired Access Token

   The flow illustrated in Figure 2 includes the following steps:

   (A)  The client requests an access token by authenticating with the
        authorization code.

   Before directing the resource owner back to server, and presenting an authorization grant.
   (B)  The authorization server authenticates the client with and validates
        the authorization code, grant, and if valid issues an access token and
        a refresh token.
   (C)  The client makes a protected resource requests to the authorization resource
        server authenticates by presenting the access token.
   (D)  The resource owner server validates the access token, and obtains authorization.  Because if valid,
        serves the request.

   (E)  Steps (C) and (D) repeat until the access token expires.  If the
        client knows the access token expired, it skips to step (G),
        otherwise it makes another protected resource owner
   only authenticates with request.
   (F)  Since the authorization server, access token is invalid, the resource
   owner's credentials are never shared server returns
        an invalid token error.
   (G)  The client requests a new access token by authenticating with
        the client.

   The authorization code provides a few important security benefits
   such as server and presenting the ability to authenticate refresh token.
   (H)  The authorization server authenticates the client and issuing validates
        the refresh token, and if valid issues a new access token directly (and
        optionally, a new refresh token).

1.6.  Notational Conventions

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   specification are to be interpreted as described in [RFC2119].

   This specification uses the client without potentially exposing it Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   Certain security-related terms are to
   others, including be understood in the sense
   defined in [RFC4949].  These terms include, but are not limited to,
   'attack', 'authentication', 'authorization', 'certificate',
   'confidentiality', 'credential', 'encryption', 'identity', 'sign',
   'signature', 'trust', 'validate', and 'verify'.

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

2.  Client Registration

   [[ Pending Consensus ]]

   Before initiating the resource owner.

1.4.2.  Implicit

   When an access token is issued to protocol, the client directly as the result
   of registers with the resource owner authorization, without an intermediary
   authorization grant (such as an
   authorization code), server.  The means through which the grant is
   considered implicit.

   When issuing an implicit grant, client registers
   with the authorization server cannot
   verify are beyond the identity scope of this
   specification, but typically involve human interaction with an HTML
   registration form.

   Client registration does not require a direct interaction between the client,
   client and the access token may be
   exposed to authorization server.  When supported by the resource owner or
   authorization server, registration can rely on other applications with access means for
   establishing trust and obtaining the required client properties (e.g.
   redirection URI, client type).  For example, registration can be
   accomplished using a self-issued or third-party-issued assertion, or
   by the authorization server performing client discovery using a
   trusted channel.

2.1.  Client Types

   OAuth defines two client types, based on their ability to
   authenticate securely with the resource owner's user-agent.

   Implicit grants improve authorization server (i.e. ability to
   maintain the responsiveness and efficiency confidentiality of some
   clients (such as a their client implemented as an in-browser application)
   since it reduces credentials):

   private
      Clients capable of maintaining the number confidentiality of round trips required to obtain an
   access token.

1.4.3.  Resource Owner Password Credentials

   The resource owner password their
      credentials (e.g. client implemented on a username and
   password) can be used directly as an authorization grant to obtain an secure server with
      restricted access token.  The credentials should only be used when there is a
   high degree to the client credentials), or capable of secure
      client authentication using other means.
   public
      Clients incapable of maintaining the confidentiality of trust between their
      credentials (e.g. clients executing on the resource owner and the client (e.g.
   its computer operating system owner's device
      such as an installed native application or a highly privileged user-agent-based
      application), and when incapable of secure client authentication via
      any other authorization grant types are not available (such as
   an authorization code).

   Even though this grant mean.

   The client type requires direct designation is based on the authorization server's
   definition of secure authentication and its acceptable exposure
   levels of client access to credentials.

2.2.  Registration Requirements

   When registering a client, the
   resource owner credentials, client developer MUST specify:

   o  the resource owner credentials are used
   for a single request and are exchanged for an access token.  Unlike client type as described in Section 2.1,
   o  the HTTP Basic authentication scheme defined client redirection URIs as described in [RFC2617], this grant
   type (when combined with a refresh token) eliminates Section 3.1.2, and
   o  any other information required by the need for authorization server (e.g.
      application name, website, description, logo image, the acceptance
      of legal terms).

2.3.  Client Identifier

   The authorization server issues the registered client to store a client
   identifier - a unique string representing the resource-owner credentials for future use.

1.4.4.  Client Credentials registration
   information provided by the client.  The client credentials can identifier is not a
   secret, it is exposed to the resource owner, and cannot not be used as an authorization grant when
   alone for client authentication.

2.4.  Client Authentication

   In addition, the client and authorization scope is limited to the protected resources under server establish a client
   authentication method suitable for the
   control client type and security
   requirements of the client, or to protected resources previously arranged authorization server.  The authorization server
   MAY accept any form of client authentication meeting its security
   requirements.

   Private clients are typically issued (or establish) a set of client
   credentials used for authenticating with the authorization server.  Client credentials are used as an server
   (e.g. password, public/private key pair).

   The authorization grant typically when server SHOULD NOT make assumptions about the client is acting on its own
   behalf (the client is also
   type or accept the resource owner).

1.4.5.  Extensions

   Additional grant types may be defined to provide a bridge between
   OAuth and other protocols.  For example,
   [I-D.ietf-oauth-saml2-bearer] defines a SAML 2.0
   [OASIS.saml-core-2.0-os] bearer assertion grant type, which can be
   used to obtain an access token.

1.5.  Refresh Token

   A refresh token is optionally issued by type information provided without establishing
   trust with the client or its developer.  The authorization server to
   the
   MUST NOT rely on client together with an access token. authentication performed by public clients.

   The client can MUST NOT use more than one authentication method in each
   request.

2.4.1.  Client Password

   Clients in possession of a client password MAY use the
   refresh token to request another access token based on the same
   authorization, without having to involve the resource owner again, or
   having HTTP Basic
   authentication scheme as defined in [RFC2617] to retain authenticate with
   the original authorization grant server.  The client identifier is used to obtain as the
   initial access token.

   A refresh token
   username, and the client password is a string representing used as the password.

   For example (extra line breaks are for display purposes only):

     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW

   Alternatively, the authorization granted to server MAY allow including the
   client by credentials in the resource owner.  The string is usually opaque to request body using the client. following
   parameters:

   client_id
         REQUIRED.  The token may denote an client identifier used issued to retrieve the
   authorization information, or self-contain client during
         the authorization
   information in a verifiable manner. registration process described by Section 2.3.
   client_secret
         REQUIRED.  The refresh token is bound to client secret.

   Including the client it was issued to, credentials in the request body using the two
   parameters is NOT RECOMMENDED, and its usage requires client
   authentication.

   The refresh token can should be used limited to obtain a new access token when clients
   unable to directly utilize the
   current HTTP Basic authentication scheme (or
   other password-based HTTP authentication schemes).

   For example, requesting to refresh an access token expires (access tokens may have a shorter
   lifetime than authorized by (Section 6) using
   the resource owner), no longer valid, or
   to obtain additional access tokens with identical or narrower scope.

  +--------+            Authorization Grant &          +---------------+
  |        |--(A)-------- Client Credentials --------->|               |
  |        |                                           |               |
  |        |<-(B)----------- Access Token -------------|               |
  |        |               & Refresh Token             |               |
  |        |                                           |               |
  |        |                            +----------+   |               |
  |        |--(C)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(D)- Protected Resource --| Resource |   | Authorization |
  | Client |                            |  Server  |   |     Server    |
  |        |--(E)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(F)- Invalid Token Error -|          |   |               |
  |        |                            +----------+   |               |
  |        |                                           |               |
  |        |                Refresh Token &            |               |
  |        |--(G)-------- Client Credentials --------->|               |
  |        |                                           |               |
  |        |<-(H)----------- Access Token -------------|               |
  +--------+           & Optional Refresh Token        +---------------+

               Figure 2: Refreshing an Expired Access Token body parameters (extra line breaks are for display purposes
   only):

     POST /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded;charset=UTF-8

     grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
     &client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw

   The flow illustrated in Figure 2 includes authorization server MUST require the following steps:

   (A)  The client use of a transport-layer
   security mechanism when sending requests an access to the token by authenticating with endpoint, as
   requests using this authentication method result in the transmission
   of clear-text credentials.

2.4.2.  Other Authentication Methods

   The authorization server using MAY support any suitable HTTP authentication
   scheme matching its client credentials, and
        presenting an authorization grant.
   (B)  The security requirements.  When using other
   authentication methods, the authorization server validates MUST define a
   mapping between the client credentials identifier (registration record) and
   authentication scheme.

2.5.  Unregistered Clients

   This specification does not exclude the authorization grant, use of unregistered clients.
   However, the use with such clients is beyond the scope of this
   specification, and if valid issues an access token requires additional security analysis and
        a refresh token.
   (C) review
   of its interoperability impact.

3.  Protocol Endpoints

   The client makes a protected resource requests authorization process utilizes two endpoints (HTTP resources):

   o  Authorization endpoint - used to obtain authorization from the
      resource
        server by presenting the access token.
   (D)  The resource server validates the owner via user-agent redirection.
   o  Token endpoint - used to exchange an authorization grant for an
      access token, and if valid,
        serves typically with client authentication.

   Not every authorization grant type utilizes both endpoints.
   Extension grant types MAY define additional endpoints as needed.

3.1.  Authorization Endpoint

   The authorization endpoint is used to interact with the request.
   (E)  Steps (C) resource
   owner and (D) repeat until the obtain authorization which is expressed explicitly as an
   authorization code (later exchanged for an access token expires.  If the
        client knows the token), or
   implicitly by direct issuance of an access token expired, it skips to step (G),
        otherwise it makes another protected resource request.
   (F)  Since token.

   The authorization server MUST first verify the access token is invalid, identity of the
   resource server returns
        an invalid token error.
   (G) owner.  The client requests a new access token by authenticating with way in which the authorization server using its client credentials,
   authenticates the resource owner (e.g. username and
        presenting password login,
   session cookies) is beyond the refresh token.

   (H) scope of this specification.

   The authorization server validates means through which the client credentials and obtains the refresh token, and if valid issues a new access token (and
        optionally, a new refresh token).

1.6.  Document Structure

   This location of the
   authorization endpoint are beyond the scope of this specification is organized into but
   the following sections:

   o  Section 2 - describes location is typically provided in the two endpoints used service documentation.  The
   endpoint URI MAY include a query component as defined by [RFC3986]
   section 3, which MUST be retained when adding additional query
   parameters.  The endpoint URI MUST NOT include a fragment component.

   Since requests to obtain and utilize the various authorization grant types.
   o  Section 3 - describes client identification and authentication endpoint result in
      general, and provides one such method for client user
   authentication
      using password credentials.
   o  Section 4 - describes the complete flow for each authorization
      grant type, including requesting authorization, authorization
      response, and requesting an access token.
   o  Section 5 - describes the common access token response used for
      all non-implicit transmission of clear-text credentials (in the
   HTTP response), the authorization grant types.
   o  Section 6 - describes server MUST require the use of a refresh token
   transport-layer security mechanism when sending requests to obtain the
   authorization endpoint.  The authorization server MUST support TLS
   1.2 as defined in [RFC5246], and MAY support additional access tokens using transport-
   layer mechanisms meeting its security requirements.

   The authorization server MUST support the same resource owner
      authorization.
   o  Section 7 - describes how access tokens are used to access
      protected resources.
   o  Section 8 - describes how to extend certain elements use of the
      protocol.
   o  Section 9 - provides a security analysis of HTTP "GET"
   method [RFC2616] for the protocol.

1.7.  Notational Conventions

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', authorization endpoint, and 'OPTIONAL' in this
   specification are to be interpreted as described in [RFC2119].

   This specification uses MAY support the Augmented Backus-Naur Form (ABNF)
   notation
   use of [RFC5234].

   Unless otherwise noted, all the protocol parameter names "POST" method as well.

   Parameters sent without a value MUST be treated as if they were
   omitted from the request.  The authorization server SHOULD ignore
   unrecognized request parameters.  Request and values
   are case sensitive.

2.  Protocol Endpoints response parameters
   MUST NOT be included more than once.

3.1.1.  Response Type

   The authorization process utilizes two endpoints (HTTP resources):

   o  Authorization endpoint - is used to obtain authorization from by the
      resource owner via user-agent redirection.

   o  Token endpoint - used to exchange an authorization grant for an
      access token, typically with client authentication.

   Not every authorization code grant
   type utilizes both endpoints.
   Extension and implicit grant types MAY define additional endpoints as needed.

2.1.  Authorization Endpoint type flows.  The authorization endpoint is used to interact with client informs the resource
   owner and obtain
   authorization which is expressed explicitly as server of the desired grant type using the following
   parameter:

   response_type
         REQUIRED.  The value MUST be one of "code" for requesting an
         authorization code (exchanged as described by Section 4.1.1, "token" for
         requesting an access token), token (implicit grant) as described by
         Section 4.2.1, or implicitly a registered extension value as described by
   direct issuance of
         Section 8.4.

   If an access token.

   The authorization server MUST first verify request is missing the identity of "response_type" parameter,
   the
   resource owner.  The way authorization server SHOULD return an error response as described
   in which Section 4.1.2.1.

3.1.2.  Redirection URI

   [[ Pending Consensus ]]

   After completing its interaction with the resource owner, the
   authorization server
   authenticates directs the resource owner (e.g. username and password login,
   session cookies) is beyond owner's user-agent back to
   the scope of this specification. client.  The means through which authorization server redirects the client obtains user-agent to the location of
   client's redirection URI previously established with the
   authorization endpoint are beyond the scope of this specification but
   is typically provided in server during the service documentation. client registration process.

   The endpoint redirection URI MUST be an absolute URI
   MAY include a query component as defined by [RFC3986]
   section 3, 4.3, MAY include a query component which MUST be retained by
   the authorization server when adding additional query parameters.

   Since requests to the authorization endpoint parameters, and
   MUST NOT include a fragment component.

3.1.2.1.  Endpoint Confidentiality

   If a redirection request will result in user
   authentication and the transmission of clear-text credentials (in an
   authorization code or access token over an open network (between the
   HTTP response),
   resource owner's user-agent and the authorization server MUST client), the client SHOULD
   require the use of a transport-layer security mechanism mechanism.

   Lack of transport-layer security can have a severe impact on the
   security of the client and the protected resources it is authorized
   to access.  The use of transport-layer security is particularly
   critical when sending requests the authorization process is used as a form of
   delegated end-user authentication by the client (e.g. third-party
   sign-in service).

3.1.2.2.  Registration Requirements

   The authorization server MUST require public clients to register
   their redirection URI, MUST require all clients to register their
   redirection URI prior to utilizing the implicit grant type, and
   SHOULD require all clients to register their redirection URI prior to
   utilizing the authorization endpoint.  The authorization server MUST support TLS
   1.2 as defined in [RFC5246], and MAY support additional transport-
   layer mechanisms meeting its security requirements. code grant type.

   The authorization server MUST support the use of SHOULD require the HTTP "GET"
   method [RFC2616] for client to provide the authorization endpoint, and
   complete redirection URI (the client MAY support the use of the "POST" method as well.

   The REQUIRED "response_type" "state" request
   parameter is used to identify
   which grant type achieve per-request customization).  The authorization
   server MAY allow the client is requesting: authorization code or
   implicit, described in Section 4.1.1 and Section 4.2.1 respectively. to register multiple redirection URIs.
   If requiring the request is missing the "response_type" parameter, registration of the
   authorization server SHOULD return an error response as described in
   Section 4.1.2.1.

   Parameters sent without a value MUST be treated as if they were
   omitted from complete redirection URI is not
   possible, the request.  The authorization server SHOULD ignore
   unrecognized request parameters.

   Request and response parameters MUST NOT repeat more than once,
   unless noted otherwise.

2.1.1.  Redirection URI

   The client directs require the resource owner's user-agent to registration of
   the
   authorization endpoint URI scheme, authority, and includes a path.

3.1.2.3.  Dynamic Configuration

   If multiple redirection URI to which the
   authorization server will redirect the user-agent back once
   authorization has URIs have been obtained (or denied).  The client MAY omit registered, if only part of
   the redirection URI has been registered, or if one no redirection URI has
   been established between registered, the client and MUST include a redirection URI with the
   authorization server via other means, such as during request using the client
   registration process.

   The "redirect_uri" request parameter.

   When a redirection URI MUST be is included in an absolute URI and MAY include a query
   component, which authorization request, the
   authorization server MUST be retained by compare and match the value received
   against at least one of the registered redirection URIs (or URI
   components) as defined in [RFC3986] section 6, if any redirection
   URIs were registered.

   If the authorization server when
   adding additional allows the client to dynamically change
   the query parameters.

   The authorization server SHOULD require component of the redirection URI, the client MUST ensure
   that manipulation of the query component by an attacker cannot lead
   to pre-register
   their an abuse of the redirection URI or at least certain components such endpoint as the
   scheme, host, port and path. an open redirector.

3.1.2.4.  Invalid Endpoint

   If an authorization request fails validation due to a missing,
   invalid, or mismatching redirection URI was registered, URI, the authorization server
   SHOULD inform the resource owner of the error, and MUST compare any redirection URI received at NOT
   automatically redirect the authorization endpoint with user-agent to the registered invalid redirection URI.

   The authorization server SHOULD NOT redirect the user-agent to
   unregistered or untrusted URIs to prevent the authorization endpoint
   from being used as an open redirector.

3.1.2.5.  Endpoint Content

   The redirection request to the client's endpoint typically results in
   an HTML document response, processed by the user-agent.  If no valid the HTML
   response is served directly as the result of the redirection request,
   any script included in the HTML document will execute with full
   access to the redirection URI is
   available, and the authorization server credentials it contains.

   The client SHOULD inform NOT include any third-party scripts in the resource owner
   directly of
   redirection endpoint response.  Instead, it should extract the error.

2.2.
   credentials from the URI and redirect the user-agent again to another
   endpoint without the credentials in the URI.

   The client MUST NOT include any untrusted third-party scripts in the
   redirection endpoint response (e.g. third-party analytics, social
   plug-ins, ad networks) without first ensuring that its own scripts
   used to extract and remove the credentials from the URI will execute
   first.

3.2.  Token Endpoint

   The token endpoint is used by the client to obtain an access token by
   authenticating with the authorization server and
   presenting its authorization grant or refresh token.  The token
   endpoint is used with every authorization grant except for the
   implicit grant type (since an access token is issued directly).

   The means through which the client obtains the location of the token
   endpoint are beyond the scope of this specification but is typically
   provided in the service documentation.  The endpoint URI MAY include
   a query component, which MUST be retained when adding additional
   query parameters.

   Since requests to the token endpoint result in the transmission of
   clear-text credentials (in the HTTP request and response), the
   authorization server MUST require the use of a transport-layer
   security mechanism when sending requests to the token endpoint.  The
   authorization server MUST support TLS 1.2 as defined in [RFC5246],
   and MAY support additional transport-layer mechanisms meeting its
   security requirements.

   The token endpoint requires client authentication as described defined in
   Section 3.  The authorization server [RFC5246],
   and MAY accept any form of client
   authentication support additional transport-layer mechanisms meeting its
   security requirements.

   The client MUST
   NOT use more than one authentication method in each request.

   The client MUST use the HTTP "POST" method when making access token
   requests.

   Parameters sent without a value MUST be treated as if they were
   omitted from the request.  The authorization server SHOULD ignore
   unrecognized request parameters.

   Request and response parameters MUST NOT repeat more than once,
   unless noted otherwise.

3.  Client Authentication

   Client credentials are used to identify and authenticate the client.
   The client credentials include a client identifier - a unique string
   issued to the client to identify itself to the authorization server.
   The client identifier is not a secret, it is exposed to the resource
   owner, and MUST NOT be used alone for client authentication.  Client
   authentication is accomplished via additional means such as a
   matching client password.

   The methods through which the client obtains its client credentials
   are beyond the scope of this specification.  However, the client
   registration process typically includes gathering relevant
   information which is used to educate the resource owner about the
   client when requesting authorization.

   Due to the nature of some clients, the authorization server should
   not make assumptions about the confidentiality of client credentials
   without establishing trust with the client.  The authorization server
   SHOULD NOT issue client credentials to clients incapable of keeping
   their credentials confidential (typically determined during the
   client registration process).

   In addition, the authorization server MAY allow unauthenticated
   access token requests when the client identity does not matter (e.g.
   anonymous client) or when the client identity is established via
   other means.  For readability purposes only, this specification is
   written under the assumption that the authorization server requires
   some form of client authentication.  However, such language does not
   affect the authorization server's discretion in allowing
   unauthenticated client requests.

3.1. parameters.  Request and response parameters
   MUST NOT be included more than once.

3.2.1.  Client Password Authentication

   [[ Pending Consensus ]]

   Clients in possession of client password credentials (the

   Private clients, clients issued client
   identifier together with a shared symmetric secret) MAY use the HTTP
   Basic credentials, or clients
   assigned other authentication scheme as defined in [RFC2617] to requirements, MUST authenticate with
   the authorization server.  The client identifier is used as the
   username, and the secret is used server as described in Section 2.4 when making
   requests to the password.

   When using the HTTP Basic token endpoint.  Client authentication scheme, the client
   identifier is included twice in the request (in used for:

   o  Enforcing the "Authorization"
   header binding of refresh tokens and in the "client_id" parameter).  The authorization server
   MUST ensure the two identifiers belong codes to
      the same client.

   For example (extra line breaks client they are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&client_id=s6BhdRkqt3&
     code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   Alternatively, the issued.  Client authentication is critical
      when an authorization server MAY allow including code is transmitted to the
   client secret in redirection URI
      endpoint over an insecure channel, or when the request body using redirection URI has
      not been registered in full.
   o  Recovery from a compromised client by disabling the following parameter:

   client_secret
         REQUIRED.  The client secret.

   The use or
      changing its credentials, by preventing an attacker from abusing
      stolen refresh tokens.  Changing a single set of the "client_secret" parameter client
      credentials is NOT RECOMMENDED, and
   should be limited to clients unable to directly utilize the HTTP
   Basic authentication scheme.

   For example (extra line breaks are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&client_id=s6BhdRkqt3&
     client_secret=gX1fBat3bV&code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   Since requests using this authentication method result in the
   transmission significantly faster than revoking an entire set of clear-text credentials, the authorization server MUST
      refresh tokens.

   o  Implementing authentication management best practices which
      require the use periodic credentials rotation.  Rotation of an entire set
      of refresh tokens can be challenging, while rotation of a transport-layer security single
      set of client credentials is significantly easier.  In addition,
      this specification does not provide a mechanism when sending
   requests to the for refresh token endpoint.

3.2.  Other Client Authentication Methods
      rotation.

   The authorization server MAY support any suitable HTTP authentication
   scheme matching its security requirements.  When using other
   authentication methods, ramifications of allowing unauthenticated access by
   public clients to the authorization server token endpoint MUST define a
   mapping between the client identifier and be considered, as well as
   the credentials used issuance of refresh tokens to
   authenticate. public clients, their scope, and
   lifetime.

4.  Obtaining Authorization

   To request an access token, the client obtains authorization from the
   resource owner.  The authorization is expressed in the form of an
   authorization grant which the client uses to request the access
   token.  OAuth defines four grant types: authorization code, implicit,
   resource owner password credentials, and client credentials.  It also
   provides an extension mechanism for defining additional grant types.

4.1.  Authorization Code

   The authorization code grant type is suitable used to obtain both access
   tokens and refresh tokens and is optimized for clients capable of
   maintaining their client credentials confidential (for authenticating
   with the authorization server) such as a client implemented on a
   secure server. private clients.  As a
   redirection-based flow, the client must be capable of interacting
   with the resource owner's user-agent (typically a web browser) and
   capable of receiving incoming requests (via redirection) from the
   authorization server.

     +----------+
     | resource |
     |   owner  |
     |          |
     +----------+
          ^
          |
         (B)
     +----|-----+          Client Identifier      +---------------+
     |         -+----(A)---         -+----(A)-- & Redirect Redirection URI ------>| ---->|               |
     |  User-   |                                 | Authorization |
     |  Agent  -+----(B)-- User authenticates --->|     Server    |
     |          |                                 |               |
     |         -+----(C)-- Authorization Code ---<|               |
     +-|----|---+                                 +---------------+
       |    |                                         ^      v
      (A)  (C)                                        |      |
       |    |                                         |      |
       ^    v                                         |      |
     +---------+                                      |      |
     |         |>---(D)-- Client Credentials, --------'      |
     |         | Authorization Code, Code ---------'      |
     |  Client |          & Redirect Redirection URI                  |
     |         |                                             |
     |         |<---(E)----- Access Token -------------------'
     +---------+       (w/ Optional Refresh Token)

                     Figure 3: Authorization Code Flow

   The flow illustrated in Figure 3 includes the following steps:

   (A)  The client initiates the flow by directing the resource owner's
        user-agent to the authorization endpoint.  The client includes
        its client identifier, requested scope, local state, and a
        redirection URI to which the authorization server will send the
        user-agent back once access is granted (or denied).
   (B)  The authorization server authenticates the resource owner (via
        the user-agent) and establishes whether the resource owner
        grants or denies the client's access request.
   (C)  Assuming the resource owner grants access, the authorization
        server redirects the user-agent back to the client using the
        redirection URI provided earlier.  The redirection URI includes
        an authorization code and any local state provided by the client
        earlier.

   (D)  The client requests an access token from the authorization
        server's token endpoint by authenticating using its client
        credentials, and includes including the authorization code
        received in the previous step.  When making the request, the
        client authenticates with the authorization server.  The client
        includes the redirection URI used to obtain the authorization
        code for verification.
   (E)  The authorization server validates authenticates the client credentials, client, validates the
        authorization code, and ensures the redirection URI received
        matches the URI used to redirect the client in step (C).  If
        valid, responds back with an access token.

4.1.1.  Authorization Request

   The client constructs the request URI by adding the following
   parameters to the query component of the authorization endpoint URI
   using the "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html401-19991224]:

   response_type
         REQUIRED.  Value MUST be set to "code".
   client_id
         REQUIRED.  The client identifier as described in Section 3. 2.3.
   redirect_uri
         REQUIRED, unless a redirection URI has been established between
         the client and authorization server via other means.  Described
         OPTIONAL, as described in Section 2.1.1. 3.1.2.
   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited, case sensitive strings.  The value is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.
   state
         OPTIONAL.  An opaque value used by the client to maintain state
         between the request and callback.  The authorization server
         includes this value when redirecting the user-agent back to the
         client.

   The client directs the resource owner to the constructed URI using an
   HTTP redirection response, or by other means available to it via the
   user-agent.

   For example, the client directs the user-agent to make the following
   HTTP request using transport-layer security (extra line breaks are
   for display purposes only):

    GET /authorize?response_type=code&client_id=s6BhdRkqt3&
         redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1

    Host: server.example.com

   The authorization server validates the request to ensure all required
   parameters are present and valid.  If the request is valid, the
   authorization server authenticates the resource owner and obtains an
   authorization decision (by asking the resource owner or by
   establishing approval via other means).

   When a decision is established, the authorization server directs the
   user-agent to the provided client redirection URI using an HTTP
   redirection response, or by other means available to it via the user-
   agent.

4.1.2.  Authorization Response

   If the resource owner grants the access request, the authorization
   server issues an authorization code and delivers it to the client by
   adding the following parameters to the query component of the
   redirection URI using the "application/x-www-form-urlencoded" format:

   code
         REQUIRED.  The authorization code generated by the
         authorization server.  The authorization code SHOULD MUST expire
         shortly after it is issued to minimize mitigate the risk of leaks.  A
         maximum authorization code lifetime of 10 minutes is
         RECOMMENDED.  The client MUST NOT reuse the authorization code.
         If an authorization code is used more than once, the
         authorization server MAY SHOULD attempt to revoke all tokens
         previously issued based on that authorization code.  The
         authorization code is bound to the client identifier and
         redirection URI.
   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server redirects the user-agent by
   sending the following HTTP response:

     HTTP/1.1 302 Found
     Location: https://client.example.com/cb?code=i1WsRn1uB1 https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA
               &state=xyz

   The client SHOULD ignore unrecognized response parameters.  The
   authorization code string size is left undefined by this
   specification.  The client should avoid making assumptions about code
   value sizes.  The authorization server should document the size of
   any value it issues.

4.1.2.1.  Error Response

   If the request fails due to a missing, invalid, or mismatching
   redirection URI, or if the client identifier provided is invalid, the
   authorization server SHOULD inform the resource owner of the error,
   and MUST NOT automatically redirect the user-agent to the invalid
   redirection URI.

   If the resource owner denies the access request or if the request
   fails for reasons other than a missing or invalid redirection URI,
   the authorization server informs the client by adding the following
   parameters to the query component of the redirection URI using the
   "application/x-www-form-urlencoded" format:

   error
         REQUIRED.  A single error code from the following:
         invalid_request
               The request is missing a required parameter, includes an
               unsupported parameter or parameter value, or is otherwise
               malformed.
         unauthorized_client
               The client is not authorized to request an authorization
               code using this method.
         access_denied
               The resource owner or authorization server denied the
               request.
         unsupported_response_type
               The authorization server does not support obtaining an
               authorization code using this method.
         invalid_scope
               The requested scope is invalid, unknown, or malformed.
         a 4xx or 5xx HTTP status code (except for 400 and 401)
         server_error
               The authorization server MAY set encountered an unexpected
               condition which prevented it from fulfilling the "error" parameter
               value request.
         temporarily_unavailable
               The authorization server is currently unable to a numerical HTTP status code from handle
               the 4xx request due to a temporary overloading or 5xx
               range, with the exception maintenance
               of the 400 (Bad Request) and
               401 (Unauthorized) status codes.  For example, if the
               service is temporarily unavailable, the authorization
               server MAY return an error response with "error" set to
               "503". server.
   error_description
         OPTIONAL.  A human-readable UTF-8 encoded text providing
         additional information, used to assist in the client developer in
         understanding and resolution
         of the error occurred. [[ add language and encoding information
         ]] that occurred.

   error_uri
         OPTIONAL.  A URI identifying a human-readable web page with
         information about the error, used to provide the resource owner client
         developer with additional information about the error.
   state
         REQUIRED if a valid "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server redirects the user-agent by
   sending the following HTTP response:

   HTTP/1.1 302 Found
   Location: https://client.example.com/cb?error=access_denied https://client.example.com/cb?error=access_denied&state=xyz

4.1.3.  Access Token Request

   The client makes a request to the token endpoint by adding the
   following parameters using the "application/x-www-form-urlencoded"
   format in the HTTP request entity-body:

   grant_type
         REQUIRED.  Value MUST be set to "authorization_code".
   client_id
         REQUIRED.  The client identifier as described in Section 3.
   code
         REQUIRED.  The authorization code received from the
         authorization server.
   redirect_uri
         REQUIRED.  The redirection URI used by
         REQUIRED, if the authorization server
         to return "redirect_uri" parameter was included in the
         authorization response request described in Section 4.1.1, and their
         values MUST be identical.

   If the previous step.

   The client includes its authentication type is private or was issued client credentials (or
   assigned other authentication requirements), the client MUST
   authenticate with the authorization server as described in
   Section 3 3.2.1.

   For example, the client makes the following HTTP using transport-
   layer security (extra line breaks are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&client_id=s6BhdRkqt3&
     code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb application/x-www-form-urlencoded;charset=UTF-8

     grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
     &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   The authorization server MUST:

   o  Validate the  require client authentication for private clients or for any
      client issued client credentials (or with other authentication
      requirements),
   o  authenticate the client if client authentication is included and
      ensure that the authorization code was issued to that client. the authenticated
      client,
   o  Verify  verify that the authorization code is valid, and
   o  ensure that the
      redirection URI matches the redirection URI used by "redirect_uri" parameter is present if the
      authorization server to deliver
      "redirect_uri" parameter was included in the initial authorization code.
      request described in Section 4.1.1, and that their values are
      identical.

4.1.4.  Access Token Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in Section 5.1.  If the request client
   authentication failed or is invalid, the authorization server returns
   an error response as described in Section 5.2.

   An example successful response:

     HTTP/1.1 200 OK
     Content-Type: application/json application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "access_token":"SlAV32hkKG",
       "access_token":"2YotnFZFEjr1zCsicMWpAA",
       "token_type":"example",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8",
       "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
       "example_parameter":"example_value"
     }

4.2.  Implicit Grant

   The implicit grant type is suitable used to obtain access tokens (it does not
   support the issuance of refresh tokens) and is optimized for public
   clients incapable of
   maintaining their client credentials confidential (for authenticating
   with the authorization server) such as client applications residing
   in known to operate a user-agent, particular redirection URI.  These clients
   are typically implemented in a browser using a scripting language
   such as JavaScript.

   As a redirection-based flow, the client must be capable of
   interacting with the resource owner's user-agent (typically a web
   browser) and capable of receiving incoming requests (via redirection)
   from the authorization server.

   Unlike the authorization code grant type in which the client makes
   separate requests for authorization and access token, the client
   receives the access token as the result of the authorization request.

   Using the

   The implicit grant type does not include client authentication
   since the client is unable to maintain their credential
   confidentiality (the client resides authentication, and
   relies on the resource owner's computer
   or device which makes presence of the client credentials accessible resource owner and
   exploitable). the registration of
   the redirection URI.  Because the access token is encoded into the
   redirection URI, it may be exposed to the resource owner and other
   applications residing on its computer or device.

     +----------+
     | Resource |
     |  Owner   |
     |          |
     +----------+
          ^
          |
         (B)
     +----|-----+          Client Identifier     +---------------+
     |         -+----(A)---         -+----(A)-- & Redirect Redirection URI ----->| --->|               |
     |  User-   |                                | Authorization |
     |  Agent  -|----(B)-- User authenticates -->|     Server    |
     |          |                                |               |
     |          |<---(C)---- Redirect          |<---(C)--- Redirection URI ------<| ----<|               |
     |          |          with Access Token     +---------------+
     |          |            in Fragment
     |          |                                +---------------+
     |          |----(D)---- Redirect          |----(D)--- Redirection URI ------>|   Web Server ---->|   Web-Hosted  |
     |          |          without Fragment      |  with     Client    |
     |          |                                |    Resource   |
     |     (F)  |<---(E)------- Script ---------<|               |
     |          |                                +---------------+
     +-|--------+
       |    |
      (A)  (G) Access Token
       |    |
       ^    v
     +---------+
     |         |
     |  Client |
     |         |
     +---------+

                       Figure 4: Implicit Grant Flow

   The flow illustrated in Figure 4 includes the following steps:

   (A)  The client initiates the flow by directing the resource owner's
        user-agent to the authorization endpoint.  The client includes
        its client identifier, requested scope, local state, and a
        redirection URI to which the authorization server will send the
        user-agent back once access is granted (or denied).
   (B)  The authorization server authenticates the resource owner (via
        the user-agent) and establishes whether the resource owner
        grants or denies the client's access request.

   (C)  Assuming the resource owner grants access, the authorization
        server redirects the user-agent back to the client using the
        redirection URI provided earlier.  The redirection URI includes
        the access token in the URI fragment.
   (D)  The user-agent follows the redirection instructions by making a
        request to the web server (does web-hosted client resource (which does not
        include the fragment).  The user-agent retains the fragment
        information locally.
   (E)  The web server web-hosted client resource returns a web page (typically an
        HTML document with an embedded script) capable of accessing the
        full redirection URI including the fragment retained by the user-
        agent,
        user-agent, and extracting the access token (and other
        parameters) contained in the fragment.
   (F)  The user-agent executes the script provided by the web server web-hosted
        client resource locally, which extracts the access token and
        passes it to the client.

4.2.1.  Authorization Request

   The client constructs the request URI by adding the following
   parameters to the query component of the authorization endpoint URI
   using the "application/x-www-form-urlencoded" format:

   response_type
         REQUIRED.  Value MUST be set to "token".
   client_id
         REQUIRED.  The client identifier as described in Section 3.
   redirect_uri
         REQUIRED, unless a redirection URI has been established between
         the client and authorization server via other means.  Described identifier as described in Section 2.3.
   redirect_uri
         OPTIONAL, as described in Section 2.1.1. 3.1.2.
   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited, case sensitive strings.  The value is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.
   state
         OPTIONAL.  An opaque value used by the client to maintain state
         between the request and callback.  The authorization server
         includes this value when redirecting the user-agent back to the
         client.

   The client directs the resource owner to the constructed URI using an
   HTTP redirection response, or by other means available to it via the
   user-agent.

   For example, the client directs the user-agent to make the following
   HTTP request using transport-layer security (extra line breaks are
   for display purposes only):

    GET /authorize?response_type=token&client_id=s6BhdRkqt3&
         redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
    Host: server.example.com

   The authorization server validates the request to ensure all required
   parameters are present and valid.  The authorization server MUST
   verify that the redirection URI to which it will redirect the access
   token matches a redirection URI registered by the client as described
   in Section 3.1.2.

   If the request is valid, the authorization server authenticates the
   resource owner and obtains an authorization decision (by asking the
   resource owner or by establishing approval via other means).

   When a decision is established, the authorization server directs the
   user-agent to the provided client redirection URI using an HTTP
   redirection response, or by other means available to it via the user-
   agent.

4.2.2.  Access Token Response

   If the resource owner grants the access request, the authorization
   server issues an access token and delivers it to the client by adding
   the following parameters to the fragment component of the redirection
   URI using the "application/x-www-form-urlencoded" format:

   access_token
         REQUIRED.  The access token issued by the authorization server.
   token_type
         REQUIRED.  The type of the token issued as described in
         Section 7.1.  Value is case insensitive.
   expires_in
         OPTIONAL.  The duration in seconds of the access token
         lifetime.  For example, the value "3600" denotes that the
         access token will expire in one hour from the time the response
         was generated.
   scope
         OPTIONAL.  The scope of the access request token expressed as a list of
         space-delimited, case sensitive strings.  The value is defined
         by the authorization server.  If the value contains multiple
         space-delimited strings, their order does not matter, and each
         string adds an additional access range to the requested scope.

         The authorization server SHOULD include the parameter if the requested
         access token scope is different from the one requested by the
         client.
   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server redirects the user-agent by
   sending the following HTTP response (URI extra line breaks are for
   display purposes only):

     HTTP/1.1 302 Found
     Location: http://example.com/rd#access_token=FJQbwq9&
               token_type=example&expires_in=3600 http://example.com/rd#access_token=2YotnFZFEjr1zCsicMWpAA
               &state=xyz&token_type=example&expires_in=3600

   Developers should note that some HTTP client implementations do not
   support the inclusion of a fragment component in the HTTP "Location"
   response header field.  Such client will require using other methods
   for redirecting the client than a 3xx redirection response.  For
   example, returning an HTML page which includes a 'continue' button
   with an action linked to the redirection URI.

   The client SHOULD ignore unrecognized response parameters.  The
   access token string size is left undefined by this specification.
   The client should avoid making assumptions about value sizes.  The
   authorization server should document the size of any value it issues.

4.2.2.1.  Error Response

   If the request fails due to a missing, invalid, or mismatching
   redirection URI, or if the client identifier provided is invalid, the
   authorization server SHOULD inform the resource owner of the error,
   and MUST NOT automatically redirect the user-agent to the invalid
   redirection URI.

   If the resource owner denies the access request or if the request
   fails for reasons other than a missing or invalid redirection URI,
   the authorization server informs the client by adding the following
   parameters to the fragment component of the redirection URI using the
   "application/x-www-form-urlencoded" format:

   error
         REQUIRED.  A single error code from the following:
         invalid_request
               The request is missing a required parameter, includes an
               unsupported parameter or parameter value, or is otherwise
               malformed.
         unauthorized_client
               The client is not authorized to request an access token
               using this method.
         access_denied
               The resource owner or authorization server denied the
               request.
         unsupported_response_type
               The authorization server does not support obtaining an
               access token using this method.
         invalid_scope
               The requested scope is invalid, unknown, or malformed.
         a 4xx or 5xx HTTP status code (except for 400 and 401)
         server_error
               The authorization server MAY set encountered an unexpected
               condition which prevented it from fulfilling the "error" parameter
               value request.
         temporarily_unavailable
               The authorization server is currently unable to a numerical HTTP status code from handle
               the 4xx request due to a temporary overloading or 5xx
               range, with the exception maintenance
               of the 400 (Bad Request) and
               401 (Unauthorized) status codes.  For example, if the
               service is temporarily unavailable, the authorization
               server MAY return an error response with "error" set to
               "503". server.
   error_description
         OPTIONAL.  A human-readable UTF-8 encoded text providing
         additional information, used to assist in the client developer in
         understanding and resolution
         of the error that occurred. [[ add language and encoding information
         ]]
   error_uri
         OPTIONAL.  A URI identifying a human-readable web page with
         information about the error, used to provide the resource owner client
         developer with additional information about the error.
   state
         REQUIRED if a valid "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server redirects the user-agent by
   sending the following HTTP response:

   HTTP/1.1 302 Found
   Location: https://client.example.com/cb#error=access_denied https://client.example.com/cb#error=access_denied&state=xyz

4.3.  Resource Owner Password Credentials

   The resource owner password credentials grant type is suitable in
   cases where the resource owner has a trust relationship with the
   client, such as its computer operating system or a highly privileged
   application.  The authorization server should take special care when
   enabling the grant type, and only when other flows are not viable.

   The grant type is suitable for clients capable of obtaining the
   resource owner credentials (username and password, typically using an
   interactive form).  It is also used to migrate existing clients using
   direct authentication schemes such as HTTP Basic or Digest
   authentication to OAuth by converting the stored credentials with to an
   access token.

     +----------+
     | Resource |
     |  Owner   |
     |          |
     +----------+
          v
          |    Resource Owner
         (A) Password Credentials
          |
          v
     +---------+                                  +---------------+
     |         |          Client Credentials      |               |
     |         |>--(B)---- & Resource Owner ----->| ------->|               |
     | Client         |         Password Credentials     | Authorization |
     | Client  |                                  |     Server    |
     |         |<--(C)---- Access Token ---------<|               |
     |         |    (w/ Optional Refresh Token)   |               |
     +---------+                                  +---------------+

            Figure 5: Resource Owner Password Credentials Flow

   The flow illustrated in Figure 5 includes the following steps:

   (A)  The resource owner provides the client with its username and
        password.
   (B)  The client requests an access token from the authorization
        server's token endpoint by authenticating using its client
        credentials, and includes including the credentials received
        from the resource owner.  When making the request, the client
        authenticates with the authorization server.

   (C)  The authorization server authenticates the client and validates
        the resource owner
        credentials and the client credentials credentials, and if valid issues an access
        token.

4.3.1.  Authorization Request and Response

   The method through which the client obtains the resource owner
   credentials is beyond the scope of this specification.  The client
   MUST discard the credentials once an access token has been obtained.

4.3.2.  Access Token Request

   The client makes a request to the token endpoint by adding the
   following parameters using the "application/x-www-form-urlencoded"
   format in the HTTP request entity-body:

   grant_type
         REQUIRED.  Value MUST be set to "password".
   client_id
         REQUIRED.  The client identifier as described in Section 3.
   username
         REQUIRED.  The resource owner username, encoded as UTF-8.
   password
         REQUIRED.  The resource owner password, encoded as UTF-8.
   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited, case sensitive strings.  The value is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   The

   If the client type is private or was issued client includes its authentication credentials (or
   assigned other authentication requirements), the client MUST
   authenticate with the authorization server as described in
   Section 3 3.2.1.

   For example, the client makes the following HTTP request using
   transport-layer security (extra line breaks are for display purposes
   only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     grant_type=password&client_id=s6BhdRkqt3&
     username=johndoe&password=A3ddj3w application/x-www-form-urlencoded;charset=UTF-8

     grant_type=password&username=johndoe&password=A3ddj3w

   The authorization server MUST:

   o  Validate  require client authentication for private clients or for any
      client issued client credentials (or with other authentication
      requirements),
   o  authenticate the client credentials. if client authentication is included, and
   o  Validate  validate the resource owner password credentials.

   Since this access token request utilizes the resource owner's
   password, the authorization server MUST protect the endpoint against
   brute force attacks.

4.3.3.  Access Token Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in Section 5.1.  If the request failed client
   authentication or is invalid, the authorization server returns an
   error response as described in Section 5.2.

   An example successful response:

     HTTP/1.1 200 OK
     Content-Type: application/json application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "access_token":"SlAV32hkKG",
       "access_token":"2YotnFZFEjr1zCsicMWpAA",
       "token_type":"example",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8",
       "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
       "example_parameter":"example_value"
     }

4.4.  Client Credentials

   The client can request an access token using only its client
   credentials (or other supported means of authentication) when the
   client is requesting access to the protected resources under its
   control, or those of another resource owner which has been previously
   arranged with the authorization server (the method of which is beyond
   the scope of this specification).

   The client credentials grant type MUST only be used by private
   clients.

     +---------+                                  +---------------+
     |         |                                  |               |
     |         |>--(A)---         |>--(A)- Client Credentials ---->| Authentication --->| Authorization |
     | Client  |                                  |     Server    |
     |         |<--(B)---- Access Token ---------<|               |
     |         |    (w/ Optional Refresh Token)                                  |               |
     +---------+                                  +---------------+

                     Figure 6: Client Credentials Flow

   The flow illustrated in Figure 6 includes the following steps:

   (A)  The client authenticates with the authorization server and
        requests an access token from the token endpoint by
        authenticating using its client credentials. endpoint.
   (B)  The authorization server validates authenticates the client credentials client, and if valid
        issues an access token.

4.4.1.  Authorization Request and Response

   Since the client credentials are authentication is used as the authorization grant,
   no additional authorization request is needed as the client is already
   in the possession of its client credentials. needed.

4.4.2.  Access Token Request

   The client makes a request to the token endpoint by adding the
   following parameters using the "application/x-www-form-urlencoded"
   format in the HTTP request entity-body:

   grant_type
         REQUIRED.  Value MUST be set to "client_credentials".
   client_id
         REQUIRED.  The client identifier as described in Section 3.
   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited, case sensitive strings.  The value is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   The client includes its authentication credentials MUST authenticate with the authorization server as
   described in Section 3 3.2.1.

   For example, the client makes the following HTTP request using
   transport-layer security (extra line breaks are for display purposes
   only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     grant_type=client_credentials&client_id=s6BhdRkqt3 application/x-www-form-urlencoded;charset=UTF-8

     grant_type=client_credentials

   The authorization server MUST validate authenticate the client credentials. client.

4.4.3.  Access Token Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in
   Section 5.1.  A refresh token SHOULD NOT be included.  If the request
   failed client authentication or is invalid, the authorization server
   returns an error response as described in Section 5.2.

   An example successful response:

     HTTP/1.1 200 OK
     Content-Type: application/json application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "access_token":"SlAV32hkKG",
       "access_token":"2YotnFZFEjr1zCsicMWpAA",
       "token_type":"example",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8",
       "example_parameter":"example_value"
     }

4.5.  Extensions

   The client uses an extension grant type by specifying the grant type
   using an absolute URI (defined by the authorization server) as the
   value of the "grant_type" parameter of the token endpoint, and by
   adding any additional parameters necessary.

   For example, to request an access token using a SAML 2.0 assertion
   grant type as defined by [I-D.ietf-oauth-saml2-bearer], the client
   makes the following HTTP request using transport-layer security (line
   breaks are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded application/x-www-form-urlencoded;charset=UTF-8

     grant_type=http%3A%2F%2Foauth.net%2Fgrant_type%2Fassertion%2F
     saml%2F2.0%2Fbearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ
     [...omitted for brevity...]V0aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in Section 5.1.  If the request failed client
   authentication or is invalid, the authorization server returns an
   error response as described in Section 5.2.

5.  Issuing an Access Token

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in Section 5.1.  If the request failed client
   authentication or is invalid, the authorization server returns an
   error response as described in Section 5.2.

5.1.  Successful Response

   The authorization server issues an access token and optional refresh
   token, and constructs the response by adding the following parameters
   to the entity body of the HTTP response with a 200 (OK) status code:

   access_token
         REQUIRED.  The access token issued by the authorization server.
   token_type
         REQUIRED.  The type of the token issued as described in
         Section 7.1.  Value is case insensitive.
   expires_in
         OPTIONAL.  The duration in seconds of the access token
         lifetime.  For example, the value "3600" denotes that the
         access token will expire in one hour from the time the response
         was generated.

   refresh_token
         OPTIONAL.  The refresh token which can be used to obtain new
         access tokens using the same authorization grant as described
         in Section 6.
   scope
         OPTIONAL.  The scope of the access request token expressed as a list of
         space-delimited, case sensitive strings.  The value is defined
         by the authorization server.  If the value contains multiple
         space-delimited strings, their order does not matter, and each
         string adds an additional access range to the requested scope.
         The authorization server SHOULD include the parameter if the requested
         access token scope is different from the one requested by the
         client.

   The parameters are included in the entity body of the HTTP response
   using the "application/json" media type as defined by [RFC4627].  The
   parameters are serialized into a JSON structure by adding each
   parameter at the highest structure level.  Parameter names and string
   values are included as JSON strings.  Numerical values are included
   as JSON numbers.

   The authorization server MUST include the HTTP "Cache-Control"
   response header field [RFC2616] with a value of "no-store" in any
   response containing tokens, secrets, or other sensitive information. information,
   as well as the "Pragma" response header field [RFC2616] with a value
   of "no-cache".

   For example:

     HTTP/1.1 200 OK
     Content-Type: application/json application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "access_token":"SlAV32hkKG",
       "access_token":"2YotnFZFEjr1zCsicMWpAA",
       "token_type":"example",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8",
       "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
       "example_parameter":"example_value"
     }

   The client SHOULD ignore unrecognized response parameters.  The sizes
   of tokens and other values received from the authorization server are
   left undefined.  The client should avoid making assumptions about
   value sizes.  The authorization server should document the size of
   any value it issues.

5.2.  Error Response

   The authorization server responds with an HTTP 400 (Bad Request)
   status code and includes the following parameters with the response:

   error
         REQUIRED.  A single error code from the following:
         invalid_request
               The request is missing a required parameter, includes an
               unsupported parameter or parameter value, repeats a
               parameter, includes multiple credentials, utilizes more
               than one mechanism for authenticating the client, or is
               otherwise malformed.
         invalid_client
               Client authentication failed (e.g. unknown client, no
               client credentials authentication included, multiple client credentials
               authentications included, or unsupported credentials type). authentication
               method).  The authorization server MAY return an HTTP 401
               (Unauthorized) status code to indicate which HTTP
               authentication schemes are supported.  If the client
               attempted to authenticate via the "Authorization" request
               header field, the authorization server MUST respond with
               an HTTP 401 (Unauthorized) status code, and include the
               "WWW-Authenticate" response header field matching the
               authentication scheme used by the client.
         invalid_grant
               The provided authorization grant is invalid, expired,
               revoked, does not match the redirection URI used in the
               authorization request, or was issued to another client.
         unauthorized_client
               The authenticated client is not authorized to use this
               authorization grant type.
         unsupported_grant_type
               The authorization grant type is not supported by the
               authorization server.
         invalid_scope
               The requested scope is invalid, unknown, malformed, or
               exceeds the scope granted by the resource owner.
   error_description
         OPTIONAL.  A human-readable UTF-8 encoded text providing
         additional information, used to assist in the client developer in
         understanding and resolution
         of the error that occurred. [[ add language and encoding information
         ]]

   error_uri
         OPTIONAL.  A URI identifying a human-readable web page with
         information about the error, used to provide the resource owner client
         developer with additional information about the error.

   The parameters are included in the entity body of the HTTP response
   using the "application/json" media type as defined by [RFC4627].  The
   parameters are serialized into a JSON structure by adding each
   parameter at the highest structure level.  Parameter names and string
   values are included as JSON strings.  Numerical values are included
   as JSON numbers.

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/json application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "error":"invalid_request"
     }

   If the authorization server encounters an error condition other than
   the 400 (Bad Request) and 401 (Unauthorized) responses described
   above (e.g. the service is temporarily unavailable), the
   authorization server SHOULD include an error response in the entity
   body, and set the "error" parameter value to the numerical HTTP
   status code returned.

   For example:

     HTTP/1.1 503 Service Unavailable
     Content-Type: application/json

     {
       "error":"503"
     }

6.  Refreshing an Access Token

   If the authorization server issued a refresh token to the client, the
   client makes a refresh request to the token endpoint by adding the
   following parameters using the "application/x-www-form-urlencoded"
   format in the HTTP request entity-body:

   grant_type
         REQUIRED.  Value MUST be set to "refresh_token".
   client_id
         REQUIRED.  The client identifier as described in Section 3.
   refresh_token
         REQUIRED.  The refresh token issued to the client.
   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited, case sensitive strings.  The value is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.  The requested scope MUST be equal or lesser
         than the scope originally granted by the resource owner, and if
         omitted is treated as equal to the scope originally granted by
         the resource owner.

   The resource owner.

   Because refresh tokens are typically long-lasting credentials used to
   request additional access tokens, the refresh token is bound to the
   client it was issued.  If the client type is private or was issued
   client includes its authentication credentials (or assigned other authentication requirements),
   the client MUST authenticate with the authorization server as
   described in Section 3. 3.2.1.

   For example, the client makes the following HTTP request using
   transport-layer security (extra line breaks are for display purposes
   only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     grant_type=refresh_token&client_id=s6BhdRkqt3&
     refresh_token=n4E9O119d application/x-www-form-urlencoded;charset=UTF-8

     grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA

   The authorization server MUST validate MUST:

   o  require client authentication for private clients or for any
      client issued client credentials (or with other authentication
      requirements),
   o  authenticate the client credentials, if client authentication is included and
      ensure
   that the refresh token was issued to the authenticated client,
   o  validate the refresh token, and
   o  verify that the resource owner's authorization is still valid.

   If valid and authorized, the authorization server issues an access
   token as described in Section 5.1.  If the request failed
   verification or is invalid, the authorization server returns an error
   response as described in Section 5.2.

   The authorization server MAY issue a new refresh token, in which
   case, case
   the client MUST discard the old refresh token and replace it with the
   new refresh token.  The authorization server MAY revoke the old
   refresh token after issuing a new refresh token to the client.  If a
   new refresh token is issued, its scope MUST be identical to that of
   the refresh token included in the request.

7.  Accessing Protected Resources

   The client accesses protected resources by presenting the access
   token to the resource server.  The resource server MUST validate the
   access token and ensure it has not expired and that its scope covers
   the requested resource.  The methods used by the resource server to
   validate the access token (as well as any error responses) are beyond
   the scope of this specification, but generally involve an interaction
   or coordination between the resource server and the authorization
   server.

   The method in which the client utilized the access token to
   authenticate with the resource server depends on the type of access
   token issued by the authorization server.  Typically, it involves
   using the HTTP "Authorization" request header field [RFC2617] with an
   authentication scheme defined by the access token type specification.

7.1.  Access Token Types

   The access token type provides the client with the information
   required to successfully utilize the access token to make a protected
   resource request (along with type-specific attributes).  The client
   MUST NOT use an access token if it does not understand or does not
   trust the token type.

   For example, the "bearer" token type defined in
   [I-D.ietf-oauth-v2-bearer] is utilized by simply including the access
   token string in the request:

     GET /resource/1 HTTP/1.1
     Host: example.com
     Authorization: Bearer 7Fjfp0ZBr1KtDRbnfVdmIw

   while the "mac" token type defined in [I-D.ietf-oauth-v2-http-mac] is
   utilized by issuing a MAC key together with the access token which is
   used to sign certain components of the HTTP requests:

     GET /resource/1 HTTP/1.1
     Host: example.com
     Authorization: MAC id="h480djs93hd8",
                        nonce="274312:dj83hs9s",
                        mac="kDZvddkndxvhGRXZhvuDjEWhGeE="

   The above examples are provided for illustration purposes only.
   Developers are advised to consult the [I-D.ietf-oauth-v2-bearer] and
   [I-D.ietf-oauth-v2-http-mac] specifications before use.

   Each access token type definition specifies the additional attributes
   (if any) sent to the client together with the "access_token" response
   parameter.  It also defines the HTTP authentication method used to
   include the access token when making a protected resource request.

8.  Extensibility

8.1.  Defining Access Token Types

   Access token types can be defined in one of two ways: registered in
   the access token type registry (following the procedures in
   Section 11.1), or use a unique absolute URI as its name.

   Types utilizing a URI name SHOULD be limited to vendor-specific
   implementations that are not commonly applicable, and are specific to
   the implementation details of the resource server where they are
   used.

   All other types MUST be registered.  Type names MUST conform to the
   type-name ABNF.  If the type definition includes a new HTTP
   authentication scheme, the type name SHOULD be identical to the HTTP
   authentication scheme name (as defined by [RFC2617]).

     type-name  = 1*name-char
     name-char   = "-" / "." / "_" / DIGIT / ALPHA

8.2.  Defining New Endpoint Parameters

   New request or response parameters for use with the authorization
   endpoint or the token endpoint are defined and registered in the
   parameters registry following the procedure in Section 11.2.

   Parameter names MUST conform to the param-name ABNF and parameter
   values syntax MUST be well-defined (e.g., using ABNF, or a reference
   to the syntax of an existing parameter).

     param-name  = 1*name-char
     name-char   = "-" / "." / "_" / DIGIT / ALPHA

   Unregistered vendor-specific parameter extensions that are not
   commonly applicable, and are specific to the implementation details
   of the authorization server where they are used SHOULD utilize a
   vendor-specific prefix that is not likely not likely to conflict with other
   registered values (e.g. begin with 'companyname_').

8.3.  Defining New Authorization Grant Types

   New authorization grant types can be defined by assigning them a
   unique absolute URI for use with the "grant_type" parameter.  If the
   extension grant type requires additional token endpoint parameters,
   they MUST be registered in the OAuth parameters registry as described
   by Section 11.2.

8.4.  Defining New Authorization Endpoint Response Types

   [[ Pending consensus ]]

   New response types for use with the authorization endpoint are
   defined and registered in the authorization endpoint response type
   registry following the procedure in Section 11.3.  Response type
   names MUST conform to the response-type ABNF.

     response-type  = response-name *( "+" response-name )
     response-name  = 1*response-char
     response-char  = "_" / DIGIT / ALPHA

   The "+" character is reserved for defining composite response types
   made up of two or more existing registered response types.  Only one
   response type of each combination may be registered and used for
   making requests.  Composite response types are treated and compared
   in the same as manner as non-composite response types.  The "+"
   notation is meant only to conflict with other
   registered values (e.g. begin with 'companyname_').

8.3.  Defining New Authorization Grant Types

   New authorization grant types can be defined by assigning them a
   unique absolute URI improve human readability and is not used
   for use with machine parsing.

   For example, an extension can define and register the "grant_type" parameter.  If "token+code"
   response type.  However, once registered, the
   extension grant type requires additional token endpoint parameters,
   they MUST same combination cannot
   be registered in the OAuth parameters registry as described
   by Section 11.2.

8.4. "code+token", or used to make an authorization
   request.

8.5.  Defining Additional Error Codes

   In cases where protocol extensions (i.e. access token types,
   extension parameters, or extension grant types) require additional
   error codes to be used with the authorization code grant error
   response (Section 4.1.2.1), the implicit grant error response
   (Section 4.2.2.1), or the token error response (Section 5.2), such
   error codes MAY be defined.

   Extension error codes MUST be registered (following the procedures in
   Section 11.3) 11.4) if the extension they are used in conjunction with is a
   registered access token type, a registered endpoint parameter, or an
   extension grant type.  Error codes used with unregistered extensions
   MAY be registered.

   Error codes MUST conform to the error-code ABNF, and SHOULD be
   prefixed by an identifying name when possible.  For example, an error
   identifying an invalid value set to the extension parameter "example"
   should be named "example_invalid".

     error-code   = ALPHA *error-char
     error-char   = "-" / "." / "_" / DIGIT / ALPHA

9.  Native Applications

   [[ Pending consensus ]]

   A native application is a client which is

   Native applications are clients installed and executes executed on the end-user's
   resource owner's device (i.e. desktop application, native mobile
   application).  Native applications are often capable of interacting
   with (or embedding) a user-agent but are limited in how such
   interactions affects their may require special consideration
   related to security, platform capabilities, and overall end-user
   experience.  In many
   cases, native applications are incapable of receiving redirection
   requests from the

   The authorization server (e.g. due to firewall rules,
   operating system restrictions). endpoint requires interaction between the client
   and the resource owner's user-agent.  Native applications can utilize OAuth in different ways, based on
   their requirements and desired end-user experience:

   o  Use the authorization code grant type flow described in
      Section 4.1 by launching invoke
   an external user-agent.  The user-agent or embed a user-agent within the application.
   For example:

   o  External user-agent - the native application can capture the
      response by providing from the authorization server using a redirection URI identifying a local (non-network) resource (registered
      with an scheme registered with the operating system to invoke the native application
      client as handler), the handler, manual copy-and-paste of the credentials,
      running a local web server, installing a user-agent plug-in, or by
      providing a redirection URI identifying a server-hosted resource
      under the native application's client's control, which in turn makes the response available to the native application (e.g. using
      the user-agent window title or other locations accessible from
      outside response
      available to the user-agent). native application.
   o  Use  Embedded user-agent - the authorization code grant type flow described in
      Section 4.1 by embedding a user-agent.  The native application obtains the response
      by directly communicating with the embedded
      user-agent.  Embedded user-agents are discouraged as they
      typically provide a less consistent user experience and do not
      enable the end-user to verify the authorization server's
      authenticity.

   Native applications SHOULD use user-agent by
      monitoring state changes emitted during the authorization code grant type flow
   without client password credentials (due to their inability to keep resource load,
      monitoring HTTP headers, or accessing the credentials confidential) to obtain short-lived access tokens,
   and use refresh tokens to maintain access. user-agent's cookies
      storage.

   When choosing between launching an external user-agent and an
   embedding a or embedded user-agent, native application developers
   should consider
   the following: consider:

   o  External user-agents may improve completion rate as the end-user resource
      owner may already have an active session with the authorization
      server removing the need to re-authenticate, and provide a
      familiar user-
      agent user-agent user experience. experience and functionality.  The end-user
      resource owner may also rely on extensions or add-ons to assist
      with authentication (e.g. password managers or 2-factor device
      reader).
   o  Embedded user-agents often may offer a better end-user flow, an improved usability, as they
      remove the need to switch context and open new windows but also
      may provide less familiar features than the external user-agent. windows.
   o  Embedded user-agents pose a security challenge because end-users resource
      owners are authenticating in an unidentified window without access
      to the visual protections offered found on by many user-agents.  Embedded of the external user-
      agents
      agents.  Embedded user-agents educate end-user to trust
      unidentified requests for authentication (making phishing attacks
      easier to execute).

   When choosing between implicit and authorization code grant types,
   the following should be considered:

   o  Native applications that use the authorization code grant type
      flow SHOULD do so without using client password credentials, due
      to the native application's inability to keep those credentials
      secure.
   o  When using the implicit grant type flow a refresh token is not
      returned.

10.  Security Considerations

   As a flexible and extensible framework, OAuth's security
   considerations depend on many factors.  The following sections
   provide implementers with security guidelines focused on three common
   client types:

   Web Application
      A web application is a client running on a web server.  End-users  Resource
      owners access the client via an HTML user interface rendered in a user-
      agent
      user-agent on the end-user's resource-owner's device.  The client credentials
      as well as any access token issued to the client are stored on the
      web server and are not exposed to or accessible by the end-user. resource
      owner.
   User-Agent-based Application
      A user-agent-based application is a client in which the client
      code is downloaded from a web server and executes within a user-
      agent on the end-user's resource owner's device.  The OAuth protocol data and
      credentials are accessible to the end-user. resource owner.  Since such
      applications directly reside within the user-agent, they can make
      seamless use of the user-agent capabilities in the end-user resource owner
      authorization process.

   Native Application
      A native application is a client which is installed and executes
      on the end-user's resource owner's device.  The OAuth protocol data and
      credentials are accessible to the end-user. resource owner.  It is assumed
      that such an any client authentication credentials included in the
      application can protect dynamically be extracted, and furthermore that rotation of the
      client authentication credentials is not practical.  Dynamically
      issued credentials, credentials such as access tokens or refresh tokens, on the
      other hand, can receive an acceptable level of protection.  At a
      minimum these credentials are protected from hostile servers which
      the application may contact.  On some platform those credentials
      might be protected from eavesdropping by other applications residing on the same
      device.

   A comprehensive OAuth security model and analysis, as well as
   background for the protocol design is provided in
   [I-D.lodderstedt-oauth-security].

10.1.  Client Authentication

   The authorization server issues client credentials to web
   applications for the purpose of authenticating them.  The
   authorization server is encouraged to consider using stronger client
   authentication means than a client password.  Application developers
   MUST ensure confidentiality of client passwords and other
   credentials.

   The authorization server MUST NOT issue client passwords or other
   credentials to native or user-agent-based applications for the
   purpose of client authentication.  The authorization server MAY issue
   a client password or other credentials for a specific installation of
   a native application on a specific device.

10.2.  Client Impersonation

   Given the inability of some clients to keep their client credentials
   confidential, a malicious client can impersonate another client and
   obtain access to protected resources.  The authorization server MUST
   authenticate the client whenever possible.  If the authorization
   server cannot authenticate the a client due to the client's
   limitations, the authorization server should utilize other means to
   protect resource owners from such malicious clients, including but
   not limited to engaging the end-user resource owner to assist in identifying
   the client and its source.

   The authorization server SHOULD enforce explicit end-user resource owner
   authentication, or prompt the end-user resource owner to authorize access
   again, providing the end-user resource owner with information about the
   client, scope, and duration of the authorization.  It is up to the end-user
   resource owner to review the information in the context of the
   current client, and authorize the request.

   The authorization server SHOULD NOT automatically, without active
   end-user
   resource owner interaction, process repeated authorization requests
   without authenticating the client or relying on other measures to
   ensure the repeated request comes from a valid client and not an
   impersonator.

   The authorization server SHOULD require the client to pre-register register its
   redirection URI and validate the value of the "redirect_uri" against
   the pre-registered registered value.  The client MUST NOT serve an open redirector
   resource which can be used by an attacker to construct an URI that
   will pass the authorization server's redirection URI matching rules,
   and will redirect the end-user's resource owner's user-agent to the attacker's
   server.

   The authorization server SHOULD issue access tokens with limited
   scope and duration to clients incapable of authenticating.

10.3.  Access Token Credentials

   Access token credentials MUST be kept confidential in transit and
   storage, and shared only among the authorization server, the resource
   servers the credentials are valid for, and the client to whom the
   credentials were issued.

   When using the implicit grant type, the access token credentials are
   transmitted in the URI fragment, which can expose the credentials to
   unauthorized parties.

   The authorization server MUST ensure that access token credentials
   cannot be generated, modified, or guessed to produce valid access
   token credentials.

   The client SHOULD request access token credentials with the minimal
   scope and duration necessary.  The authorization server SHOULD take
   the client identity into account when choosing to honor the requested
   scope, and MAY issue credentials with a lesser scope than requested.

10.4.  Refresh Tokens

   Authorization servers MAY issue refresh tokens to web and native
   applications.

   Refresh tokens MUST be kept confidential in transit and storage, and
   shared only among the authorization server and the client to whom the
   refresh tokens were issued.  The authorization server MUST maintain
   the link between a refresh token and the client to whom it was
   issued.

   The authorization server MUST verify the link between the refresh
   token and client identity whenever the client's identity can be
   authenticated.  When client authentication is not possible, the
   authorization server SHOULD deploy other means to detect refresh
   token abuse.

   The authorization server MUST ensure that refresh tokens cannot be
   generated, modified, or guessed to produce valid refresh tokens.

10.5.  Request Confidentiality

   Access token credentials, refresh tokens, resource owner passwords,
   and client secrets MUST NOT be transmitted in the clear.
   Authorization codes SHOULD NOT be transmitted in the clear.

10.6.  Endpoints Authenticity

   In order to prevent man-in-the-middle and phishing attacks, the
   authorization server MUST implement and require TLS with server-side server
   authentication in all exchanges. exchanges as described by [RFC2818].  The
   client MUST verify validate the authorization server's TLS certificate, as well as the respective certificate chain. in
   accordance with its requirements for authentication of the server's
   identity.

10.7.  Credentials Guessing Attacks

   The authorization server MUST prevent attackers from guessing access
   tokens, authorization codes, refresh tokens, resource owner
   passwords, and client secrets.

   When generating tokens and other secrets not intended for direct
   human utilization, the authorization server MUST use a reasonable
   level of entropy in order to mitigate the risk of guessing attacks.
   When creating secrets intended for human usage, the authorization
   server MUST utilize other means to protect those secrets.

10.8.  Phishing Attacks

   Native applications SHOULD use external browsers instead of embedding
   browsers within the application when requesting end-user resource owner
   authorization.  External browsers offer a familiar user experience
   and a trusted environment in which end-users resource owners can confirm the
   authenticity of the authorization server.

   To reduce the risk of phishing attacks, the authorization servers
   MUST utilize TLS to allow user-agents to validate the authorization
   server's identity.  Service providers should educate their end-users
   about the risks of phishing attacks and how they can verify the
   authorization server's identity.

10.9.  Authorization Codes

   The transmission of authorization codes SHOULD be made over a secure
   channel, and the client SHOULD implement TLS for use with its
   redirection URI if the URI identifies a network resource.
   Authorization codes MUST be kept confidential.  Since authorization
   codes are transmitted via user-agent redirections, they could
   potentially be disclosed through user-agent history and HTTP referrer
   headers.

   Authorization codes operate as plaintext bearer credentials, used to
   verify that the end-user resource owner who granted authorization at the
   authorization server, is the same end-user resource owner returning to the
   client to complete the process.  Therefore, if the client relies on
   the authorization code for its own end-user resource owner authentication, the
   client redirection endpoint MUST require TLS.

   Authorization codes SHOULD MUST be short lived and MUST be single use.  If the
   authorization server observes multiple attempts to exchange an
   authorization code for an access token, the authorization server
   SHOULD attempt to revoke all access tokens already granted based on
   the compromised authorization code.

   If the client can be authenticated, the authorization servers MUST
   authenticate the client and ensure that the authorization code was
   issued to the same client.

10.10.  Session Fixation  Authorization Code Leakage

   Session fixation attacks leverage the authorization code grant type,
   by tricking an end-user a resource owner to authorize access to a legitimate
   client, but to a client account under the control of the attacker.
   The only difference between a valid flow and the attack flow is in
   how the victim reached the authorization server to grant access.
   Once at the authorization server, the victim is prompted with a
   normal, valid request on behalf of a legitimate and familiar client.
   The attacker then uses the victim's authorization to gain access to
   the information authorized by the victim.

   In order to prevent such an attack, authorization servers MUST ensure
   that the redirection URI used to obtain the authorization code, is
   the same as the redirection URI provided when exchanging the
   authorization code for an access token.  The authorization server
   SHOULD require the client to pre-register register their redirection URI and if
   provided, MUST validate the redirection URI received in the
   authorization request against the pre-registered registered value.

10.11.  Redirection URI Validation

   [[ Add specific recommendations about redirection validation and
   matching ]]

10.12.  Resource Owner Password Credentials

   The resource owner password credentials grant type is often used for
   legacy or migration reasons.  It reduces the overall risk of storing
   username and password in the client, but does not eliminate the need
   to expose highly privileged credentials to the client.

   This grant type carries a higher risk than the other grant types
   because it maintains the password anti-pattern OAuth seeks to avoid.
   The client could abuse the password or the password could
   unintentionally be disclosed to an attacker (e.g. via log files or
   other records kept by the client).

   Additionally, because the resource owner does not have control over
   the authorization process (the resource owner involvement ends when
   it hands over its credentials to the client), the client can obtain
   access tokens with a broader scope and longer duration than desired
   by the resource owner.  The authorization server SHOULD restrict the
   scope and duration of access tokens issued via this grant type.

   The authorization server and client SHOULD minimize use and client SHOULD minimize use of this grant
   type and utilize other grant types whenever possible.

10.13.  Cross-Site Request Forgery

   Cross-site request forgery (CSRF) is a web-based attack whereby HTTP
   requests are transmitted from the user-agent of an end-user the
   server trusts or has authenticated.  CSRF attacks on the
   authorization endpoint can allow an attacker to obtain authorization
   without the consent of the resource owner.

   The "state" request parameter SHOULD be used to mitigate against CSRF
   attacks, particularly for login CSRF attacks.  CSRF attacks against
   the client's redirection URI allow an attacker to inject their own
   authorization code or access token, which can result in the client
   using an access token associated with the attacker's account rather
   than the victim's.  Depending on the nature of the client and the
   protected resources, this can have undesirable and damaging effects.

   It is strongly RECOMMENDED that the client includes the "state"
   request parameter with authorization requests to the authorization
   server.  The "state" request parameter MUST contain a non-guessable
   value, and the client MUST keep it in a location accessible only by
   the client or the user-agent (i.e., protected by same-origin policy).

   For example, using a DOM variable (protected by JavaScript or other
   DOM-binding language's enforcement of SOP), HTTP cookie, or HTML5
   client-side storage.  The authorization server includes the value of this grant
   type and utilize other grant types whenever possible.

10.13.  XSRF/CSRF Prevention

   [[ Add text with reference to
   the 'state' "state" parameter ]] when redirecting the user-agent back to the
   client which MUST then ensure the received value matches the stored
   value.

10.14.  Clickjacking

11.  IANA Considerations

11.1.  The OAuth Access Token Type Registry

   This specification establishes the OAuth access token type registry.

   Access token types are registered on the advice of one or more
   Designated Experts (appointed by the IESG or their delegate), with a
   Specification Required (using terminology from [RFC5226]).  However,
   to allow for the allocation of values prior to publication, the
   Designated Expert(s) may approve registration once they are satisfied
   that such a specification will be published.

   Registration requests should be sent to the [TBD]@ietf.org mailing
   list for review and comment, with an appropriate subject (e.g.,
   "Request for access toke type: example"). [[ Note to RFC-EDITOR: The
   name of the mailing list should be determined in consultation with
   the IESG and IANA.  Suggested name: oauth-ext-review. ]]

   Within at most 14 days of the request, the Designated Expert(s) will
   either approve or deny the registration request, communicating this
   decision to the review list and IANA.  Denials should include an
   explanation and, if applicable, suggestions as to how to make the
   request successful.

   Decisions (or lack thereof) made by the Designated Expert can be
   first appealed to Application Area Directors (contactable using
   app-ads@tools.ietf.org email address or directly by looking up their
   email addresses on http://www.iesg.org/ website) and, if the
   appellant is not satisfied with the response, to the full IESG (using
   the iesg@iesg.org mailing list).

   IANA should only accept registry updates from the Designated
   Expert(s), and should direct all requests for registration to the
   review mailing list.

11.1.1.  Registration Template

   Type name:
      The name requested (e.g., "example").
   Additional Token Endpoint Response Parameters:
      Additional response parameters returned together with the
      "access_token" parameter.  New parameters MUST be separately
      registered in the OAuth parameters registry as described by
      Section 11.2.
   HTTP Authentication Scheme(s):
      The HTTP authentication scheme name(s), if any, used to
      authenticate protected resources requests using access token of
      this type.
   Change controller:
      For standards-track RFCs, state "IETF".  For others, give the name
      of the responsible party.  Other details (e.g., postal address,
      e-mail address, home page URI) may also be included.
   Specification document(s):
      Reference to document that specifies the parameter, preferably
      including a URI that can be used to retrieve a copy of the
      document.  An indication of the relevant sections may also be
      included, but is not required.

11.2.  The OAuth Parameters Registry

   This specification establishes the OAuth parameters registry.

   Additional parameters for inclusion in the authorization endpoint
   request, the authorization endpoint response, the token endpoint
   request, or the token endpoint response, are registered on the advice
   of one or more Designated Experts (appointed by the IESG or their
   delegate), with a Specification Required (using terminology from
   [RFC5226]).  However, to allow for the allocation of values prior to
   publication, the Designated Expert(s) may approve registration once
   they are satisfied that such a specification will be published.

   Registration requests should be sent to the [TBD]@ietf.org mailing
   list for review and comment, with an appropriate subject (e.g.,
   "Request for parameter: example"). [[ Note to RFC-EDITOR: The name of
   the mailing list should be determined in consultation with the IESG
   and IANA.  Suggested name: oauth-ext-review. ]]

   Within at most 14 days of the request, the Designated Expert(s) will
   either approve or deny the registration request, communicating this
   decision to the review list and IANA.  Denials should include an
   explanation and, if applicable, suggestions as to how to make the
   request successful.

   Decisions (or lack thereof) made by the Designated Expert can be
   first appealed to Application Area Directors (contactable using
   app-ads@tools.ietf.org email address or directly by looking up their
   email addresses on http://www.iesg.org/ website) and, if the
   appellant is not satisfied with the response, to the full IESG (using
   the iesg@iesg.org mailing list).

   IANA should only accept registry updates from the Designated
   Expert(s), and should direct all requests for registration to the
   review mailing list.

11.2.1.  Registration Template

   Parameter name:
      The name requested (e.g., "example").
   Parameter usage location:
      The location(s) where parameter can be used.  The possible
      locations are: authorization request, authorization response,
      token request, or token response.
   Change controller:
      For standards-track RFCs, state "IETF".  For others, give the name
      of the responsible party.  Other details (e.g., postal address,
      e-mail address, home page URI) may also be included.
   Specification document(s):
      Reference to document that specifies the parameter, preferably
      including a URI that can be used to retrieve a copy of the
      document.  An indication of the relevant sections may also be
      included, but is not required.

11.2.2.  Initial Registry Contents

   The OAuth Parameters Registry's initial contents are:

   o  Parameter name: client_id
   o  Parameter usage location: authorization request, token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: client_secret
   o  Parameter usage location: token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: response_type
   o  Parameter usage location: authorization request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: redirect_uri
   o  Parameter usage location: authorization request, token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: scope
   o  Parameter usage location: authorization request, authorization
      response, token request, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: state
   o  Parameter usage location: authorization request, authorization
      response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: code
   o  Parameter usage location: authorization response, token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: error_description
   o  Parameter usage location: authorization response, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: error_uri
   o  Parameter usage location: authorization response, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: grant_type
   o  Parameter usage location: token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: access_token
   o  Parameter usage location: authorization response, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: token_type
   o  Parameter usage location: authorization response, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: expires_in
   o  Parameter usage location: authorization response, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: username
   o  Parameter usage location: token document ]]

   o  Parameter name: username
   o  Parameter usage location: token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: password
   o  Parameter usage location: token request
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter name: refresh_token
   o  Parameter usage location: token request, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

11.3.  The OAuth Authorization Endpoint Response Type Registry

   This specification establishes the OAuth authorization endpoint
   response type registry.

   Additional response type for use with the authorization endpoint are
   registered on the advice of one or more Designated Experts (appointed
   by the IESG or their delegate), with a Specification Required (using
   terminology from [RFC5226]).  However, to allow for the allocation of
   values prior to publication, the Designated Expert(s) may approve
   registration once they are satisfied that such a specification will
   be published.

   Registration requests should be sent to the [TBD]@ietf.org mailing
   list for review and comment, with an appropriate subject (e.g.,
   "Request for response type: example"). [[ Note to RFC-EDITOR: The
   name of the mailing list should be determined in consultation with
   the IESG and IANA.  Suggested name: oauth-ext-review. ]]

   Within at most 14 days of the request, the Designated Expert(s) will
   either approve or deny the registration request, communicating this
   decision to the review list and IANA.  Denials should include an
   explanation and, if applicable, suggestions as to how to make the
   request
   o successful.

   Decisions (or lack thereof) made by the Designated Expert can be
   first appealed to Application Area Directors (contactable using
   app-ads@tools.ietf.org email address or directly by looking up their
   email addresses on http://www.iesg.org/ website) and, if the
   appellant is not satisfied with the response, to the full IESG (using
   the iesg@iesg.org mailing list).

   IANA should only accept registry updates from the Designated
   Expert(s), and should direct all requests for registration to the
   review mailing list.

11.3.1.  Registration Template

   Response type name:
      The name requested (e.g., "example").
   Change controller: IETF
   o
      For standards-track RFCs, state "IETF".  For others, give the name
      of the responsible party.  Other details (e.g., postal address,
      e-mail address, home page URI) may also be included.
   Specification document(s): [[ this
      Reference to document ]] that specifies the type, preferably
      including a URI that can be used to retrieve a copy of the
      document.  An indication of the relevant sections may also be
      included, but is not required.

11.3.2.  Initial Registry Contents

   The OAuth Authorization Endpoint Response Type Registry's initial
   contents are:

   o  Parameter  Response type name: password
   o  Parameter usage location: token request code
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

   o  Parameter  Response type name: refresh_token
   o  Parameter usage location: token request, token response
   o  Change controller: IETF
   o  Specification document(s): [[ this document ]]

11.3.

11.4.  The OAuth Extensions Error Registry

   This specification establishes the OAuth extensions error registry.

   Additional error codes used together with other protocol extensions
   (i.e. extension grant types, access token types, or extension
   parameters) are registered on the advice of one or more Designated
   Experts (appointed by the IESG or their delegate), with a
   Specification Required (using terminology from [RFC5226]).  However,
   to allow for the allocation of values prior to publication, the
   Designated Expert(s) may approve registration once they are satisfied
   that such a specification will be published.

   Registration requests should be sent to the [TBD]@ietf.org mailing
   list for review and comment, with an appropriate subject (e.g.,
   "Request for error code: example"). [[ Note to RFC-EDITOR: The name
   of the mailing list should be determined in consultation with the
   IESG and IANA.  Suggested name: oauth-ext-review. ]]

   Within at most 14 days of the request, the Designated Expert(s) will
   either approve or deny the registration request, communicating this
   decision to the review list and IANA.  Denials should include an
   explanation and, if applicable, suggestions as to how to make the
   request successful.

   Decisions (or lack thereof) made by the Designated Expert can be
   first appealed to Application Area Directors (contactable using
   app-ads@tools.ietf.org email address or directly by looking up their
   email addresses on http://www.iesg.org/ website) and, if the
   appellant is not satisfied with the response, to the full IESG (using
   the iesg@iesg.org mailing list).

   IANA should only accept registry updates from the Designated
   Expert(s), and should direct all requests for registration to the
   review mailing list.

11.3.1.

11.4.1.  Registration Template

   Error name:
      The name requested (e.g., "example").
   Error usage location:
      The location(s) where the error can be used.  The possible
      locations are: authorization code grant error response
      (Section 4.1.2.1), implicit grant error response
      (Section 4.2.2.1), or token error response (Section 5.2).
   Related protocol extension:
      The name of the extension grant type, access token type, or
      extension parameter, the error code is used in conjunction with.
   Change controller:
      For standards-track RFCs, state "IETF".  For others, give the name
      of the responsible party.  Other details (e.g., postal address,
      e-mail address, home page URI) may also be included.
   Specification document(s):
      Reference to document that specifies the error code, preferably
      including a URI that can be used to retrieve a copy of the
      document.  An indication of the relevant sections may also be
      included, but is not required.

12.  Acknowledgements

   The initial OAuth 2.0 protocol specification was edited by David
   Recordon, based on two previous publications: the OAuth 1.0 community
   specification [RFC5849], and OAuth WRAP (OAuth Web Resource
   Authorization Profiles) [I-D.draft-hardt-oauth-01].  The Security
   Considerations section was drafted by Torsten Lodderstedt, Mark
   McGloin, Phil Hunt, and Anthony Nadalin.

   The OAuth 1.0 community specification was edited by Eran Hammer-Lahav
   and authored by Mark Atwood, Dirk Balfanz, Darren Bounds, Richard M.
   Conlan, Blaine Cook, Leah Culver, Breno de Medeiros, Brian Eaton,
   Kellan Elliott-McCrea, Larry Halff, Eran Hammer-Lahav, Ben Laurie,
   Chris Messina, John Panzer, Sam Quigley, David Recordon, Eran
   Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy
   Smith.

   The OAuth WRAP specification was edited by Dick Hardt and authored by
   Brian Eaton, Yaron Goland, Dick Hardt, and Allen Tom.

   This specification is the work of the OAuth Working Group which
   includes dozens of active and dedicated participants.  In particular,
   the following individuals contributed ideas, feedback, and wording
   which shaped and formed the final specification:

   Michael Adams, Andrew Arnott, Dirk Balfanz, Scott Cantor, Blaine
   Cook, Brian Campbell, Brian Eaton, Leah Culver, Bill de hOra, Brian
   Eaton, Brian Ellin, Igor Faynberg, George Fletcher, Tim Freeman, Evan
   Gilbert, Yaron Goland, Brent Goldman, Kristoffer Gronowski, Justin
   Hart, Dick Hardt, Craig Heath, Phil Hunt, Michael B. Jones, John
   Kemp, Mark Kent, Raffi Krikorian, Chasen Le Hara, Rasmus Lerdorf,
   Torsten Lodderstedt, Hui-
   Lan Hui-Lan Lu, Paul Madsen, Alastair Mair, Eve
   Maler, James Manger, Mark McGloin, Laurence Miao, Chuck Mortimore,
   Anthony Nadalin, Justin Richer, Peter Saint-
   Andre, Saint-Andre, Nat Sakimura, Rob
   Sayre, Marius Scurtescu, Naitik Shah, Luke Shepard, Vlad Skvortsov,
   Justin Smith, Jeremy Suriel, Christian Stuebner, Paul Tarjan, Allen
   Tom, Franklin Tse, Nick Walker, Shane Weeden, and Skylar Woodward.

Appendix A.  Editor's Notes

   While many people contributed to this specification throughout its
   long journey, the editor would like to acknowledge and thank a few
   individuals for their outstanding and invaluable efforts leading up
   to the publication of this specification.  It is these individuals
   without whom this work would not have existed, existed or reached its
   successful conclusion.

   David Recordon for continuously being one of OAuth's most valuable
   assets, bringing pragmatism and urgency to the work, and helping
   shape it from its very beginning, as well as being one of the best
   collaborators I had the pleasure of working with.

   Mark Nottingham for introducing OAuth to the IETF and setting the
   community on this course.  Lisa Dusseault for her support and
   guidance as the Application area director.  Blaine Cook, Peter Saint-
   Andre, and Hannes Tschofenig for their work as working group chairs.

   James Manger for his creative ideas and always insightful feedback.
   Brian Campbell, Torsten Lodderstedt, Chuck Mortimore, Justin Richer,
   Marius Scurtescu, and Luke Shepard for their continued participation
   and valuable feedback.

   Special thanks goes to Mike Curtis and Yahoo! for their unconditional
   support of this work for over three years.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [W3C.REC-html401-19991224]
              Hors, A.,
              Jacobs, I., Hors, A., and D. Raggett, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

13.2.  Informative References

   [I-D.draft-hardt-oauth-01]
              Hardt, D., Ed., Tom, A., Eaton, B., and Y. Goland, "OAuth
              Web Resource Authorization Profiles", January 2010.

   [I-D.ietf-oauth-saml2-bearer]
              Campbell, B. and C. Mortimore, "SAML 2.0 Bearer Assertion
              Grant Type Profile for OAuth 2.0",
              draft-ietf-oauth-saml2-bearer-03 (work in progress),
              February 2011.

   [I-D.ietf-oauth-v2-bearer]
              Jones, M., Hardt, D., and D. Recordon, "The OAuth 2.0
              Protocol: Bearer Tokens", draft-ietf-oauth-v2-bearer-04
              (work in progress), March 2011.

   [I-D.ietf-oauth-v2-http-mac]
              Hammer-Lahav, E., Barth, A., and B. Adida, "HTTP
              Authentication: MAC Access Authentication",
              draft-ietf-oauth-v2-http-mac-00 (work in progress),
              May 2011.

   [I-D.lodderstedt-oauth-security]
              Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations",
              draft-lodderstedt-oauth-security-01 (work in progress),
              March 2011.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

Authors' Addresses

   Eran Hammer-Lahav (editor)
   Yahoo!

   Email: eran@hueniverse.com
   URI:   http://hueniverse.com

   David Recordon
   Facebook

   Email: dr@fb.com
   URI:   http://www.davidrecordon.com/

   Dick Hardt
   Microsoft

   Email: dick.hardt@gmail.com
   URI:   http://dickhardt.org/