draft-ietf-oauth-mtls-17.txt   rfc8705.txt 
OAuth Working Group B. Campbell Internet Engineering Task Force (IETF) B. Campbell
Internet-Draft Ping Identity Request for Comments: 8705 Ping Identity
Intended status: Standards Track J. Bradley Category: Standards Track J. Bradley
Expires: February 23, 2020 Yubico ISSN: 2070-1721 Yubico
N. Sakimura N. Sakimura
Nomura Research Institute Nomura Research Institute
T. Lodderstedt T. Lodderstedt
YES.com AG YES.com AG
August 22, 2019 February 2020
OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
Access Tokens Access Tokens
draft-ietf-oauth-mtls-17
Abstract Abstract
This document describes OAuth client authentication and certificate- This document describes OAuth client authentication and certificate-
bound access and refresh tokens using mutual Transport Layer Security bound access and refresh tokens using mutual Transport Layer Security
(TLS) authentication with X.509 certificates. OAuth clients are (TLS) authentication with X.509 certificates. OAuth clients are
provided a mechanism for authentication to the authorization server provided a mechanism for authentication to the authorization server
using mutual TLS, based on either self-signed certificates or public using mutual TLS, based on either self-signed certificates or public
key infrastructure (PKI). OAuth authorization servers are provided a key infrastructure (PKI). OAuth authorization servers are provided a
mechanism for binding access tokens to a client's mutual-TLS mechanism for binding access tokens to a client's mutual-TLS
certificate, and OAuth protected resources are provided a method for certificate, and OAuth protected resources are provided a method for
ensuring that such an access token presented to it was issued to the ensuring that such an access token presented to it was issued to the
client presenting the token. client presenting the token.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on February 23, 2020. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8705.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Requirements Notation and Conventions . . . . . . . . . . 5 1.1. Requirements Notation and Conventions
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Terminology
2. Mutual TLS for OAuth Client Authentication . . . . . . . . . 5 2. Mutual TLS for OAuth Client Authentication
2.1. PKI Mutual-TLS Method . . . . . . . . . . . . . . . . . . 6 2.1. PKI Mutual-TLS Method
2.1.1. PKI Method Metadata Value . . . . . . . . . . . . . . 7 2.1.1. PKI Method Metadata Value
2.1.2. Client Registration Metadata . . . . . . . . . . . . 7 2.1.2. Client Registration Metadata
2.2. Self-Signed Certificate Mutual-TLS Method . . . . . . . . 8 2.2. Self-Signed Certificate Mutual-TLS Method
2.2.1. Self-Signed Method Metadata Value . . . . . . . . . . 8 2.2.1. Self-Signed Method Metadata Value
2.2.2. Client Registration Metadata . . . . . . . . . . . . 8 2.2.2. Client Registration Metadata
3. Mutual-TLS Client Certificate-Bound Access Tokens . . . . . . 9 3. Mutual-TLS Client Certificate-Bound Access Tokens
3.1. JWT Certificate Thumbprint Confirmation Method . . . . . 10 3.1. JWT Certificate Thumbprint Confirmation Method
3.2. Confirmation Method for Token Introspection . . . . . . . 11 3.2. Confirmation Method for Token Introspection
3.3. Authorization Server Metadata . . . . . . . . . . . . . . 12 3.3. Authorization Server Metadata
3.4. Client Registration Metadata . . . . . . . . . . . . . . 12 3.4. Client Registration Metadata
4. Public Clients and Certificate-Bound Tokens . . . . . . . . . 13 4. Public Clients and Certificate-Bound Tokens
5. Metadata for Mutual-TLS Endpoint Aliases . . . . . . . . . . 13 5. Metadata for Mutual-TLS Endpoint Aliases
6. Implementation Considerations . . . . . . . . . . . . . . . . 15 6. Implementation Considerations
6.1. Authorization Server . . . . . . . . . . . . . . . . . . 15 6.1. Authorization Server
6.2. Resource Server . . . . . . . . . . . . . . . . . . . . . 16 6.2. Resource Server
6.3. Certificate Expiration and Bound Access Tokens . . . . . 16 6.3. Certificate Expiration and Bound Access Tokens
6.4. Implicit Grant Unsupported . . . . . . . . . . . . . . . 16 6.4. Implicit Grant Unsupported
6.5. TLS Termination . . . . . . . . . . . . . . . . . . . . . 17 6.5. TLS Termination
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations
7.1. Certificate-Bound Refresh Tokens . . . . . . . . . . . . 17 7.1. Certificate-Bound Refresh Tokens
7.2. Certificate Thumbprint Binding . . . . . . . . . . . . . 17 7.2. Certificate Thumbprint Binding
7.3. TLS Versions and Best Practices . . . . . . . . . . . . . 18 7.3. TLS Versions and Best Practices
7.4. X.509 Certificate Spoofing . . . . . . . . . . . . . . . 18 7.4. X.509 Certificate Spoofing
7.5. X.509 Certificate Parsing and Validation Complexity . . . 18 7.5. X.509 Certificate Parsing and Validation Complexity
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19 8. Privacy Considerations
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. IANA Considerations
9.1. JWT Confirmation Methods Registration . . . . . . . . . . 19 9.1. JWT Confirmation Methods Registration
9.2. Authorization Server Metadata Registration . . . . . . . 19 9.2. Authorization Server Metadata Registration
9.3. Token Endpoint Authentication Method Registration . . . . 20 9.3. Token Endpoint Authentication Method Registration
9.4. Token Introspection Response Registration . . . . . . . . 20 9.4. Token Introspection Response Registration
9.5. Dynamic Client Registration Metadata Registration . . . . 21 9.5. Dynamic Client Registration Metadata Registration
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 10. References
10.1. Normative References . . . . . . . . . . . . . . . . . . 22 10.1. Normative References
10.2. Informative References . . . . . . . . . . . . . . . . . 24 10.2. Informative References
Appendix A. Example "cnf" Claim, Certificate and JWK . . . . . . 25 Appendix A. Example "cnf" Claim, Certificate, and JWK
Appendix B. Relationship to Token Binding . . . . . . . . . . . 26 Appendix B. Relationship to Token Binding
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 26 Acknowledgements
Appendix D. Document(s) History . . . . . . . . . . . . . . . . 27 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction 1. Introduction
The OAuth 2.0 Authorization Framework [RFC6749] enables third-party The OAuth 2.0 Authorization Framework [RFC6749] enables third-party
client applications to obtain delegated access to protected client applications to obtain delegated access to protected
resources. In the prototypical abstract OAuth flow, illustrated in resources. In the prototypical abstract OAuth flow, illustrated in
Figure 1, the client obtains an access token from an entity known as Figure 1, the client obtains an access token from an entity known as
an authorization server and then uses that token when accessing an authorization server and then uses that token when accessing
protected resources, such as HTTPS APIs. protected resources, such as HTTPS APIs.
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| | | | | |
| | v | | v
| | +---------------+ | | +---------------+
| | | (C) | | | | (C) |
| | | | | | | |
| |<--(B)-- Use the access token -->| Protected | | |<--(B)-- Use the access token -->| Protected |
| | | Resource | | | | Resource |
| | | | | | | |
+--------+ +---------------+ +--------+ +---------------+
Figure 1: Abstract OAuth 2.0 Protocol Flow Figure 1: Abstract OAuth 2.0 Protocol Flow
The flow illustrated in Figure 1 includes the following steps: The flow illustrated in Figure 1 includes the following steps:
(A) The client makes an HTTPS "POST" request to the authorization (A) The client makes an HTTPS "POST" request to the authorization
server and presents a credential representing the authorization server and presents a credential representing the authorization
grant. For certain types of clients (those that have been grant. For certain types of clients (those that have been
issued or otherwise established a set of client credentials) the issued or otherwise established a set of client credentials) the
request must be authenticated. In the response, the request must be authenticated. In the response, the
authorization server issues an access token to the client. authorization server issues an access token to the client.
(B) The client includes the access token when making a request to (B) The client includes the access token when making a request to
access a protected resource. access a protected resource.
(C) The protected resource validates the access token in order to (C) The protected resource validates the access token in order to
authorize the request. In some cases, such as when the token is authorize the request. In some cases, such as when the token is
self-contained and cryptographically secured, the validation can self-contained and cryptographically secured, the validation can
be done locally by the protected resource. Other cases require be done locally by the protected resource. Other cases require
that the protected resource call out to the authorization server that the protected resource call out to the authorization server
to determine the state of the token and obtain meta-information to determine the state of the token and obtain metainformation
about it. about it.
Layering on the abstract flow above, this document standardizes Layering on the abstract flow above, this document standardizes
enhanced security options for OAuth 2.0 utilizing client-certificate- enhanced security options for OAuth 2.0 utilizing client-certificate-
based mutual TLS. Section 2 provides options for authenticating the based mutual TLS. Section 2 provides options for authenticating the
request in step (A). Step (C) is supported with semantics to express request in Step (A). Step (C) is supported with semantics to express
the binding of the token to the client certificate for both local and the binding of the token to the client certificate for both local and
remote processing in Section 3.1 and Section 3.2 respectively. This remote processing in Sections 3.1 and 3.2, respectively. This
ensures that, as described in Section 3, protected resource access in ensures that, as described in Section 3, protected resource access in
step (B) is only possible by the legitimate client using a Step (B) is only possible by the legitimate client using a
certificate-bound token and holding the private key corresponding to certificate-bound token and holding the private key corresponding to
the certificate. the certificate.
OAuth 2.0 defines a shared-secret method of client authentication but OAuth 2.0 defines a shared-secret method of client authentication but
also allows for definition and use of additional client also allows for defining and using additional client authentication
authentication mechanisms when interacting directly with the mechanisms when interacting directly with the authorization server.
authorization server. This document describes an additional This document describes an additional mechanism of client
mechanism of client authentication utilizing mutual-TLS certificate- authentication utilizing mutual-TLS certificate-based authentication
based authentication, which provides better security characteristics that provides better security characteristics than shared secrets.
than shared secrets. While [RFC6749] documents client authentication While [RFC6749] documents client authentication for requests to the
for requests to the token endpoint, extensions to OAuth 2.0 (such as token endpoint, extensions to OAuth 2.0 (such as Introspection
Introspection [RFC7662], Revocation [RFC7009], and the Backchannel [RFC7662], Revocation [RFC7009], and the Backchannel Authentication
Authentication Endpoint in [OpenID.CIBA]) define endpoints that also Endpoint in [OpenID.CIBA]) define endpoints that also utilize client
utilize client authentication and the mutual TLS methods defined authentication, and the mutual-TLS methods defined herein are
herein are applicable to those endpoints as well. applicable to those endpoints as well.
Mutual-TLS certificate-bound access tokens ensure that only the party Mutual-TLS certificate-bound access tokens ensure that only the party
in possession of the private key corresponding to the certificate can in possession of the private key corresponding to the certificate can
utilize the token to access the associated resources. Such a utilize the token to access the associated resources. Such a
constraint is sometimes referred to as key confirmation, proof-of- constraint is sometimes referred to as key confirmation, proof-of-
possession, or holder-of-key and is unlike the case of the bearer possession, or holder-of-key and is unlike the case of the bearer
token described in [RFC6750], where any party in possession of the token described in [RFC6750], where any party in possession of the
access token can use it to access the associated resources. Binding access token can use it to access the associated resources. Binding
an access token to the client's certificate prevents the use of an access token to the client's certificate prevents the use of
stolen access tokens or replay of access tokens by unauthorized stolen access tokens or replay of access tokens by unauthorized
parties. parties.
Mutual-TLS certificate-bound access tokens and mutual-TLS client Mutual-TLS certificate-bound access tokens and mutual-TLS client
authentication are distinct mechanisms, which are complementary but authentication are distinct mechanisms that are complementary but
don't necessarily need to be deployed or used together. don't necessarily need to be deployed or used together.
Additional client metadata parameters are introduced by this document Additional client metadata parameters are introduced by this document
in support of certificate-bound access tokens and mutual-TLS client in support of certificate-bound access tokens and mutual-TLS client
authentication. The authorization server can obtain client metadata authentication. The authorization server can obtain client metadata
via the Dynamic Client Registration Protocol [RFC7591], which defines via the Dynamic Client Registration Protocol [RFC7591], which defines
mechanisms for dynamically registering OAuth 2.0 client metadata with mechanisms for dynamically registering OAuth 2.0 client metadata with
authorization servers. Also the metadata defined by RFC7591, and authorization servers. Also the metadata defined by [RFC7591], and
registered extensions to it, imply a general data model for clients registered extensions to it, imply a general data model for clients
that is useful for authorization server implementations even when the that is useful for authorization server implementations, even when
Dynamic Client Registration Protocol isn't in play. Such the Dynamic Client Registration Protocol isn't in play. Such
implementations will typically have some sort of user interface implementations will typically have some sort of user interface
available for managing client configuration. available for managing client configuration.
1.1. Requirements Notation and Conventions 1.1. Requirements Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
1.2. Terminology 1.2. Terminology
Throughout this document the term "mutual TLS" refers to the process Throughout this document the term "mutual TLS" refers to the process
whereby, in addition to the normal TLS server authentication with a whereby, in addition to the normal TLS server authentication with a
certificate, a client presents its X.509 certificate and proves certificate, a client presents its X.509 certificate and proves
possession of the corresponding private key to a server when possession of the corresponding private key to a server when
negotiating a TLS session. In contemporary versions of TLS [RFC8446] negotiating a TLS session. In contemporary versions of TLS [RFC5246]
[RFC5246] this requires that the client send the Certificate and [RFC8446], this requires that the client send the Certificate and
CertificateVerify messages during the handshake and for the server to CertificateVerify messages during the handshake and for the server to
verify the CertificateVerify and Finished messages. verify the CertificateVerify and Finished messages.
2. Mutual TLS for OAuth Client Authentication 2. Mutual TLS for OAuth Client Authentication
This section defines, as an extension of OAuth 2.0, Section 2.3 This section defines, as an extension of Section 2.3 of OAuth 2.0
[RFC6749], two distinct methods of using mutual-TLS X.509 client [RFC6749], two distinct methods of using mutual-TLS X.509 client
certificates as client credentials. The requirement of mutual TLS certificates as client credentials. The requirement of mutual TLS
for client authentication is determined by the authorization server for client authentication is determined by the authorization server,
based on policy or configuration for the given client (regardless of based on policy or configuration for the given client (regardless of
whether the client was dynamically registered, statically configured, whether the client was dynamically registered, statically configured,
or otherwise established). or otherwise established).
In order to utilize TLS for OAuth client authentication, the TLS In order to utilize TLS for OAuth client authentication, the TLS
connection between the client and the authorization server MUST have connection between the client and the authorization server MUST have
been established or reestablished with mutual-TLS X.509 certificate been established or re-established with mutual-TLS X.509 certificate
authentication (i.e. the Client Certificate and Certificate Verify authentication (i.e., the client Certificate and CertificateVerify
messages are sent during the TLS Handshake). messages are sent during the TLS handshake).
For all requests to the authorization server utilizing mutual-TLS For all requests to the authorization server utilizing mutual-TLS
client authentication, the client MUST include the "client_id" client authentication, the client MUST include the "client_id"
parameter, described in OAuth 2.0, Section 2.2 [RFC6749]. The parameter described in Section 2.2 of OAuth 2.0 [RFC6749]. The
presence of the "client_id" parameter enables the authorization presence of the "client_id" parameter enables the authorization
server to easily identify the client independently from the content server to easily identify the client independently from the content
of the certificate. The authorization server can locate the client of the certificate. The authorization server can locate the client
configuration using the client identifier and check the certificate configuration using the client identifier and check the certificate
presented in the TLS Handshake against the expected credentials for presented in the TLS handshake against the expected credentials for
that client. The authorization server MUST enforce the binding that client. The authorization server MUST enforce the binding
between client and certificate as described in either Section 2.1 or between client and certificate, as described in either Section 2.1 or
Section 2.2 below. If no certificate is presented or that which is 2.2 below. If no certificate is presented, or that which is
presented doesn't match that which is expected for the given presented doesn't match that which is expected for the given
"client_id", the authorization server returns a normal OAuth 2.0 "client_id", the authorization server returns a normal OAuth 2.0
error response per Section 5.2 of RFC6749 [RFC6749] with the error response per Section 5.2 of [RFC6749] with the "invalid_client"
"invalid_client" error code to indicate failed client authentication. error code to indicate failed client authentication.
2.1. PKI Mutual-TLS Method 2.1. PKI Mutual-TLS Method
The PKI (public key infrastructure) method of mutual-TLS OAuth client The PKI (public key infrastructure) method of mutual-TLS OAuth client
authentication adheres to the way in which X.509 certificates are authentication adheres to the way in which X.509 certificates are
traditionally used for authentication. It relies on a validated traditionally used for authentication. It relies on a validated
certificate chain [RFC5280] and a single subject distinguished name certificate chain [RFC5280] and a single subject distinguished name
(DN) or a single subject alternative name (SAN) to authenticate the (DN) or a single subject alternative name (SAN) to authenticate the
client. Only one subject name value of any type is used for each client. Only one subject name value of any type is used for each
client. The TLS handshake is utilized to validate the client's client. The TLS handshake is utilized to validate the client's
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In order to convey the expected subject of the certificate, the In order to convey the expected subject of the certificate, the
following metadata parameters are introduced for the OAuth 2.0 following metadata parameters are introduced for the OAuth 2.0
Dynamic Client Registration Protocol [RFC7591] in support of the PKI Dynamic Client Registration Protocol [RFC7591] in support of the PKI
method of mutual-TLS client authentication. A client using the method of mutual-TLS client authentication. A client using the
"tls_client_auth" authentication method MUST use exactly one of the "tls_client_auth" authentication method MUST use exactly one of the
below metadata parameters to indicate the certificate subject value below metadata parameters to indicate the certificate subject value
that the authorization server is to expect when authenticating the that the authorization server is to expect when authenticating the
respective client. respective client.
tls_client_auth_subject_dn tls_client_auth_subject_dn
An [RFC4514] string representation of the expected subject A string representation -- as defined in [RFC4514] -- of the
distinguished name of the certificate, which the OAuth client will expected subject distinguished name of the certificate that the
use in mutual-TLS authentication. OAuth client will use in mutual-TLS authentication.
tls_client_auth_san_dns tls_client_auth_san_dns
A string containing the value of an expected dNSName SAN entry in A string containing the value of an expected dNSName SAN entry in
the certificate, which the OAuth client will use in mutual-TLS the certificate that the OAuth client will use in mutual-TLS
authentication. authentication.
tls_client_auth_san_uri tls_client_auth_san_uri
A string containing the value of an expected A string containing the value of an expected
uniformResourceIdentifier SAN entry in the certificate, which the uniformResourceIdentifier SAN entry in the certificate that the
OAuth client will use in mutual-TLS authentication. OAuth client will use in mutual-TLS authentication.
tls_client_auth_san_ip tls_client_auth_san_ip
A string representation of an IP address in either dotted decimal A string representation of an IP address in either dotted decimal
notation (for IPv4) or colon-delimited hexadecimal (for IPv6, as notation (for IPv4) or colon-delimited hexadecimal (for IPv6, as
defined in [RFC5952]) that is expected to be present as an defined in [RFC5952]) that is expected to be present as an
iPAddress SAN entry in the certificate, which the OAuth client iPAddress SAN entry in the certificate that the OAuth client will
will use in mutual-TLS authentication. Per section 8 of [RFC5952] use in mutual-TLS authentication. Per Section 8 of [RFC5952], the
the IP address comparison of the value in this parameter and the IP address comparison of the value in this parameter and the SAN
SAN entry in the certificate is to be done in binary format. entry in the certificate is to be done in binary format.
tls_client_auth_san_email tls_client_auth_san_email
A string containing the value of an expected rfc822Name SAN entry A string containing the value of an expected rfc822Name SAN entry
in the certificate, which the OAuth client will use in mutual-TLS in the certificate that the OAuth client will use in mutual-TLS
authentication. authentication.
2.2. Self-Signed Certificate Mutual-TLS Method 2.2. Self-Signed Certificate Mutual-TLS Method
This method of mutual-TLS OAuth client authentication is intended to This method of mutual-TLS OAuth client authentication is intended to
support client authentication using self-signed certificates. As a support client authentication using self-signed certificates. As a
prerequisite, the client registers its X.509 certificates (using prerequisite, the client registers its X.509 certificates (using
"jwks" defined in [RFC7591]) or a reference to a trusted source for "jwks" defined in [RFC7591]) or a reference to a trusted source for
its X.509 certificates (using "jwks_uri" from [RFC7591]) with the its X.509 certificates (using "jwks_uri" from [RFC7591]) with the
authorization server. During authentication, TLS is utilized to authorization server. During authentication, TLS is utilized to
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Endpoint Authentication Methods" registry [IANA.OAuth.Parameters]. Endpoint Authentication Methods" registry [IANA.OAuth.Parameters].
self_signed_tls_client_auth self_signed_tls_client_auth
Indicates that client authentication to the authorization server Indicates that client authentication to the authorization server
will occur using mutual TLS with the client utilizing a self- will occur using mutual TLS with the client utilizing a self-
signed certificate. signed certificate.
2.2.2. Client Registration Metadata 2.2.2. Client Registration Metadata
For the Self-Signed Certificate method of binding a certificate with For the Self-Signed Certificate method of binding a certificate with
a client using mutual TLS client authentication, the existing a client using mutual-TLS client authentication, the existing
"jwks_uri" or "jwks" metadata parameters from [RFC7591] are used to "jwks_uri" or "jwks" metadata parameters from [RFC7591] are used to
convey the client's certificates via JSON Web Key (JWK) in a JWK Set convey the client's certificates via JSON Web Key (JWK) in a JWK Set
(JWKS) [RFC7517]. The "jwks" metadata parameter is a JWK Set [RFC7517]. The "jwks" metadata parameter is a JWK Set containing the
containing the client's public keys as an array of JWKs while the client's public keys as an array of JWKs, while the "jwks_uri"
"jwks_uri" parameter is a URL that references a client's JWK Set. A parameter is a URL that references a client's JWK Set. A certificate
certificate is represented with the "x5c" parameter of an individual is represented with the "x5c" parameter of an individual JWK within
JWK within the set. Note that the members of the JWK representing the set. Note that the members of the JWK representing the public
the public key (e.g. "n" and "e" for RSA, "x" and "y" for EC) are key (e.g., "n" and "e" for RSA, "x" and "y" for Elliptic Curve (EC))
required parameters per [RFC7518] so will be present even though they are required parameters per [RFC7518] so will be present even though
are not utilized in this context. Also note that that Section 4.7 of they are not utilized in this context. Also note that Section 4.7 of
[RFC7517] requires that the key in the first certificate of the "x5c" [RFC7517] requires that the key in the first certificate of the "x5c"
parameter match the public key represented by those other members of parameter match the public key represented by those other members of
the JWK. the JWK.
3. Mutual-TLS Client Certificate-Bound Access Tokens 3. Mutual-TLS Client Certificate-Bound Access Tokens
When mutual TLS is used by the client on the connection to the token When mutual TLS is used by the client on the connection to the token
endpoint, the authorization server is able to bind the issued access endpoint, the authorization server is able to bind the issued access
token to the client certificate. Such a binding is accomplished by token to the client certificate. Such a binding is accomplished by
associating the certificate with the token in a way that can be associating the certificate with the token in a way that can be
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mutual TLS during protected resource access to serve purely as a mutual TLS during protected resource access to serve purely as a
proof-of-possession mechanism. Other methods of associating a proof-of-possession mechanism. Other methods of associating a
certificate with an access token are possible, per agreement by the certificate with an access token are possible, per agreement by the
authorization server and the protected resource, but are beyond the authorization server and the protected resource, but are beyond the
scope of this specification. scope of this specification.
In order for a resource server to use certificate-bound access In order for a resource server to use certificate-bound access
tokens, it must have advance knowledge that mutual TLS is to be used tokens, it must have advance knowledge that mutual TLS is to be used
for some or all resource accesses. In particular, the access token for some or all resource accesses. In particular, the access token
itself cannot be used as input to the decision of whether or not to itself cannot be used as input to the decision of whether or not to
request mutual TLS, since from the TLS perspective those are request mutual TLS because (from the TLS perspective) it is
"Application Data", only exchanged after the TLS handshake has been "Application Data", only exchanged after the TLS handshake has been
completed, and the initial CertificateRequest occurs during the completed, and the initial CertificateRequest occurs during the
handshake, before the Application Data is available. Although handshake, before the Application Data is available. Although
subsequent opportunities for a TLS client to present a certificate subsequent opportunities for a TLS client to present a certificate
may be available, e.g., via TLS 1.2 renegotiation [RFC5246] or TLS may be available, e.g., via TLS 1.2 renegotiation [RFC5246] or TLS
1.3 post-handshake authentication [RFC8446], this document makes no 1.3 post-handshake authentication [RFC8446], this document makes no
provision for their usage. It is expected to be common that a provision for their usage. It is expected to be common that a
mutual-TLS-using resource server will require mutual TLS for all mutual-TLS-using resource server will require mutual TLS for all
resources hosted thereupon, or will serve mutual-TLS-protected and resources hosted thereupon or will serve mutual-TLS-protected and
regular resources on separate hostname+port combinations, though regular resources on separate hostname and port combinations, though
other workflows are possible. How resource server policy is other workflows are possible. How resource server policy is
synchronized with the AS is out of scope for this document. synchronized with the authorization server (AS) is out of scope for
this document.
Within the scope of an mutual-TLS-protected resource-access flow, the Within the scope of a mutual-TLS-protected resource-access flow, the
client makes protected resource requests as described in [RFC6750], client makes protected resource requests, as described in [RFC6750],
however, those requests MUST be made over a mutually authenticated however, those requests MUST be made over a mutually authenticated
TLS connection using the same certificate that was used for mutual TLS connection using the same certificate that was used for mutual
TLS at the token endpoint. TLS at the token endpoint.
The protected resource MUST obtain, from its TLS implementation The protected resource MUST obtain, from its TLS implementation
layer, the client certificate used for mutual TLS and MUST verify layer, the client certificate used for mutual TLS and MUST verify
that the certificate matches the certificate associated with the that the certificate matches the certificate associated with the
access token. If they do not match, the resource access attempt MUST access token. If they do not match, the resource access attempt MUST
be rejected with an error per [RFC6750] using an HTTP 401 status code be rejected with an error, per [RFC6750], using an HTTP 401 status
and the "invalid_token" error code. code and the "invalid_token" error code.
Metadata to convey server and client capabilities for mutual-TLS Metadata to convey server and client capabilities for mutual-TLS
client certificate-bound access tokens is defined in Section 3.3 and client certificate-bound access tokens is defined in Sections 3.3 and
Section 3.4 respectively. 3.4, respectively.
3.1. JWT Certificate Thumbprint Confirmation Method 3.1. JWT Certificate Thumbprint Confirmation Method
When access tokens are represented as JSON Web Tokens (JWT)[RFC7519], When access tokens are represented as JSON Web Tokens (JWT)
the certificate hash information SHOULD be represented using the [RFC7519], the certificate hash information SHOULD be represented
"x5t#S256" confirmation method member defined herein. using the "x5t#S256" confirmation method member defined herein.
To represent the hash of a certificate in a JWT, this specification To represent the hash of a certificate in a JWT, this specification
defines the new JWT Confirmation Method [RFC7800] member "x5t#S256" defines the new JWT Confirmation Method [RFC7800] member "x5t#S256"
for the X.509 Certificate SHA-256 Thumbprint. The value of the for the X.509 Certificate SHA-256 Thumbprint. The value of the
"x5t#S256" member is a base64url-encoded [RFC4648] SHA-256 [SHS] hash "x5t#S256" member is a base64url-encoded [RFC4648] SHA-256 [SHS] hash
(a.k.a. thumbprint, fingerprint or digest) of the DER encoding [X690] (a.k.a., thumbprint, fingerprint, or digest) of the DER encoding
of the X.509 certificate [RFC5280]. The base64url-encoded value MUST [X690] of the X.509 certificate [RFC5280]. The base64url-encoded
omit all trailing pad '=' characters and MUST NOT include any line value MUST omit all trailing pad '=' characters and MUST NOT include
breaks, whitespace, or other additional characters. any line breaks, whitespace, or other additional characters.
The following is an example of a JWT payload containing an "x5t#S256" The following is an example of a JWT payload containing an "x5t#S256"
certificate thumbprint confirmation method. The new JWT content certificate thumbprint confirmation method. The new JWT content
introduced by this specification is the "cnf" confirmation method introduced by this specification is the "cnf" confirmation method
claim at the bottom of the example that has the "x5t#S256" claim at the bottom of the example that has the "x5t#S256"
confirmation method member containing the value that is the hash of confirmation method member containing the value that is the hash of
the client certificate to which the access token is bound. the client certificate to which the access token is bound.
{ {
"iss": "https://server.example.com", "iss": "https://server.example.com",
skipping to change at page 11, line 22 skipping to change at line 479
} }
} }
Figure 2: Example JWT Claims Set with an X.509 Certificate Thumbprint Figure 2: Example JWT Claims Set with an X.509 Certificate Thumbprint
Confirmation Method Confirmation Method
3.2. Confirmation Method for Token Introspection 3.2. Confirmation Method for Token Introspection
OAuth 2.0 Token Introspection [RFC7662] defines a method for a OAuth 2.0 Token Introspection [RFC7662] defines a method for a
protected resource to query an authorization server about the active protected resource to query an authorization server about the active
state of an access token as well as to determine meta-information state of an access token as well as to determine metainformation
about the token. about the token.
For a mutual-TLS client certificate-bound access token, the hash of For a mutual-TLS client certificate-bound access token, the hash of
the certificate to which the token is bound is conveyed to the the certificate to which the token is bound is conveyed to the
protected resource as meta-information in a token introspection protected resource as metainformation in a token introspection
response. The hash is conveyed using the same "cnf" with "x5t#S256" response. The hash is conveyed using the same "cnf" with "x5t#S256"
member structure as the certificate SHA-256 thumbprint confirmation member structure as the certificate SHA-256 thumbprint confirmation
method, described in Section 3.1, as a top-level member of the method, described in Section 3.1, as a top-level member of the
introspection response JSON. The protected resource compares that introspection response JSON. The protected resource compares that
certificate hash to a hash of the client certificate used for mutual- certificate hash to a hash of the client certificate used for mutual-
TLS authentication and rejects the request, if they do not match. TLS authentication and rejects the request if they do not match.
The following is an example of an introspection response for an The following is an example of an introspection response for an
active token with an "x5t#S256" certificate thumbprint confirmation active token with an "x5t#S256" certificate thumbprint confirmation
method. The new introspection response content introduced by this method. The new introspection response content introduced by this
specification is the "cnf" confirmation method at the bottom of the specification is the "cnf" confirmation method at the bottom of the
example that has the "x5t#S256" confirmation method member containing example that has the "x5t#S256" confirmation method member containing
the value that is the hash of the client certificate to which the the value that is the hash of the client certificate to which the
access token is bound. access token is bound.
HTTP/1.1 200 OK HTTP/1.1 200 OK
skipping to change at page 12, line 19 skipping to change at line 514
"active": true, "active": true,
"iss": "https://server.example.com", "iss": "https://server.example.com",
"sub": "ty.webb@example.com", "sub": "ty.webb@example.com",
"exp": 1493726400, "exp": 1493726400,
"nbf": 1493722800, "nbf": 1493722800,
"cnf":{ "cnf":{
"x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2" "x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
} }
} }
Figure 3: Example Introspection Response for a Certificate-Bound Figure 3: Example Introspection Response for a Certificate-Bound
Access Token Access Token
3.3. Authorization Server Metadata 3.3. Authorization Server Metadata
This document introduces the following new authorization server This document introduces the following new authorization server
metadata [RFC8414] parameter to signal the server's capability to metadata [RFC8414] parameter to signal the server's capability to
issue certificate bound access tokens: issue certificate-bound access tokens:
tls_client_certificate_bound_access_tokens tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value indicating server support for mutual-TLS OPTIONAL. Boolean value indicating server support for mutual-TLS
client certificate-bound access tokens. If omitted, the default client certificate-bound access tokens. If omitted, the default
value is "false". value is "false".
3.4. Client Registration Metadata 3.4. Client Registration Metadata
The following new client metadata parameter is introduced to convey The following new client metadata parameter is introduced to convey
the client's intention to use certificate bound access tokens: the client's intention to use certificate-bound access tokens:
tls_client_certificate_bound_access_tokens tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value used to indicate the client's intention OPTIONAL. Boolean value used to indicate the client's intention
to use mutual-TLS client certificate-bound access tokens. If to use mutual-TLS client certificate-bound access tokens. If
omitted, the default value is "false". omitted, the default value is "false".
Note that, if a client that has indicated the intention to use Note that if a client that has indicated the intention to use mutual-
mutual-TLS client certificate-bound tokens makes a request to the TLS client certificate-bound tokens makes a request to the token
token endpoint over a non-mutual-TLS connection, it is at the endpoint over a non-mutual-TLS connection, it is at the authorization
authorization server's discretion as to whether to return an error or server's discretion as to whether to return an error or issue an
issue an unbound token. unbound token.
4. Public Clients and Certificate-Bound Tokens 4. Public Clients and Certificate-Bound Tokens
Mutual-TLS OAuth client authentication and certificate-bound access Mutual-TLS OAuth client authentication and certificate-bound access
tokens can be used independently of each other. Use of certificate- tokens can be used independently of each other. Use of certificate-
bound access tokens without mutual-TLS OAuth client authentication, bound access tokens without mutual-TLS OAuth client authentication,
for example, is possible in support of binding access tokens to a TLS for example, is possible in support of binding access tokens to a TLS
client certificate for public clients (those without authentication client certificate for public clients (those without authentication
credentials associated with the "client_id"). The authorization credentials associated with the "client_id"). The authorization
server would configure the TLS stack in the same manner as for the server would configure the TLS stack in the same manner as for the
Self-Signed Certificate method such that it does not verify that the Self-Signed Certificate method such that it does not verify that the
certificate presented by the client during the handshake is signed by certificate presented by the client during the handshake is signed by
a trusted CA. Individual instances of a client would create a self- a trusted CA. Individual instances of a client would create a self-
signed certificate for mutual TLS with both the authorization server signed certificate for mutual TLS with both the authorization server
and resource server. The authorization server would not use the and resource server. The authorization server would not use the
mutual-TLS certificate to authenticate the client at the OAuth layer mutual-TLS certificate to authenticate the client at the OAuth layer
but would bind the issued access token to that certificate, for which but would bind the issued access token to the certificate for which
the client has proven possession of the corresponding private key. the client has proven possession of the corresponding private key.
The access token is then bound to the certificate and can only be The access token is then bound to the certificate and can only be
used by the client possessing the certificate and corresponding used by the client possessing the certificate and corresponding
private key and utilizing them to negotiate mutual TLS on connections private key and utilizing them to negotiate mutual TLS on connections
to the resource server. When the authorization server issues a to the resource server. When the authorization server issues a
refresh token to such a client, it SHOULD also bind the refresh token refresh token to such a client, it SHOULD also bind the refresh token
to the respective certificate. And check the binding when the to the respective certificate and check the binding when the refresh
refresh token is presented to get new access tokens. The token is presented to get new access tokens. The implementation
implementation details of the binding the refresh token are at the details of the binding of the refresh token are at the discretion of
discretion of the authorization server. the authorization server.
5. Metadata for Mutual-TLS Endpoint Aliases 5. Metadata for Mutual-TLS Endpoint Aliases
The process of negotiating client certificate-based mutual TLS The process of negotiating client certificate-based mutual TLS
involves a TLS server requesting a certificate from the TLS client involves a TLS server requesting a certificate from the TLS client
(the client does not provide one unsolicited). Although a server can (the client does not provide one unsolicited). Although a server can
be configured such that client certificates are optional, meaning be configured such that client certificates are optional, meaning
that the connection is allowed to continue when the client does not that the connection is allowed to continue when the client does not
provide a certificate, the act of a server requesting a certificate provide a certificate, the act of a server requesting a certificate
can result in undesirable behavior from some clients. This is can result in undesirable behavior from some clients. This is
particularly true of web browsers as TLS clients, which will particularly true of web browsers as TLS clients, which will
typically present the end-user with an intrusive certificate typically present the end user with an intrusive certificate
selection interface when the server requests a certificate. selection interface when the server requests a certificate.
Authorization servers supporting both clients using mutual TLS and Authorization servers supporting both clients using mutual TLS and
conventional clients MAY chose to isolate the server side mutual-TLS conventional clients MAY chose to isolate the server side mutual-TLS
behavior to only clients intending to do mutual TLS, thus avoiding behavior to only clients intending to do mutual TLS, thus avoiding
any undesirable effects it might have on conventional clients. The any undesirable effects it might have on conventional clients. The
following authorization server metadata parameter is introduced to following authorization server metadata parameter is introduced to
facilitate such separation: facilitate such separation:
mtls_endpoint_aliases mtls_endpoint_aliases
OPTIONAL. A JSON object containing alternative authorization OPTIONAL. A JSON object containing alternative authorization
server endpoints that, when present, an OAuth client intending to server endpoints that, when present, an OAuth client intending to
do mutual TLS uses in preference to the conventional endpoints. do mutual TLS uses in preference to the conventional endpoints.
The parameter value itself consists of one or more endpoint The parameter value itself consists of one or more endpoint
parameters, such as "token_endpoint", "revocation_endpoint", parameters, such as "token_endpoint", "revocation_endpoint",
"introspection_endpoint", etc., conventionally defined for the "introspection_endpoint", etc., conventionally defined for the top
top-level of authorization server metadata. An OAuth client level of authorization server metadata. An OAuth client intending
intending to do mutual TLS (for OAuth client authentication and/or to do mutual TLS (for OAuth client authentication and/or to
to acquire or use certificate-bound tokens) when making a request acquire or use certificate-bound tokens) when making a request
directly to the authorization server MUST use the alias URL of the directly to the authorization server MUST use the alias URL of the
endpoint within the "mtls_endpoint_aliases", when present, in endpoint within the "mtls_endpoint_aliases", when present, in
preference to the endpoint URL of the same name at top-level of preference to the endpoint URL of the same name at the top level
metadata. When an endpoint is not present in of metadata. When an endpoint is not present in
"mtls_endpoint_aliases", then the client uses the conventional "mtls_endpoint_aliases", then the client uses the conventional
endpoint URL defined at the top-level of the authorization server endpoint URL defined at the top level of the authorization server
metadata. Metadata parameters within "mtls_endpoint_aliases" that metadata. Metadata parameters within "mtls_endpoint_aliases" that
do not define endpoints to which an OAuth client makes a direct do not define endpoints to which an OAuth client makes a direct
request have no meaning and SHOULD be ignored. request have no meaning and SHOULD be ignored.
Below is an example of an authorization server metadata document with Below is an example of an authorization server metadata document with
the "mtls_endpoint_aliases" parameter, which indicates aliases for the "mtls_endpoint_aliases" parameter, which indicates aliases for
the token, revocation, and introspection endpoints that an OAuth the token, revocation, and introspection endpoints that an OAuth
client intending to do mutual TLS would in preference to the client intending to do mutual TLS would use in preference to the
conventional token, revocation, and introspection endpoints. Note conventional token, revocation, and introspection endpoints. Note
that the endpoints in "mtls_endpoint_aliases" use a different host that the endpoints in "mtls_endpoint_aliases" use a different host
than their conventional counterparts, which allows the authorization than their conventional counterparts, which allows the authorization
server (via TLS "server_name" extension [RFC6066] or actual distinct server (via TLS "server_name" extension [RFC6066] or actual distinct
hosts) to differentiate its TLS behavior as appropriate. hosts) to differentiate its TLS behavior as appropriate.
{ {
"issuer": "https://server.example.com", "issuer": "https://server.example.com",
"authorization_endpoint": "https://server.example.com/authz", "authorization_endpoint": "https://server.example.com/authz",
"token_endpoint": "https://server.example.com/token", "token_endpoint": "https://server.example.com/token",
"introspection_endpoint": "https://server.example.com/introspect", "introspection_endpoint": "https://server.example.com/introspect",
"revocation_endpoint": "https://server.example.com/revo", "revocation_endpoint": "https://server.example.com/revo",
"jwks_uri": "https://server.example.com/jwks", "jwks_uri": "https://server.example.com/jwks",
"response_types_supported": ["code"], "response_types_supported": ["code"],
"response_modes_supported": ["fragment","query","form_post"], "response_modes_supported": ["fragment","query","form_post"],
"grant_types_supported": ["authorization_code", "refresh_token"], "grant_types_supported": ["authorization_code", "refresh_token"],
"token_endpoint_auth_methods_supported": "token_endpoint_auth_methods_supported":
["tls_client_auth","client_secret_basic","none"], ["tls_client_auth","client_secret_basic","none"],
"tls_client_certificate_bound_access_tokens": true "tls_client_certificate_bound_access_tokens": true,
"mtls_endpoint_aliases": { "mtls_endpoint_aliases": {
"token_endpoint": "https://mtls.example.com/token", "token_endpoint": "https://mtls.example.com/token",
"revocation_endpoint": "https://mtls.example.com/revo", "revocation_endpoint": "https://mtls.example.com/revo",
"introspection_endpoint": "https://mtls.example.com/introspect" "introspection_endpoint": "https://mtls.example.com/introspect"
} }
} }
Figure 4: Example Authorization Server Metadata with Mutual-TLS Figure 4: Example Authorization Server Metadata with Mutual-TLS
Endpoint Aliases Endpoint Aliases
6. Implementation Considerations 6. Implementation Considerations
6.1. Authorization Server 6.1. Authorization Server
The authorization server needs to set up its TLS configuration The authorization server needs to set up its TLS configuration
appropriately for the OAuth client authentication methods it appropriately for the OAuth client authentication methods it
supports. supports.
An authorization server that supports mutual-TLS client An authorization server that supports mutual-TLS client
authentication and other client authentication methods or public authentication and other client authentication methods or public
clients in parallel would make mutual TLS optional (i.e. allowing a clients in parallel would make mutual TLS optional (i.e., allowing a
handshake to continue after the server requests a client certificate handshake to continue after the server requests a client certificate
but the client does not send one). but the client does not send one).
In order to support the Self-Signed Certificate method alone, the In order to support the Self-Signed Certificate method alone, the
authorization server would configure the TLS stack in such a way that authorization server would configure the TLS stack in such a way that
it does not verify whether the certificate presented by the client it does not verify whether the certificate presented by the client
during the handshake is signed by a trusted CA certificate. during the handshake is signed by a trusted CA certificate.
As described in Section 3, the authorization server binds the issued As described in Section 3, the authorization server binds the issued
access token to the TLS client certificate, which means that it will access token to the TLS client certificate, which means that it will
only issue certificate-bound tokens for a certificate which the only issue certificate-bound tokens for a certificate that the client
client has proven possession of the corresponding private key. has proven possession of the corresponding private key.
The authorization server may also consider hosting the token The authorization server may also consider hosting the token endpoint
endpoint, and other endpoints requiring client authentication, on a and other endpoints requiring client authentication on a separate
separate host name or port in order to prevent unintended impact on host name or port in order to prevent unintended impact on the TLS
the TLS behavior of its other endpoints, e.g. the authorization behavior of its other endpoints, e.g., the authorization endpoint.
endpoint. As described in Section 5, it may further isolate any As described in Section 5, it may further isolate any potential
potential impact of the server requesting client certificates by impact of the server requesting client certificates by offering a
offering a distinct set of endpoints on a separate host or port, distinct set of endpoints on a separate host or port, which are
which are aliases for the originals that a client intending to do aliases for the originals that a client intending to do mutual TLS
mutual TLS will use in preference to the conventional endpoints. will use in preference to the conventional endpoints.
6.2. Resource Server 6.2. Resource Server
OAuth divides the roles and responsibilities such that the resource OAuth divides the roles and responsibilities such that the resource
server relies on the authorization server to perform client server relies on the authorization server to perform client
authentication and obtain resource owner (end-user) authorization. authentication and obtain resource-owner (end-user) authorization.
The resource server makes authorization decisions based on the access The resource server makes authorization decisions based on the access
token presented by the client but does not directly authenticate the token presented by the client but does not directly authenticate the
client per se. The manner in which an access token is bound to the client per se. The manner in which an access token is bound to the
client certificate and how a protected resource verifies the proof- client certificate and how a protected resource verifies the proof-
of-possession decouples that from the specific method that the client of-possession decouples that from the specific method that the client
used to authenticate with the authorization server. Mutual TLS used to authenticate with the authorization server. Mutual TLS
during protected resource access can therefore serve purely as a during protected resource access can, therefore, serve purely as a
proof-of-possession mechanism. As such, it is not necessary for the proof-of-possession mechanism. As such, it is not necessary for the
resource server to validate the trust chain of the client's resource server to validate the trust chain of the client's
certificate in any of the methods defined in this document. The certificate in any of the methods defined in this document. The
resource server would therefore configure the TLS stack in a way that resource server would, therefore, configure the TLS stack in a way
it does not verify whether the certificate presented by the client that it does not verify whether the certificate presented by the
during the handshake is signed by a trusted CA certificate. client during the handshake is signed by a trusted CA certificate.
6.3. Certificate Expiration and Bound Access Tokens 6.3. Certificate Expiration and Bound Access Tokens
As described in Section 3, an access token is bound to a specific As described in Section 3, an access token is bound to a specific
client certificate, which means that the same certificate must be client certificate, which means that the same certificate must be
used for mutual TLS on protected resource access. It also implies used for mutual TLS on protected resource access. It also implies
that access tokens are invalidated when a client updates the that access tokens are invalidated when a client updates the
certificate, which can be handled similar to expired access tokens certificate, which can be handled similarly to expired access tokens
where the client requests a new access token (typically with a where the client requests a new access token (typically with a
refresh token) and retries the protected resource request. refresh token) and retries the protected resource request.
6.4. Implicit Grant Unsupported 6.4. Implicit Grant Unsupported
This document describes binding an access token to the client This document describes binding an access token to the client
certificate presented on the TLS connection from the client to the certificate presented on the TLS connection from the client to the
authorization server's token endpoint, however, such binding of authorization server's token endpoint, however, such binding of
access tokens issued directly from the authorization endpoint via the access tokens issued directly from the authorization endpoint via the
implicit grant flow is explicitly out of scope. End users interact implicit grant flow is explicitly out of scope. End users interact
directly with the authorization endpoint using a web browser and the directly with the authorization endpoint using a web browser, and the
use of client certificates in user's browsers bring operational and use of client certificates in user's browsers bring operational and
usability issues, which make it undesirable to support certificate- usability issues that make it undesirable to support certificate-
bound access tokens issued in the implicit grant flow. bound access tokens issued in the implicit grant flow.
Implementations wanting to employ certificate-bound access tokens Implementations wanting to employ certificate-bound access tokens
should utilize grant types that involve the client making an access should utilize grant types that involve the client making an access
token request directly to the token endpoint (e.g. the authorization token request directly to the token endpoint (e.g., the authorization
code and refresh token grant types). code and refresh token grant types).
6.5. TLS Termination 6.5. TLS Termination
An authorization server or resource server MAY choose to terminate An authorization server or resource server MAY choose to terminate
TLS connections at a load balancer, reverse proxy, or other network TLS connections at a load balancer, reverse proxy, or other network
intermediary. How the client certificate metadata is securely intermediary. How the client certificate metadata is securely
communicated between the intermediary and the application server in communicated between the intermediary and the application server, in
this case is out of scope of this specification. this case, is out of scope of this specification.
7. Security Considerations 7. Security Considerations
7.1. Certificate-Bound Refresh Tokens 7.1. Certificate-Bound Refresh Tokens
The OAuth 2.0 Authorization Framework [RFC6749] requires that an The OAuth 2.0 Authorization Framework [RFC6749] requires that an
authorization server bind refresh tokens to the client to which they authorization server (AS) bind refresh tokens to the client to which
were issued and that confidential clients (those having established they were issued and that confidential clients (those having
authentication credentials with the authorization server) established authentication credentials with the AS) authenticate to
authenticate to the AS when presenting a refresh token. As a result, the AS when presenting a refresh token. As a result, refresh tokens
refresh tokens are indirectly certificate-bound by way of the client are indirectly certificate-bound by way of the client ID and the
ID and the associated requirement for (certificate-based) associated requirement for (certificate-based) authentication to the
authentication to the authorization server when issued to clients AS when issued to clients utilizing the "tls_client_auth" or
utilizing the "tls_client_auth" or "self_signed_tls_client_auth" "self_signed_tls_client_auth" methods of client authentication.
methods of client authentication. Section 4 describes certificate- Section 4 describes certificate-bound refresh tokens issued to public
bound refresh tokens issued to public clients (those without clients (those without authentication credentials associated with the
authentication credentials associated with the "client_id"). "client_id").
7.2. Certificate Thumbprint Binding 7.2. Certificate Thumbprint Binding
The binding between the certificate and access token specified in The binding between the certificate and access token specified in
Section 3.1 uses a cryptographic hash of the certificate. It relies Section 3.1 uses a cryptographic hash of the certificate. It relies
on the hash function having sufficient second-preimage resistance so on the hash function having sufficient second-preimage resistance so
as to make it computationally infeasible to find or create another as to make it computationally infeasible to find or create another
certificate that produces to the same hash output value. The SHA-256 certificate that produces to the same hash output value. The SHA-256
hash function was used because it meets the aforementioned hash function was used because it meets the aforementioned
requirement while being widely available. If, in the future, requirement while being widely available. If, in the future,
certificate thumbprints need to be computed using hash function(s) certificate thumbprints need to be computed using hash function(s)
other than SHA-256, it is suggested that additional related JWT other than SHA-256, it is suggested that, for additional related JWT
confirmation methods members be defined for that purpose and confirmation methods, members be defined for that purpose and
registered in the IANA "JWT Confirmation Methods" registry registered in the IANA "JWT Confirmation Methods" registry
[IANA.JWT.Claims] for JWT "cnf" member values. [IANA.JWT.Claims] for JWT "cnf" member values.
Community knowledge about the strength of various algorithms and Community knowledge about the strength of various algorithms and
feasible attacks can change suddenly, and experience shows that a feasible attacks can change suddenly, and experience shows that a
document about security is a point-in-time statement. Readers are document about security is a point-in-time statement. Readers are
advised to seek out any errata or updates that apply to this advised to seek out any errata or updates that apply to this
document. document.
7.3. TLS Versions and Best Practices 7.3. TLS Versions and Best Practices
In the abstract this document is applicable with any TLS version This document is applicable with any TLS version supporting
supporting certificate-based client authentication. Both TLS 1.3 certificate-based client authentication. Both TLS 1.3 [RFC8446] and
[RFC8446] and TLS 1.2 [RFC5246] are cited herein because, at the time TLS 1.2 [RFC5246] are cited herein, because, at the time of writing,
of writing, 1.3 is the newest version while 1.2 is the most widely 1.3 is the newest version, while 1.2 is the most widely deployed.
deployed. General implementation and security considerations for General implementation and security considerations for TLS, including
TLS, including version recommendations, can be found in [BCP195]. version recommendations, can be found in [BCP195].
TLS certificate validation (for both client and server certificates) TLS certificate validation (for both client and server certificates)
requires a local database of trusted certificate authorities (CAs). requires a local database of trusted certificate authorities (CAs).
Decisions about what CAs to trust and how to make such a Decisions about what CAs to trust and how to make such a
determination of trust are out of scope for this document. determination of trust are out of scope for this document.
7.4. X.509 Certificate Spoofing 7.4. X.509 Certificate Spoofing
If the PKI method of client authentication is used, an attacker could If the PKI method of client authentication is used, an attacker could
try to impersonate a client using a certificate with the same subject try to impersonate a client using a certificate with the same subject
(DN or SAN) but issued by a different CA, which the authorization (DN or SAN) but issued by a different CA that the authorization
server trusts. To cope with that threat, the authorization server server trusts. To cope with that threat, the authorization server
SHOULD only accept as trust anchors a limited number of CAs whose SHOULD only accept, as trust anchors, a limited number of CAs whose
certificate issuance policy meets its security requirements. There certificate issuance policy meets its security requirements. There
is an assumption then that the client and server agree out of band on is an assumption then that the client and server agree out of band on
the set of trust anchors that the server uses to create and validate the set of trust anchors that the server uses to create and validate
the certificate chain. Without this assumption the use of a subject the certificate chain. Without this assumption the use of a subject
to identify the client certificate would open the server up to to identify the client certificate would open the server up to
certificate spoofing attacks. certificate spoofing attacks.
7.5. X.509 Certificate Parsing and Validation Complexity 7.5. X.509 Certificate Parsing and Validation Complexity
Parsing and validation of X.509 certificates and certificate chains Parsing and validation of X.509 certificates and certificate chains
is complex and implementation mistakes have previously exposed is complex, and implementation mistakes have previously exposed
security vulnerabilities. Complexities of validation include (but security vulnerabilities. Complexities of validation include (but
are not limited to) [CX5P] [DCW] [RFC5280]: are not limited to) [CX5P] [DCW] [RFC5280]:
o checking of Basic Constraints, basic and extended Key Usage * checking of basic constraints, basic and extended key usage
constraints, validity periods, and critical extensions; constraints, validity periods, and critical extensions;
o handling of embedded NUL bytes in ASN.1 counted-length strings, * handling of embedded NUL bytes in ASN.1 counted-length strings and
and non-canonical or non-normalized string representations in non-canonical or non-normalized string representations in subject
subject names; names;
o handling of wildcard patterns in subject names; * handling of wildcard patterns in subject names;
o recursive verification of certificate chains and checking * recursive verification of certificate chains and checking
certificate revocation. certificate revocation.
For these reasons, implementors SHOULD use an established and well- For these reasons, implementors SHOULD use an established and well-
tested X.509 library (such as one used by an established TLS library) tested X.509 library (such as one used by an established TLS library)
for validation of X.509 certificate chains and SHOULD NOT attempt to for validation of X.509 certificate chains and SHOULD NOT attempt to
write their own X.509 certificate validation procedures. write their own X.509 certificate validation procedures.
8. Privacy Considerations 8. Privacy Considerations
In TLS versions prior to 1.3, the client's certificate is sent In TLS versions prior to 1.3, the client's certificate is sent
unencrypted in the initial handshake and can potentially be used by unencrypted in the initial handshake and can potentially be used by
third parties to monitor, track, and correlate client activity. This third parties to monitor, track, and correlate client activity. This
is likely of little concern for clients that act on behalf of a is likely of little concern for clients that act on behalf of a
significant number of end-users because individual user activity will significant number of end users because individual user activity will
not be discernible amidst the client activity as a whole. However, not be discernible amidst the client activity as a whole. However,
clients that act on behalf of a single end-user, such as a native clients that act on behalf of a single end user, such as a native
application on a mobile device, should use TLS version 1.3 whenever application on a mobile device, should use TLS version 1.3 whenever
possible or consider the potential privacy implications of using possible or consider the potential privacy implications of using
mutual TLS on earlier versions. mutual TLS on earlier versions.
9. IANA Considerations 9. IANA Considerations
9.1. JWT Confirmation Methods Registration 9.1. JWT Confirmation Methods Registration
This specification requests registration of the following value in Per this specification, the following value has been registered in
the IANA "JWT Confirmation Methods" registry [IANA.JWT.Claims] for the IANA "JWT Confirmation Methods" registry [IANA.JWT.Claims] for
JWT "cnf" member values established by [RFC7800]. JWT "cnf" member values established by [RFC7800].
o Confirmation Method Value: "x5t#S256" Confirmation Method Value: "x5t#S256"
o Confirmation Method Description: X.509 Certificate SHA-256 Confirmation Method Description: X.509 Certificate SHA-256
Thumbprint Thumbprint
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 3.1 of [[ this specification ]] Specification Document(s): Section 3.1 of RFC 8705
9.2. Authorization Server Metadata Registration 9.2. Authorization Server Metadata Registration
This specification requests registration of the following values in Per this specification, the following values have been registered in
the IANA "OAuth Authorization Server Metadata" registry the IANA "OAuth Authorization Server Metadata" registry
[IANA.OAuth.Parameters] established by [RFC8414]. [IANA.OAuth.Parameters] established by [RFC8414].
o Metadata Name: "tls_client_certificate_bound_access_tokens" Metadata Name: "tls_client_certificate_bound_access_tokens"
o Metadata Description: Indicates authorization server support for Metadata Description: Indicates authorization server support for
mutual-TLS client certificate-bound access tokens. mutual-TLS client certificate-bound access tokens.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 3.3 of [[ this specification ]] Specification Document(s): Section 3.3 of RFC 8705
o Metadata Name: "mtls_endpoint_aliases"
o Metadata Description: JSON object containing alternative Metadata Name: "mtls_endpoint_aliases"
Metadata Description: JSON object containing alternative
authorization server endpoints, which a client intending to do authorization server endpoints, which a client intending to do
mutual TLS will use in preference to the conventional endpoints. mutual TLS will use in preference to the conventional endpoints.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 5 of [[ this specification ]] Specification Document(s): Section 5 of RFC 8705
9.3. Token Endpoint Authentication Method Registration 9.3. Token Endpoint Authentication Method Registration
This specification requests registration of the following values in Per this specification, the following values have been registered in
the IANA "OAuth Token Endpoint Authentication Methods" registry the IANA "OAuth Token Endpoint Authentication Methods" registry
[IANA.OAuth.Parameters] established by [RFC7591]. [IANA.OAuth.Parameters] established by [RFC7591].
o Token Endpoint Authentication Method Name: "tls_client_auth" Token Endpoint Authentication Method Name: "tls_client_auth"
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.1 of [[ this specification Specification Document(s): Section 2.1.1 of RFC 8705
]]
o Token Endpoint Authentication Method Name: Token Endpoint Authentication Method Name: "self_signed_tls_client_
"self_signed_tls_client_auth" auth"
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.2.1 of [[ this specification Specification Document(s): Section 2.2.1 of RFC 8705
]]
9.4. Token Introspection Response Registration 9.4. Token Introspection Response Registration
Proof-of-Possession Key Semantics for JSON Web Tokens [RFC7800] "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)"
defined the "cnf" (confirmation) claim, which enables confirmation [RFC7800] defined the "cnf" (confirmation) claim that enables
key information to be carried in a JWT. However, the same proof-of- confirmation key information to be carried in a JWT. However, the
possession semantics are also useful for introspected access tokens same proof-of-possession semantics are also useful for introspected
whereby the protected resource obtains the confirmation key data as access tokens whereby the protected resource obtains the confirmation
meta-information of a token introspection response and uses that key data as metainformation of a token introspection response and
information in verifying proof-of-possession. Therefore this uses that information in verifying proof-of-possession. Therefore,
specification defines and registers proof-of-possession semantics for this specification defines and registers proof-of-possession
OAuth 2.0 Token Introspection [RFC7662] using the "cnf" structure. semantics for OAuth 2.0 Token Introspection [RFC7662] using the "cnf"
When included as a top-level member of an OAuth token introspection structure. When included as a top-level member of an OAuth token
response, "cnf" has the same semantics and format as the claim of the introspection response, "cnf" has the same semantics and format as
same name defined in [RFC7800]. While this specification only the claim of the same name defined in [RFC7800]. While this
explicitly uses the "x5t#S256" confirmation method member (see specification only explicitly uses the "x5t#S256" confirmation method
Section 3.2), it needs to define and register the higher level "cnf" member (see Section 3.2), it needs to define and register the higher-
structure as an introspection response member in order to define and level "cnf" structure as an introspection response member in order to
use the more specific certificate thumbprint confirmation method. define and use the more specific certificate thumbprint confirmation
method.
As such, this specification requests registration of the following As such, the following values have been registered in the IANA "OAuth
value in the IANA "OAuth Token Introspection Response" registry Token Introspection Response" registry [IANA.OAuth.Parameters]
[IANA.OAuth.Parameters] established by [RFC7662]. established by [RFC7662].
o Claim Name: "cnf" Claim Name: "cnf"
o Claim Description: Confirmation Claim Description: Confirmation
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): [RFC7800] and [[ this specification ]] Specification Document(s): [RFC7800] and RFC 8705
9.5. Dynamic Client Registration Metadata Registration 9.5. Dynamic Client Registration Metadata Registration
This specification requests registration of the following client Per this specification, the following client metadata definitions
metadata definitions in the IANA "OAuth Dynamic Client Registration have been registered in the IANA "OAuth Dynamic Client Registration
Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591]: Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591]:
o Client Metadata Name: "tls_client_certificate_bound_access_tokens" Client Metadata Name: "tls_client_certificate_bound_access_tokens"
o Client Metadata Description: Indicates the client's intention to Client Metadata Description: Indicates the client's intention to use
use mutual-TLS client certificate-bound access tokens. mutual-TLS client certificate-bound access tokens.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 3.4 of [[ this specification ]] Specification Document(s): Section 3.4 of RFC 8705
o Client Metadata Name: "tls_client_auth_subject_dn" Client Metadata Name: "tls_client_auth_subject_dn"
o Client Metadata Description: String value specifying the expected Client Metadata Description: String value specifying the expected
subject DN of the client certificate. subject DN of the client certificate.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.2 of [[ this specification Specification Document(s): Section 2.1.2 of RFC 8705
]]
o Client Metadata Name: "tls_client_auth_san_dns" Client Metadata Name: "tls_client_auth_san_dns"
o Client Metadata Description: String value specifying the expected Client Metadata Description: String value specifying the expected
dNSName SAN entry in the client certificate. dNSName SAN entry in the client certificate.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.2 of [[ this specification Specification Document(s): Section 2.1.2 of RFC 8705
]]
o Client Metadata Name: "tls_client_auth_san_uri" Client Metadata Name: "tls_client_auth_san_uri"
o Client Metadata Description: String value specifying the expected Client Metadata Description: String value specifying the expected
uniformResourceIdentifier SAN entry in the client certificate. uniformResourceIdentifier SAN entry in the client certificate.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.2 of [[ this specification Specification Document(s): Section 2.1.2 of RFC 8705
]]
o Client Metadata Name: "tls_client_auth_san_ip" Client Metadata Name: "tls_client_auth_san_ip"
o Client Metadata Description: String value specifying the expected Client Metadata Description: String value specifying the expected
iPAddress SAN entry in the client certificate. iPAddress SAN entry in the client certificate.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.2 of [[ this specification Specification Document(s): Section 2.1.2 of RFC 8705
]]
o Client Metadata Name: "tls_client_auth_san_email" Client Metadata Name: "tls_client_auth_san_email"
o Client Metadata Description: String value specifying the expected Client Metadata Description: String value specifying the expected
rfc822Name SAN entry in the client certificate. rfc822Name SAN entry in the client certificate.
o Change Controller: IESG Change Controller: IESG
o Specification Document(s): Section 2.1.2 of [[ this specification Specification Document(s): Section 2.1.2 of RFC 8705
]]
10. References 10. References
10.1. Normative References 10.1. Normative References
[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre, [BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, May 2015,
2015, <http://www.rfc-editor.org/info/bcp195>. <https://www.rfc-editor.org/info/bcp195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol [RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names", (LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006, RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://www.rfc-editor.org/info/rfc4514>. <https://www.rfc-editor.org/info/rfc4514>.
skipping to change at page 23, line 40 skipping to change at line 1032
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414, Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018, DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>. <https://www.rfc-editor.org/info/rfc8414>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[SHS] National Institute of Standards and Technology, "Secure [SHS] National Institute of Standards and Technology (NIST),
Hash Standard (SHS)", FIPS PUB 180-4, March 2012, "Secure Hash Standard (SHS)", FIPS PUB 180-4,
<http://csrc.nist.gov/publications/fips/fips180-4/ DOI 10.6028/NIST.FIPS.180-4, August 2015,
fips-180-4.pdf>. <https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[X690] International Telephone and Telegraph Consultative [X690] ITU-T, "Information Technology - ASN.1 encoding rules:
Committee, "ASN.1 encoding rules: Specification of basic Specification of Basic Encoding Rules (BER), Canonical
encoding Rules (BER), Canonical encoding rules (CER) and Encoding Rules (CER) and Distinguished Encoding Rules
Distinguished encoding rules (DER)", CCITT Recommendation (DER)", ITU-T Recommendation X.690, August 2015.
X.690, July 2015.
10.2. Informative References 10.2. Informative References
[CX5P] Wong, D., "Common x509 certificate validation/creation [CX5P] Wong, D., "Common x509 certificate validation/creation
pitfalls", September 2016, pitfalls", September 2016,
<https://www.cryptologie.net/article/374/ <https://www.cryptologie.net/article/374/common-x509-
common-x509-certificate-validationcreation-pitfalls>. certificate-validationcreation-pitfalls>.
[DCW] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh, [DCW] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
D., and V. Shmatikov, "The Most Dangerous Code in the D., and V. Shmatikov, "The Most Dangerous Code in the
World: Validating SSL Certificates in Non-Browser World: Validating SSL Certificates in Non-Browser
Software", Software", DOI 10.1145/2382196.2382204, October 2012,
<http://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf>. <http://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf>.
[I-D.ietf-oauth-token-binding]
Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", draft-ietf-oauth-token-
binding-06 (work in progress), March 2018.
[IANA.JWT.Claims] [IANA.JWT.Claims]
IANA, "JSON Web Token Claims", IANA, "JSON Web Token Claims",
<http://www.iana.org/assignments/jwt>. <https://www.iana.org/assignments/jwt>.
[IANA.OAuth.Parameters] [IANA.OAuth.Parameters]
IANA, "OAuth Parameters", IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>. <https://www.iana.org/assignments/oauth-parameters>.
[OpenID.CIBA] [OpenID.CIBA]
Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B. Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B.
Campbell, "OpenID Connect Client Initiated Backchannel Campbell, "OpenID Connect Client Initiated Backchannel
Authentication Flow - Core 1.0", January 2019, Authentication Flow - Core 1.0", 16 January 2019,
<https://openid.net/specs/openid-client-initiated- <https://openid.net/specs/openid-client-initiated-
backchannel-authentication-core-1_0.html>. backchannel-authentication-core-1_0.html>.
[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol [RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, (LDAP): Syntaxes and Matching Rules", RFC 4517,
DOI 10.17487/RFC4517, June 2006, DOI 10.17487/RFC4517, June 2006,
<https://www.rfc-editor.org/info/rfc4517>. <https://www.rfc-editor.org/info/rfc4517>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, Address Text Representation", RFC 5952,
skipping to change at page 25, line 13 skipping to change at line 1094
<https://www.rfc-editor.org/info/rfc6066>. <https://www.rfc-editor.org/info/rfc6066>.
[RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
August 2013, <https://www.rfc-editor.org/info/rfc7009>. August 2013, <https://www.rfc-editor.org/info/rfc7009>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015, DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>. <https://www.rfc-editor.org/info/rfc7518>.
Appendix A. Example "cnf" Claim, Certificate and JWK [TOKEN] Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", Work in Progress, Internet-
Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
<https://tools.ietf.org/html/draft-ietf-oauth-token-
binding-08>.
For reference, an "x5t#S256" value and the X.509 Certificate from Appendix A. Example "cnf" Claim, Certificate, and JWK
For reference, an "x5t#S256" value and the X.509 certificate from
which it was calculated are provided in the following examples, which it was calculated are provided in the following examples,
Figure 5 and Figure 6 respectively. A JWK representation of the Figures 5 and 6, respectively. A JWK representation of the
certificate's public key along with the "x5c" member is also provided certificate's public key along with the "x5c" member is also provided
in Figure 7. in Figure 7.
"cnf":{"x5t#S256":"A4DtL2JmUMhAsvJj5tKyn64SqzmuXbMrJa0n761y5v0"} "cnf":{"x5t#S256":"A4DtL2JmUMhAsvJj5tKyn64SqzmuXbMrJa0n761y5v0"}
Figure 5: x5t#S256 Confirmation Claim Figure 5: x5t#S256 Confirmation Claim
-----BEGIN CERTIFICATE----- -----BEGIN CERTIFICATE-----
MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTAx MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTAx
ODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGByqG ODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGByqG
skipping to change at page 25, line 51 skipping to change at line 1138
"x5c":[ "x5c":[
"MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTA "MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTA
xODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGBy xODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGBy
qGSM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZ qGSM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZ
jjJ/Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0E jjJ/Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0E
AwIDSQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2 AwIDSQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2
eXZOVbUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY=" eXZOVbUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY="
] ]
} }
Figure 7: JSON Web Key Figure 7: JSON Web Key
Appendix B. Relationship to Token Binding Appendix B. Relationship to Token Binding
OAuth 2.0 Token Binding [I-D.ietf-oauth-token-binding] enables the OAuth 2.0 Token Binding [TOKEN] enables the application of Token
application of Token Binding to the various artifacts and tokens Binding to the various artifacts and tokens employed throughout
employed throughout OAuth. That includes binding of an access token OAuth. That includes binding of an access token to a Token Binding
to a Token Binding key, which bears some similarities in motivation key, which bears some similarities in motivation and design to the
and design to the mutual-TLS client certificate-bound access tokens mutual-TLS client certificate-bound access tokens defined in this
defined in this document. Both documents define what is often called document. Both documents define what is often called a proof-of-
a proof-of-possession security mechanism for access tokens, whereby a possession security mechanism for access tokens, whereby a client
client must demonstrate possession of cryptographic keying material must demonstrate possession of cryptographic keying material when
when accessing a protected resource. The details differ somewhat accessing a protected resource. The details differ somewhat between
between the two documents but both have the authorization server bind the two documents but both have the authorization server bind the
the access token that it issues to an asymmetric key pair held by the access token that it issues to an asymmetric key pair held by the
client. The client then proves possession of the private key from client. The client then proves possession of the private key from
that pair with respect to the TLS connection over which the protected that pair with respect to the TLS connection over which the protected
resource is accessed. resource is accessed.
Token Binding uses bare keys that are generated on the client, which Token Binding uses bare keys that are generated on the client, which
avoids many of the difficulties of creating, distributing, and avoids many of the difficulties of creating, distributing, and
managing certificates used in this specification. However, at the managing certificates used in this specification. However, at the
time of writing, Token Binding is fairly new and there is relatively time of writing, Token Binding is fairly new, and there is relatively
little support for it in available application development platforms little support for it in available application development platforms
and tooling. Until better support for the underlying core Token and tooling. Until better support for the underlying core Token
Binding specifications exists, practical implementations of OAuth 2.0 Binding specifications exists, practical implementations of OAuth 2.0
Token Binding are infeasible. Mutual TLS, on the other hand, has Token Binding are infeasible. Mutual TLS, on the other hand, has
been around for some time and enjoys widespread support in web been around for some time and enjoys widespread support in web
servers and development platforms. As a consequence, OAuth 2.0 servers and development platforms. As a consequence, OAuth 2.0
Mutual-TLS Client Authentication and Certificate-Bound Access Tokens Mutual-TLS Client Authentication and Certificate-Bound Access Tokens
can be built and deployed now using existing platforms and tools. In can be built and deployed now using existing platforms and tools. In
the future, the two specifications are likely to be deployed in the future, the two specifications are likely to be deployed in
parallel for solving similar problems in different environments. parallel for solving similar problems in different environments.
Authorization servers may even support both specifications Authorization servers may even support both specifications
simultaneously using different proof-of-possession mechanisms for simultaneously using different proof-of-possession mechanisms for
tokens issued to different clients. tokens issued to different clients.
Appendix C. Acknowledgements Acknowledgements
Scott "not Tomlinson" Tomilson and Matt Peterson were involved in Scott "not Tomlinson" Tomilson and Matt Peterson were involved in
design and development work on a mutual-TLS OAuth client design and development work on a mutual-TLS OAuth client
authentication implementation, which predates this document. authentication implementation that predates this document.
Experience and learning from that work informed some of the content Experience and learning from that work informed some of the content
of this document. of this document.
This specification was developed within the OAuth Working Group under This specification was developed within the OAuth Working Group under
the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with
Eric Rescorla, Benjamin Kaduk, and Roman Danyliw serving as Security Eric Rescorla, Benjamin Kaduk, and Roman Danyliw serving as Security
Area Directors. Additionally, the following individuals contributed Area Directors. Additionally, the following individuals contributed
ideas, feedback, and wording that helped shape this specification: ideas, feedback, and wording that helped shape this specification:
Vittorio Bertocci, Sergey Beryozkin, Ralph Bragg, Sophie Bremer, Vittorio Bertocci, Sergey Beryozkin, Ralph Bragg, Sophie Bremer,
Roman Danyliw, Vladimir Dzhuvinov, Samuel Erdtman, Evan Gilman, Leif Roman Danyliw, Vladimir Dzhuvinov, Samuel Erdtman, Evan Gilman, Leif
Johansson, Michael Jones, Phil Hunt, Benjamin Kaduk, Takahiko Johansson, Michael Jones, Phil Hunt, Benjamin Kaduk, Takahiko
Kawasaki, Sean Leonard, Kepeng Li, Neil Madden, James Manger, Jim Kawasaki, Sean Leonard, Kepeng Li, Neil Madden, James Manger, Jim
Manico, Nov Matake, Sascha Preibisch, Eric Rescorla, Justin Richer, Manico, Nov Matake, Sascha Preibisch, Eric Rescorla, Justin Richer,
Vincent Roca, Filip Skokan, Dave Tonge, and Hannes Tschofenig. Vincent Roca, Filip Skokan, Dave Tonge, and Hannes Tschofenig.
Appendix D. Document(s) History
[[ to be removed by the RFC Editor before publication as an RFC ]]
draft-ietf-oauth-mtls-17
o Updates from IESG ballot position comments.
draft-ietf-oauth-mtls-16
o Editorial updates from last call review.
draft-ietf-oauth-mtls-15
o Editorial updates from second AD review.
draft-ietf-oauth-mtls-14
o Editorial clarifications around there being only a single subject
registered/configured per client for the tls_client_auth method.
o Add a brief explanation about how, with tls_client_auth and
self_signed_tls_client_auth, refresh tokens are certificate-bound
indirectly via the client authentication.
o Add mention of refresh tokens in the abstract.
draft-ietf-oauth-mtls-13
o Add an abstract protocol flow and diagram to serve as an overview
of OAuth in general and baseline to describe the various ways in
which the mechanisms defined herein are intended to be used.
o A little bit less of that German influence.
o Rework the TLS references a bit and, in the Terminology section,
clean up the description of what messages are sent and verified in
the handshake to do 'mutual TLS'.
o Move the explanation about "cnf" introspection registration into
the IANA Considerations.
o Add CIBA as an informational reference and additional example of
an OAuth extension that defines an endpoint that utilizes client
authentication.
o Shorten a few of the section titles.
o Add new client metadata values to allow for the use of a SAN in
the PKI MTLS client authentication method.
o Add privacy considerations attempting to discuss the implications
of the client cert being sent in the clear in TLS 1.2.
o Changed the 'Certificate Bound Access Tokens Without Client
Authentication' section to 'Public Clients and Certificate-Bound
Tokens' and moved it up to be a top level section while adding
discussion of binding refresh tokens for public clients.
o Reword/restructure the main PKI method section somewhat to
(hopefully) improve readability.
o Reword/restructure the Self-Signed method section a bit to
(hopefully) make it more comprehensible.
o Reword the AS and RS Implementation Considerations somewhat to
(hopefully) improve readability.
o Clarify that the protected resource obtains the client certificate
used for mutual TLS from its TLS implementation layer.
o Add Security Considerations section about the certificate
thumbprint binding that includes the hash algorithm agility
recommendation.
o Add an "mtls_endpoint_aliases" AS metadata parameter that is a
JSON object containing alternative authorization server endpoints,
which a client intending to do mutual TLS will use in preference
to the conventional endpoints.
o Minor editorial updates.
draft-ietf-oauth-mtls-12
o Add an example certificate, JWK, and confirmation method claim.
o Minor editorial updates based on implementer feedback.
o Additional Acknowledgements.
draft-ietf-oauth-mtls-11
o Editorial updates.
o Mention/reference TLS 1.3 RFC8446 in the TLS Versions and Best
Practices section.
draft-ietf-oauth-mtls-10
o Update draft-ietf-oauth-discovery reference to RFC8414
draft-ietf-oauth-mtls-09
o Change "single certificates" to "self-signed certificates" in the
Abstract
draft-ietf-oauth-mtls-08
o Incorporate clarifications and editorial improvements from Justin
Richer's WGLC review
o Drop the use of the "sender constrained" terminology per WGLC
feedback from Neil Madden (including changing the metadata
parameters from mutual_tls_sender_constrained_access_tokens to
tls_client_certificate_bound_access_tokens)
o Add a new security considerations section on X.509 parsing and
validation per WGLC feedback from Neil Madden and Benjamin Kaduk
o Note that a server can terminate TLS at a load balancer, reverse
proxy, etc. but how the client certificate metadata is securely
communicated to the backend is out of scope per WGLC feedback
o Note that revocation checking is at the discretion of the AS per
WGLC feedback
o Editorial updates and clarifications
o Update draft-ietf-oauth-discovery reference to -10 and draft-ietf-
oauth-token-binding to -06
o Add folks involved in WGLC feedback to the acknowledgements list
draft-ietf-oauth-mtls-07
o Update to use the boilerplate from RFC 8174
draft-ietf-oauth-mtls-06
o Add an appendix section describing the relationship of this
document to OAuth Token Binding as requested during the Singapore
meeting https://datatracker.ietf.org/doc/minutes-100-oauth/
o Add an explicit note that the implicit flow is not supported for
obtaining certificate bound access tokens as discussed at the
Singapore meeting https://datatracker.ietf.org/doc/minutes-
100-oauth/
o Add/incorporate text to the Security Considerations on Certificate
Spoofing as suggested https://mailarchive.ietf.org/arch/msg/oauth/
V26070X-6OtbVSeUz_7W2k94vCo
o Changed the title to be more descriptive
o Move the Security Considerations section to before the IANA
Considerations
o Elaborated on certificate-bound access tokens a bit more in the
Abstract
o Update draft-ietf-oauth-discovery reference to -08
draft-ietf-oauth-mtls-05
o Editorial fixes
draft-ietf-oauth-mtls-04
o Change the name of the 'Public Key method' to the more accurate
'Self-Signed Certificate method' and also change the associated
authentication method metadata value to
"self_signed_tls_client_auth".
o Removed the "tls_client_auth_root_dn" client metadata field as
discussed in https://mailarchive.ietf.org/arch/msg/oauth/
swDV2y0be6o8czGKQi1eJV-g8qc
o Update draft-ietf-oauth-discovery reference to -07
o Clarify that MTLS client authentication isn't exclusive to the
token endpoint and can be used with other endpoints, e.g. RFC
7009 revocation and 7662 introspection, that utilize client
authentication as discussed in
https://mailarchive.ietf.org/arch/msg/oauth/
bZ6mft0G7D3ccebhOxnEYUv4puI
o Reorganize the document somewhat in an attempt to more clearly
make a distinction between mTLS client authentication and
certificate-bound access tokens as well as a more clear
delineation between the two (PKI/Public key) methods for client
authentication
o Editorial fixes and clarifications
draft-ietf-oauth-mtls-03
o Introduced metadata and client registration parameter to publish
and request support for mutual TLS sender constrained access
tokens
o Added description of two methods of binding the cert and client,
PKI and Public Key.
o Indicated that the "tls_client_auth" authentication method is for
the PKI method and introduced "pub_key_tls_client_auth" for the
Public Key method
o Added implementation considerations, mainly regarding TLS stack
configuration and trust chain validation, as well as how to to do
binding of access tokens to a TLS client certificate for public
clients, and considerations around certificate-bound access tokens
o Added new section to security considerations on cert spoofing
o Add text suggesting that a new cnf member be defined in the
future, if hash function(s) other than SHA-256 need to be used for
certificate thumbprints
draft-ietf-oauth-mtls-02
o Fixed editorial issue https://mailarchive.ietf.org/arch/msg/oauth/
U46UMEh8XIOQnvXY9pHFq1MKPns
o Changed the title (hopefully "Mutual TLS Profile for OAuth 2.0" is
better than "Mutual TLS Profiles for OAuth Clients").
draft-ietf-oauth-mtls-01
o Added more explicit details of using RFC 7662 token introspection
with mutual TLS sender constrained access tokens.
o Added an IANA OAuth Token Introspection Response Registration
request for "cnf".
o Specify that tls_client_auth_subject_dn and
tls_client_auth_root_dn are RFC 4514 String Representation of
Distinguished Names.
o Changed tls_client_auth_issuer_dn to tls_client_auth_root_dn.
o Changed the text in the Section 3 to not be specific about using a
hash of the cert.
o Changed the abbreviated title to 'OAuth Mutual TLS' (previously
was the acronym MTLSPOC).
draft-ietf-oauth-mtls-00
o Created the initial working group version from draft-campbell-
oauth-mtls
draft-campbell-oauth-mtls-01
o Fix some typos.
o Add to the acknowledgements list.
draft-campbell-oauth-mtls-00
o Add a Mutual TLS sender constrained protected resource access
method and a x5t#S256 cnf method for JWT access tokens (concepts
taken in part from draft-sakimura-oauth-jpop-04).
o Fixed "token_endpoint_auth_methods_supported" to
"token_endpoint_auth_method" for client metadata.
o Add "tls_client_auth_subject_dn" and "tls_client_auth_issuer_dn"
client metadata parameters and mention using "jwks_uri" or "jwks".
o Say that the authentication method is determined by client policy
regardless of whether the client was dynamically registered or
statically configured.
o Expand acknowledgements to those that participated in discussions
around draft-campbell-oauth-tls-client-auth-00
o Add Nat Sakimura and Torsten Lodderstedt to the author list.
draft-campbell-oauth-tls-client-auth-00
o Initial draft.
Authors' Addresses Authors' Addresses
Brian Campbell Brian Campbell
Ping Identity Ping Identity
Email: brian.d.campbell@gmail.com Email: brian.d.campbell@gmail.com
John Bradley John Bradley
Yubico Yubico
Email: ve7jtb@ve7jtb.com Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/ URI: http://www.thread-safe.com/
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