Open Authentication Protocol                                  A. Parecki
Internet-Draft                                                      Okta
Intended status: Best Current Practice                          D. Waite
Expires: January 9, 25, 2020                                  Ping Identity
                                                           July 08, 24, 2019

                    OAuth 2.0 for Browser-Based Apps


   OAuth 2.0 authorization requests from browser-based apps must be made
   using the authorization code grant with the PKCE extension, and
   should not be issued a client secret when registered.

   This specification details the security considerations that must be
   taken into account when developing browser-based applications, as
   well as best practices for how they can securely implement OAuth 2.0.

Status of This Memo

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

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 25, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4   3
   5.  First-Party Applications  . . . . . . . . . . . . . . . . . .   5   4
   6.  Application Architecture Patterns . . . . . . . . . . . . . .   5
     6.1.  Apps Served from a Common Domain as the Resource Server .   6   5
     6.2.  Apps Served from  JavaScript Applications with a Dynamic Application Server Backend  . . . . . . . . .   6
     6.3.  Apps Served from  JavaScript Applications without a Static Web Server  . . Backend . . . . . . . .   8
   7.  Authorization Code Flow . . . . . . . . . . . . . . . . . . .   9
     7.1.  Initiating the Authorization Request from a Browser-Based
           Application . . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Handling the Authorization Code Redirect  . . . . . . . .   9
   8.  Refresh Tokens  . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     9.1.  Registration of Browser-Based Apps  . . . . . . . . . . .  10
     9.2.  Client Authentication . . . . . . . . . . . . . . . . . .  10
     9.3.  Client Impersonation  . . . . . . . . . . . . . . . . . .  11
     9.4.  Cross-Site Request Forgery Protections  . . . . . . . . .  11
     9.5.  Authorization Server Mix-Up Mitigation  . . . . . . . . .  11
     9.6.  Cross-Domain Requests . . . . . . . . . . . . . . . . . .  12
     9.7.  Content-Security Policy . . . . . . . . . . . . . . . . .  12
     9.8.  OAuth Implicit Grant Authorization Flow . . . . . . . . .  12
       9.8.1.  Threat: Interception of the Redirect URI  . . . . . .  13
       9.8.2.  Threat: Access Token Leak in Browser History  . . . .  13
       9.8.3.  Threat: Manipulation of Scripts . . . . . . . . . . .  13
       9.8.4.  Threat: Access Token Leak to Third Party Scripts  . .  13
       9.8.5.  Countermeasures . . . . . . . . . . . . . . . . . . .  14
       9.8.6.  Disadvantages of the Implicit Flow  . . . . . . . . .  14
       9.8.7.  Historic Note . . . . . . . . . . . . . . . . . . . .  15
     9.9.  Additional Security Considerations  . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Server Support Checklist . . . . . . . . . . . . . .  16
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  16
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  17  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17  18

1.  Introduction

   This specification describes the current best practices for
   implementing OAuth 2.0 authorization flows in applications running
   entirely in a browser.

   For native application developers using OAuth 2.0 and OpenID Connect,
   an IETF BCP (best current practice) was published that guides
   integration of these technologies.  This document is formally known
   as [RFC8252] or BCP 212, but nicknamed "AppAuth" after the OpenID
   Foundation-sponsored set of libraries that assist developers in
   adopting these practices.

   AppAuth steers developers away from performing user authorization via
   embedding user agents such as browser controls into native apps,
   instead insisting that an external agent (such as the system browser)
   be used.  The RFC continues on

   [RFC8252] makes specific recommendations for how to promote capabilities and
   supplemental specifications beyond the base securely
   implement OAuth 2.0 and OpenID
   Connect specifications to improve baseline security, such as
   [RFC7636], also known as PKCE. in native applications, including incorporating
   additional OAuth extensions where needed.

   OAuth 2.0 for Browser-Based Apps addresses the similarities between
   implementing OAuth for native apps as well as browser-based apps, and
   includes additional considerations when running in a browser.  This
   is primarily focused on OAuth, except where OpenID Connect provides
   additional considerations.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

3.  Terminology

   In addition to the terms defined in referenced specifications, this
   document uses the following terms:

   "OAuth":  In this document, "OAuth" refers to OAuth 2.0, [RFC6749].

   "Browser-based application":  An application that is dynamically
      downloaded and executed in a web browser, usually written in
      JavaScript.  Also sometimes referred to as a "single-page
      application", or "SPA".

4.  Overview

   At the time that OAuth 2.0 RFC 6749 was created, browser-based
   JavaScript applications needed a solution that strictly complied with
   the same-origin policy.  Common deployments of OAuth 2.0 involved an
   application running on a different domain than the authorization
   server, so it was historically not possible to use the authorization
   code flow which would require a cross-origin POST request.  This was
   the principal motivation for the definition of the implicit flow,
   which returns the access token in the front channel via the fragment
   part of the URL, bypassing the need for a cross-origin POST request.

   However, there are several drawbacks to the implicit flow, generally
   involving vulnerabilities associated with the exposure of the access
   token in the URL.  See Section 9.8 for an analysis of these attacks
   and the drawbacks of using the implicit flow in browsers.  Additional
   attacks and security considerations can be found in

   In recent years, widespread adoption of Cross-Origin Resource Sharing
   (CORS), which enables exceptions to the same-origin policy, allows
   browser-based apps to use the OAuth 2.0 authorization code flow and
   make a POST request to exchange the authorization code for an access
   token at the token endpoint.  In this flow, the access token is never
   exposed in the less secure front-channel.  Furthermore, adding PKCE
   to the flow assures that even if an authorization code is
   intercepted, it is unusable by an attacker.

   For this reason, and from other lessons learned, the current best
   practice for browser-based applications is to use the OAuth 2.0
   authorization code flow with PKCE.

   Applications should:

   o  Use the OAuth 2.0 authorization code flow with the PKCE extension

   o  Use the OAuth 2.0 state parameter to carry one-time use CSRF

   o  Register one or more redirect URIs, and not vary the redirect URI
      per authorization request

   OAuth 2.0 servers should: must:

   o  Require exact matching of registered redirect URIs

5.  First-Party Applications

   While OAuth and OpenID Connect were was initially created to allow third-
   party third-party applications
   to access an API on behalf of a user, they have
   both it has proven to be useful in a
   first-party scenario as well.  First-
   party  First-party apps are applications
   where by the same organization that provides both the API being accessed by and the

   For example, a web email client provided by the operator of the email
   account, or a mobile banking application created by bank itself.
   (Note that there is no requirement that the application actually be
   developed by the same company; a mobile banking application developed
   by a contractor that is branded as the bank's application is still
   considered a first-party application.)  The first-party app
   consideration is about the user's relationship to the application and
   the service.

   To conform to this best practice, first-party applications using
   OAuth or OpenID Connect MUST use the OAuth Authorization Code flow as
   described later in this document or use the OAuth Password grant.

   It is strongly RECOMMENDED that applications use the Authorization
   Code flow over the Password grant for several reasons.  By
   redirecting to the authorization server, this provides the
   authorization server the opportunity to prompt the user for multi-
   factor authentication options, take advantage of single-sign-on
   sessions, or use third-party identity providers.  In contrast, the
   Password grant does not provide any built-in mechanism for these, and
   must be extended with custom code.

6.  Application Architecture Patterns

   There are three primary architectural patterns available when
   building browser-based applications.

   o  a JavaScript application served from a common domain as the
      resource server

   o  JavaScript served from  a dynamic JavaScript application server with a backend

   o  JavaScript served from  a static web server JavaScript application with no backend, accessing resource
      servers directly

   These three architectures have different use cases and

6.1.  Apps Served from a Common Domain as the Resource Server

   For simple system architectures, such as when the JavaScript
   application is served from a domain that can share cookies with the
   domain of the API (resource server), it is likely OAuth adds additional attack
   vectors that could be avoided with a better decision
   to avoid different solution.

   In particular, using OAuth entirely, any redirect-based mechanism of obtaining an
   access token enables the redirect-based attacks described in
   [oauth-security-topics], but if the application, AS and just API share a
   domain, then it is unnecessary to use session authentication a redirect mechanism to
   communicate directly between them.

   An additional concern with handling access tokens in a browser is
   that there is no secure storage mechanism where JavaScript code can
   keep the API. access token to be later used in an API request.  Using an
   OAuth and OpenID Connect provide very little benefit flow results in this
   deployment scenario, so the JavaScript code getting an access token,
   needing to store it somewhere, and then retrieve it to make an API
   request.  Instead, it is recommended more secure to reconsider whether you
   need OAuth or OpenID Connect at all in use an HTTP-only cookie
   between the JavaScript application and API so that the JavaScript
   code can't access the cookie value itself.

   In this case.  Session
   authentication has situation, it is best to avoid letting the benefit of having fewer moving parts JavaScript code
   ever see the access token, and fewer
   attack vectors. instead use a mechanism such as the
   "JavaScript Applications with a Backend" pattern described below, to
   keep access tokens out of the browser.

   If your JavaScript application has no backend, but still shares a
   domain with the resource server, then it may be best to avoid using
   OAuth and OpenID Connect were entirely.

   OAuth was created primarily for third-party or federated access to
   APIs, so it may not be the best solution in a same-domain scenario.

6.2.  Apps Served from  JavaScript Applications with a Dynamic Application Server Backend
   |             |
   |   Server    |
   |             |

      ^     +
      |(A)  |(B)
      |     |
      +     v

   +-------------+             +--------------+
   |             | +---------> |              |
   | Application |   (C)       |   Resource   |
   |   Server    |             |    Server    |
   |             | <---------+ |              |
   +-------------+   (D)       +--------------+

       ^    +
       |    |
       |    | browser
       |    | cookie
       |    |
       +    v

   |             |
   |   Browser   |
   |             |

   In this architecture, the JavaScript code is loaded from a dynamic
   Application Server that also has the ability to execute code itself.
   This enables the ability to keep all of the steps involved in
   obtaining an access token outside of the JavaScript application.

   (Common examples of this architecture are an Angular front-end with a
   .NET backend, or a React front-end with a Spring Boot backend.)

   The Application Server SHOULD be considered a confidential client,
   and issued its own client secret.  The Application Server SHOULD use
   the OAuth 2.0 authorization code grant to initiate a request request for an
   access token.  Upon handling the redirect from the Authorization
   Server, the Application Server will request an access token using the
   authorization code returned (A), which will be returned to the
   Application Server (B).  The Application Server utilizes its own
   session with the browser to store the access token.

   When the JavaScript application in the browser wants to make a
   request to the Resource Server, it MUST instead make the request to
   the Application Server, and the Application Server will make the
   request with the access token to the Resource Server (C), and forward
   the response (D) back to the browser.

   Security of the connection between code running in the browser and
   this Application Server is assumed to utilize browser-level
   protection mechanisms.  Details are out of scope of this document,
   but many recommendations can be found at the OWASP Foundation
   (, such as setting an HTTP-only and Secure
   cookie to authenticate the session between the browser and
   Application Server.

   In this scenario, the session between the browser and Application
   Server MAY be either a session cookie provided by the Application
   Server, OR the access token itself.  Note that if the access token is
   used as the session identifier, this exposes the access token to the
   end user even if it is not available to the JavaScript application,
   so some authorization servers may wish to limit the capabilities of
   these clients to mitigate risk.

6.3.  Apps Served from  JavaScript Applications without a Static Web Server Backend

                         +---------------+           +--------------+
                         |               |           |              |
                         | Authorization |           |   Resource   |
                         |    Server     |           |    Server    |
                         |               |           |              |
                         +---------------+           +--------------+

                                ^     +                 ^     +
                                |     |                 |     |
                                |(B)  |(C)              |(D)  |(E)
                                |     |                 |     |
                                |     |                 |     |
                                +     v                 +     v

   +-----------------+         +-------------------------------+
   |                 |   (A)   |                               |
   | Static Web Host | +-----> |           Browser             |
   |                 |         |                               |
   +-----------------+         +-------------------------------+

   In this architecture, the JavaScript code is first loaded from a
   static web host into the browser (A).  The application then runs in
   the browser, and is considered a public client since it has no
   ability to be issued a client secret.

   The code in the browser then initiates the authorization code flow
   with the PKCE extension (described in Section 7) (B) above, and
   obtains an access token via a POST request (C).  The JavaScript app
   is then responsible for storing the access token securely using
   appropriate browser APIs.

   When the JavaScript application in the browser wants to make a
   request to the Resource Server, it can include the access token in
   the request (D) and make the request directly.

   In this scenario, the Authorization Server and Resource Server MUST
   support the necessary CORS headers to enable the JavaScript code to
   make this POST request from the domain on which the script is
   executing.  (See Section 9.6 for additional details.)

7.  Authorization Code Flow

   Public browser-based apps needing user authorization create an
   authorization request URI with that use the authorization code grant type per
   described in Section 4.1 of OAuth 2.0 [RFC6749], using a redirect URI capable of
   being received by the app. [RFC6749] MUST also follow
   these additional requirements described in this section.

7.1.  Initiating the Authorization Request from a Browser-Based

   Public browser-based apps MUST implement the Proof Key for Code
   Exchange (PKCE [RFC7636]) extension to OAuth, and authorization
   servers MUST support PKCE for such clients.

   The PKCE extension prevents an attack where the authorization code is
   intercepted and exchanged for an access token by a malicious client,
   by providing the authorization server with a way to verify the same
   client instance that exchanges the authorization code is the same one
   that initiated the flow.

   Browser-based apps MUST use the OAuth 2.0 "state" parameter to
   protect themselves against Cross-Site Request Forgery and
   authorization code swap attacks and MUST use a unique value for each
   authorization request, and MUST verify the returned state in the
   authorization response matches the original state the app created.

7.2.  Handling the Authorization Code Redirect

   Authorization servers MUST require an exact match of a registered
   redirect URI.

8.  Refresh Tokens

   Refresh tokens provide a way for applications to obtain a new access
   token when the initial access token expires. [oauth-security-topics]
   describes some additional requirements around refresh tokens on top
   of the recommendations of [RFC6749].

   For public clients, the risk of a leaked refresh token is much
   greater than leaked access tokens, since an attacker can potentially
   continue using the stoken stolen refresh token to obtain new access tokens
   without being detectable by the authorization server.  Additionally, browser-
   browser-based applications provide many attack vectors by which a
   refresh token can be leaked.  As such, these applications are
   considered a higher risk for handling refresh tokens.

   Authorization servers SHOULD NOT issue refresh tokens to browser-
   based applications.

   If an authorization server does choose to issue refresh tokens to
   browser-based applications, then it MUST issue a new refresh token
   with every access token refresh response.  Doing this mitigates the
   risk of a leaked refresh token, as a leaked refresh token can be
   detected if both the attacker and the legitimate client attempt to
   use the same refresh token.  Authorization servers MUST follow the
   additional refresh token replay mitigation techniques described in

9.  Security Considerations

9.1.  Registration of Browser-Based Apps

   Browser-based applications are considered public clients as defined
   by section 2.1 of OAuth 2.0 [RFC6749], and MUST be registered with
   the authorization server as such.  Authorization servers MUST record
   the client type in the client registration details in order to
   identify and process requests accordingly.

   Authorization servers MUST require that browser-based applications
   register one or more redirect URIs.

9.2.  Client Authentication

   Since a browser-based application's source code is delivered to the
   end-user's browser, it cannot contain provisioned secrets.  As such,
   a browser-based app with native OAuth support is considered a public
   client as defined by Section 2.1 of OAuth 2.0 [RFC6749].

   Secrets that are statically included as part of an app distributed to
   multiple users should not be treated as confidential secrets, as one
   user may inspect their copy and learn the shared secret.  For this
   reason, and those stated in Section 5.3.1 of [RFC6819], it is NOT
   RECOMMENDED for authorization servers to require client
   authentication of browser-based applications using a shared secret,
   as this serves little value beyond client identification which is
   already provided by the client_id request parameter.

   Authorization servers that still require a statically included shared
   secret for SPA clients MUST treat the client as a public client, and
   not accept the secret as proof of the client's identity.  Without
   additional measures, such clients are subject to client impersonation
   (see Section 9.3 below).

9.3.  Client Impersonation

   As stated in Section 10.2 of OAuth 2.0 [RFC6749], the authorization
   server SHOULD NOT process authorization requests automatically
   without user consent or interaction, except when the identity of the
   client can be assured.  Even when the user has previously approved an
   authorization request for a given client_id, the request SHOULD be
   processed as if no previous request had been approved, unless the
   identity of the client can be proven.

   If authorization servers restrict redirect URIs to a fixed set of
   absolute HTTPS URIs without wildcard domains, paths, or query string
   components, this exact match of registered absolute HTTPS URIs MAY be
   accepted by authorization servers as proof of identity of the client
   for the purpose of deciding whether to automatically process an
   authorization request when a previous request for the client_id has
   already been approved.

9.4.  Cross-Site Request Forgery Protections

   Section 5.3.5 of [RFC6819] recommends using the "state" parameter to
   link client requests and responses to prevent CSRF (Cross-Site
   Request Forgery) attacks.  To conform to this best practice, use of
   the "state" parameter is REQUIRED, as described in Section 7.1.

9.5.  Authorization Server Mix-Up Mitigation

   The security considerations around the authorization server mix-up
   that are referenced in Section 8.10 of [RFC8252] also apply to
   browser-based apps.

   Clients MUST use a unique redirect URI for each authorization server
   used by the application.  The client MUST store the redirect URI
   along with the session data (e.g. along with "state") and MUST verify
   that the URI on which the authorization response was received exactly

9.6.  Cross-Domain Requests

   To complete the authorization code flow, the browser-based
   application will need to exchange the authorization code for an
   access token at the token endpoint.  If the authorization server
   provides additional endpoints to the application, such as metadata
   URLs, dynamic client registration, revocation, introspection,
   discovery or user info endpoints, these endpoints may also be
   accessed by the browser-based app.  Since these requests will be made
   from a browser, authorization servers MUST support the necessary CORS
   headers (defined in [Fetch]) to allow the browser to make the

   This specification does not include guidelines for deciding whether a
   CORS policy for the token endpoint should be a wildcard origin or
   more restrictive.  Note, however, that the browser will attempt to
   GET or POST to the API endpoint before knowing any CORS policy; it
   simply hides the succeeding or failing result from JavaScript if the
   policy does not allow sharing.  If POSTs in particular from
   unsupported single-page applications are to be rejected as errors per
   authorization server security policy, such rejection is typically
   done based on the Origin request header.

9.7.  Content-Security Policy

   A browser-based application that wishes to use either long-lived
   refresh tokens or privileged scopes SHOULD restrict its JavaScript
   execution to a set of statically hosted scripts via a Content
   Security Policy ([CSP2]) or similar mechanism.  A strong Content
   Security Policy can limit the potential attack vectors for malicious
   JavaScript to be executed on the page.

9.8.  OAuth Implicit Grant Authorization Flow

   The OAuth 2.0 Implicit grant authorization flow (defined in
   Section 4.2 of OAuth 2.0 [RFC6749]) works by receiving an access
   token in the HTTP redirect (front-channel) immediately without the
   code exchange step.  In this case, the access token is returned in
   the fragment part of the redirect URI, providing an attacker with
   several opportunities to intercept and steal the access token.
   Several attacks on the implicit flow are described by [RFC6819] and
   [oauth-security-topics], not all of which have sufficient mitigation

9.8.1.  Threat: Interception of the Redirect URI

   If an attacker is able to cause the authorization response to be sent
   to a URI under his control, he will directly get access to the
   fragment carrying the access token.  A method of performing this
   attack is described in detail in [oauth-security-topics].

9.8.2.  Threat: Access Token Leak in Browser History

   An attacker could obtain the access token from the browser's history.
   The countermeasures recommended by [RFC6819] are limited to using
   short expiration times for tokens, and indicating that browsers
   should not cache the response.  Neither of these fully prevent this
   attack, they only reduce the potential damage.

   Additionally, many browsers now also sync browser history to cloud
   services and to multiple devices, providing an even wider attack
   surface to extract access tokens out of the URL.

   This is discussed in more detail in Section 4.3.2 of

9.8.3.  Threat: Manipulation of Scripts

   An attacker could modify the page or inject scripts into the browser
   via various means, including when the browser's HTTPS connection is
   being man-in-the-middled by for example a corporate network.  While
   this type of attack is typically out of scope of basic security
   recommendations to prevent, in the case of browser-based apps it is
   much easier to perform this kind of attack, where an injected script
   can suddenly have access to everything on the page.

   The risk of a malicious script running on the page is far greater
   when the application uses a known standard way of obtaining access
   tokens, namely that the attacker can always look at the
   window.location to find an access token.  This threat profile is very
   different compared to an attacker specifically targeting an
   individual application by knowing where or how an access token
   obtained via the authorization code flow may end up being stored.

9.8.4.  Threat: Access Token Leak to Third Party Scripts

   It is relatively common to use third-party scripts in browser-based
   apps, such as analytics tools, crash reporting, and even things like
   a Facebook or Twitter "like" button.  In these situations, the author
   of the application may not be able to be fully aware of the entirety
   of the code running in the application.  When an access token is
   returned in the fragment, it is visible to any third-party scripts on
   the page.

9.8.5.  Countermeasures

   In addition to the countermeasures described by [RFC6819] and
   [oauth-security-topics], using the authorization code with PKCE
   avoids these attacks.

   When PKCE is used, if an authorization code is stolen in transport,
   the attacker is unable to do anything with the authorization code.

9.8.6.  Disadvantages of the Implicit Flow

   There are several additional reasons the Implicit flow is
   disadvantageous compared to using the standard Authorization Code

   o  OAuth 2.0 provides no mechanism for a client to verify that an
      access token was issued to it, which could lead to misuse and
      possible impersonation attacks if a malicious party hands off an
      access token it retrieved through some other means to the client.

   o  Returning an access token in the front channel redirect gives the
      authorization server little assurance that the access token will
      actually end up at the application, since there are many ways this
      redirect may fail or be intercepted.

   o  Supporting the implicit flow requires additional code, more upkeep
      and understanding of the related security considerations, while
      limiting the authorization server to just the authorization code
      flow reduces the attack surface of the implementation.

   o  If the JavaScript application gets wrapped into a native app, then
      [RFC8252] also requires the use of the authorization code flow
      with PKCE anyway.

   In OpenID Connect, the id_token is sent in a known format (as a JWT),
   and digitally signed.  Performing OpenID Connect using the
   authorization code flow also provides the additional benefit of the
   client not needing to verify the JWT signature, as the token will
   have been fetched over an HTTPS connection directly from the
   authorization server.  However, returning an id_token using the
   Implicit flow requires the client validate the JWT signature, as
   malicious parties could otherwise craft and supply fraudulent

9.8.7.  Historic Note

   Historically, the Implicit flow provided an advantage to single-page
   apps since JavaScript could always arbitrarily read and manipulate
   the fragment portion of the URL without triggering a page reload.
   Now with
   This was necessary in order to remove the access token from the URL
   after it was obtained by the app.

   Modern browsers now have the Session History API (described in
   "Session history and navigation" of [HTML]), browsers have which provides a
   mechanism to modify the path and query string component of the URL
   without triggering a page reload, so this
   overloaded reload.  This means modern browser-based
   apps can use of the fragment portion is no longer needed. unmodified OAuth 2.0 authorization code flow, since
   they have the ability to remove the authorization code from the query
   string without triggering a page reload thanks to the Session History

9.9.  Additional Security Considerations

   The OWASP Foundation ( maintains a set of
   security recommendations and best practices for web applications, and
   it is RECOMMENDED to follow these best practices when creating an
   OAuth 2.0 Browser-Based application.

10.  IANA Considerations

   This document does not require any IANA actions.

11.  References

11.1.  Normative References

   [CSP2]     West, M., Barth, A., and D. Veditz, "Content Security
              Policy", December 2016.

   [Fetch]    whatwg, "Fetch", 2018.

              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", November 2018. July 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,

11.2.  Informative References

   [HTML]     whatwg, "HTML", 2018.

Appendix A.  Server Support Checklist

   OAuth servers that support browser-based apps MUST:

   1.  Require "https" scheme redirect URIs.

   2.  Require exact matching of registered redirect URIs.

   3.  Support PKCE [RFC7636].  Required to protect authorization code
       grants sent to public clients.  See Section 7.1

   4.  Support cross-domain requests at the token endpoint in order to
       allow browsers to make the authorization code exchange request.
       See Section 9.6

   5.  Not assume that browser-based clients can keep a secret, and
       SHOULD NOT issue secrets to applications of this type.

Appendix B.  Document History

   [[ To be removed from the final specification ]]

   o  Updated the historic note about the fragment URL clarifying that
      the Session History API means browsers can use the unmodified
      authorization code flow

   o  Rephrased "Authorization Code Flow" intro paragraph to better lead
      into the next two sections

   o  Softened "is likely a better decision to avoid using OAuth
      entirely" to "it may be..." for common-domain deployments

   o  Updated abstract to not be limited to public clients, since the
      later sections talk about confidential clients

   o  Removed references to avoiding OpenID Connect for same-domain

   o  Updated headers to better describe architectures (Apps Served from
      a Static Web Server -> JavaScript Applications without a Backend)

   o  Expanded "same-domain architecture" section to better explain the
      problems that OAuth has in this scenario

   o  Referenced Security BCP in implicit flow attacks where possible

   o  Minor typo corrections


   o  Rewrote overview section incorporating feedback from Leo Tohill

   o  Updated summary recommendation bullet points to split out
      application and server requirements

   o  Removed the allowance on hostname-only redirect URI matching, now
      requiring exact redirect URI matching

   o  Updated section 6.2 to drop reference of SPA with a backend
      component being a public client

   o  Expanded the architecture section to explicitly mention three
      architectural patterns available to JS apps


   o  Incorporated feedback from Torsten Lodderstedt

   o  Updated abstract
   o  Clarified the definition of browser-based apps to not exclude
      applications cached in the browser, e.g. via Service Workers

   o  Clarified use of the state parameter for CSRF protection

   o  Added background information about the original reason the
      implicit flow was created due to lack of CORS support

   o  Clarified the same-domain use case where the SPA and API share a
      cookie domain

   o  Moved historic note about the fragment URL into the Overview

Appendix C.  Acknowledgements

   The authors would like to acknowledge the work of William Denniss and
   John Bradley, whose recommendation for native apps informed many of
   the best practices for browser-based applications.  The authors would
   also like to thank Hannes Tschofenig and Torsten Lodderstedt, the
   attendees of the Internet Identity Workshop 27 session at which this
   BCP was originally proposed, and the following individuals who
   contributed ideas, feedback, and wording that shaped and formed the
   final specification:

   Annabelle Backman, Brian Campbell, Brock Allen, Christian Mainka,
   Daniel Fett, George Fletcher, Hannes Tschofenig, Janak Amarasena,
   John Bradley, Joseph Heenan, Justin Richer, Karl McGuinness, Leo
   Tohill, Tomek Stojecki, Torsten Lodderstedt, and Vittorio Bertocci.

Authors' Addresses

   Aaron Parecki


   David Waite
   Ping Identity