Internet Engineering Task Force (IETF) B. Campbell
Request for Comments: 8705 Ping Identity
Category: Standards Track J. Bradley
ISSN: 2070-1721 Yubico
N. Sakimura
Nomura Research Institute
T. Lodderstedt
YES.com AG
February 2020
OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
Access Tokens
Abstract
This document describes OAuth client authentication and certificate-
bound access and refresh tokens using mutual Transport Layer Security
(TLS) authentication with X.509 certificates. OAuth clients are
provided a mechanism for authentication to the authorization server
using mutual TLS, based on either self-signed certificates or public
key infrastructure (PKI). OAuth authorization servers are provided a
mechanism for binding access tokens to a client's mutual-TLS
certificate, and OAuth protected resources are provided a method for
ensuring that such an access token presented to it was issued to the
client presenting the token.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
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 (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Notation and Conventions
1.2. Terminology
2. Mutual TLS for OAuth Client Authentication
2.1. PKI Mutual-TLS Method
2.1.1. PKI Method Metadata Value
2.1.2. Client Registration Metadata
2.2. Self-Signed Certificate Mutual-TLS Method
2.2.1. Self-Signed Method Metadata Value
2.2.2. Client Registration Metadata
3. Mutual-TLS Client Certificate-Bound Access Tokens
3.1. JWT Certificate Thumbprint Confirmation Method
3.2. Confirmation Method for Token Introspection
3.3. Authorization Server Metadata
3.4. Client Registration Metadata
4. Public Clients and Certificate-Bound Tokens
5. Metadata for Mutual-TLS Endpoint Aliases
6. Implementation Considerations
6.1. Authorization Server
6.2. Resource Server
6.3. Certificate Expiration and Bound Access Tokens
6.4. Implicit Grant Unsupported
6.5. TLS Termination
7. Security Considerations
7.1. Certificate-Bound Refresh Tokens
7.2. Certificate Thumbprint Binding
7.3. TLS Versions and Best Practices
7.4. X.509 Certificate Spoofing
7.5. X.509 Certificate Parsing and Validation Complexity
8. Privacy Considerations
9. IANA Considerations
9.1. JWT Confirmation Methods Registration
9.2. Authorization Server Metadata Registration
9.3. Token Endpoint Authentication Method Registration
9.4. Token Introspection Response Registration
9.5. Dynamic Client Registration Metadata Registration
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Example "cnf" Claim, Certificate, and JWK
Appendix B. Relationship to Token Binding
Acknowledgements
Authors' Addresses
1. Introduction
The OAuth 2.0 Authorization Framework [RFC6749] enables third-party
client applications to obtain delegated access to protected
resources. In the prototypical abstract OAuth flow, illustrated in
Figure 1, the client obtains an access token from an entity known as
an authorization server and then uses that token when accessing
protected resources, such as HTTPS APIs.
+--------+ +---------------+
| | | |
| |<--(A)-- Get an access token --->| Authorization |
| | | Server |
| | | |
| | +---------------+
| | ^
| | |
| |
| | (C) |
| Client | Validate the
| | access token |
| |
| | |
| | v
| | +---------------+
| | | (C) |
| | | |
| |<--(B)-- Use the access token -->| Protected |
| | | Resource |
| | | |
+--------+ +---------------+
Figure 1: Abstract OAuth 2.0 Protocol Flow
The flow illustrated in Figure 1 includes the following steps:
(A) The client makes an HTTPS "POST" request to the authorization
server and presents a credential representing the authorization
grant. For certain types of clients (those that have been
issued or otherwise established a set of client credentials) the
request must be authenticated. In the response, the
authorization server issues an access token to the client.
(B) The client includes the access token when making a request to
access a protected resource.
(C) The protected resource validates the access token in order to
authorize the request. In some cases, such as when the token is
self-contained and cryptographically secured, the validation can
be done locally by the protected resource. Other cases require
that the protected resource call out to the authorization server
to determine the state of the token and obtain metainformation
about it.
Layering on the abstract flow above, this document standardizes
enhanced security options for OAuth 2.0 utilizing client-certificate-
based mutual TLS. Section 2 provides options for authenticating the
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
remote processing in Sections 3.1 and 3.2, respectively. This
ensures that, as described in Section 3, protected resource access in
Step (B) is only possible by the legitimate client using a
certificate-bound token and holding the private key corresponding to
the certificate.
OAuth 2.0 defines a shared-secret method of client authentication but
also allows for defining and using additional client authentication
mechanisms when interacting directly with the authorization server.
This document describes an additional mechanism of client
authentication utilizing mutual-TLS certificate-based authentication
that provides better security characteristics than shared secrets.
While [RFC6749] documents client authentication for requests to the
token endpoint, extensions to OAuth 2.0 (such as Introspection
[RFC7662], Revocation [RFC7009], and the Backchannel Authentication
Endpoint in [OpenID.CIBA]) define endpoints that also utilize client
authentication, and the mutual-TLS methods defined herein are
applicable to those endpoints as well.
Mutual-TLS certificate-bound access tokens ensure that only the party
in possession of the private key corresponding to the certificate can
utilize the token to access the associated resources. Such a
constraint is sometimes referred to as key confirmation, proof-of-
possession, or holder-of-key and is unlike the case of the bearer
token described in [RFC6750], where any party in possession of the
access token can use it to access the associated resources. Binding
an access token to the client's certificate prevents the use of
stolen access tokens or replay of access tokens by unauthorized
parties.
Mutual-TLS certificate-bound access tokens and mutual-TLS client
authentication are distinct mechanisms that are complementary but
don't necessarily need to be deployed or used together.
Additional client metadata parameters are introduced by this document
in support of certificate-bound access tokens and mutual-TLS client
authentication. The authorization server can obtain client metadata
via the Dynamic Client Registration Protocol [RFC7591], which defines
mechanisms for dynamically registering OAuth 2.0 client metadata with
authorization servers. Also the metadata defined by [RFC7591], and
registered extensions to it, imply a general data model for clients
that is useful for authorization server implementations, even when
the Dynamic Client Registration Protocol isn't in play. Such
implementations will typically have some sort of user interface
available for managing client configuration.
1.1. Requirements Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
Throughout this document the term "mutual TLS" refers to the process
whereby, in addition to the normal TLS server authentication with a
certificate, a client presents its X.509 certificate and proves
possession of the corresponding private key to a server when
negotiating a TLS session. In contemporary versions of TLS [RFC5246]
[RFC8446], this requires that the client send the Certificate and
CertificateVerify messages during the handshake and for the server to
verify the CertificateVerify and Finished messages.
2. Mutual TLS for OAuth Client Authentication
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
certificates as client credentials. The requirement of mutual TLS
for client authentication is determined by the authorization server,
based on policy or configuration for the given client (regardless of
whether the client was dynamically registered, statically configured,
or otherwise established).
In order to utilize TLS for OAuth client authentication, the TLS
connection between the client and the authorization server MUST have
been established or re-established with mutual-TLS X.509 certificate
authentication (i.e., the client Certificate and CertificateVerify
messages are sent during the TLS handshake).
For all requests to the authorization server utilizing mutual-TLS
client authentication, the client MUST include the "client_id"
parameter described in Section 2.2 of OAuth 2.0 [RFC6749]. The
presence of the "client_id" parameter enables the authorization
server to easily identify the client independently from the content
of the certificate. The authorization server can locate the client
configuration using the client identifier and check the certificate
presented in the TLS handshake against the expected credentials for
that client. The authorization server MUST enforce the binding
between client and certificate, as described in either Section 2.1 or
2.2 below. If no certificate is presented, or that which is
presented doesn't match that which is expected for the given
"client_id", the authorization server returns a normal OAuth 2.0
error response per Section 5.2 of [RFC6749] with the "invalid_client"
error code to indicate failed client authentication.
2.1. PKI Mutual-TLS Method
The PKI (public key infrastructure) method of mutual-TLS OAuth client
authentication adheres to the way in which X.509 certificates are
traditionally used for authentication. It relies on a validated
certificate chain [RFC5280] and a single subject distinguished name
(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. The TLS handshake is utilized to validate the client's
possession of the private key corresponding to the public key in the
certificate and to validate the corresponding certificate chain. The
client is successfully authenticated if the subject information in
the certificate matches the single expected subject configured or
registered for that particular client (note that a predictable
treatment of DN values, such as the distinguishedNameMatch rule from
[RFC4517], is needed in comparing the certificate's subject DN to the
client's registered DN). Revocation checking is possible with the
PKI method but if and how to check a certificate's revocation status
is a deployment decision at the discretion of the authorization
server. Clients can rotate their X.509 certificates without the need
to modify the respective authentication data at the authorization
server by obtaining a new certificate with the same subject from a
trusted certificate authority (CA).
2.1.1. PKI Method Metadata Value
For the PKI method of mutual-TLS client authentication, this
specification defines and registers the following authentication
method metadata value into the "OAuth Token Endpoint Authentication
Methods" registry [IANA.OAuth.Parameters].
tls_client_auth
Indicates that client authentication to the authorization server
will occur with mutual TLS utilizing the PKI method of associating
a certificate to a client.
2.1.2. Client Registration Metadata
In order to convey the expected subject of the certificate, the
following metadata parameters are introduced for the OAuth 2.0
Dynamic Client Registration Protocol [RFC7591] in support of the PKI
method of mutual-TLS client authentication. A client using the
"tls_client_auth" authentication method MUST use exactly one of the
below metadata parameters to indicate the certificate subject value
that the authorization server is to expect when authenticating the
respective client.
tls_client_auth_subject_dn
A string representation -- as defined in [RFC4514] -- of the
expected subject distinguished name of the certificate that the
OAuth client will use in mutual-TLS authentication.
tls_client_auth_san_dns
A string containing the value of an expected dNSName SAN entry in
the certificate that the OAuth client will use in mutual-TLS
authentication.
tls_client_auth_san_uri
A string containing the value of an expected
uniformResourceIdentifier SAN entry in the certificate that the
OAuth client will use in mutual-TLS authentication.
tls_client_auth_san_ip
A string representation of an IP address in either dotted decimal
notation (for IPv4) or colon-delimited hexadecimal (for IPv6, as
defined in [RFC5952]) that is expected to be present as an
iPAddress SAN entry in the certificate that the OAuth client will
use in mutual-TLS authentication. Per Section 8 of [RFC5952], the
IP address comparison of the value in this parameter and the SAN
entry in the certificate is to be done in binary format.
tls_client_auth_san_email
A string containing the value of an expected rfc822Name SAN entry
in the certificate that the OAuth client will use in mutual-TLS
authentication.
2.2. Self-Signed Certificate Mutual-TLS Method
This method of mutual-TLS OAuth client authentication is intended to
support client authentication using self-signed certificates. As a
prerequisite, the client registers its X.509 certificates (using
"jwks" defined in [RFC7591]) or a reference to a trusted source for
its X.509 certificates (using "jwks_uri" from [RFC7591]) with the
authorization server. During authentication, TLS is utilized to
validate the client's possession of the private key corresponding to
the public key presented within the certificate in the respective TLS
handshake. In contrast to the PKI method, the client's certificate
chain is not validated by the server in this case. The client is
successfully authenticated if the certificate that it presented
during the handshake matches one of the certificates configured or
registered for that particular client. The Self-Signed Certificate
method allows the use of mutual TLS to authenticate clients without
the need to maintain a PKI. When used in conjunction with a
"jwks_uri" for the client, it also allows the client to rotate its
X.509 certificates without the need to change its respective
authentication data directly with the authorization server.
2.2.1. Self-Signed Method Metadata Value
For the Self-Signed Certificate method of mutual-TLS client
authentication, this specification defines and registers the
following authentication method metadata value into the "OAuth Token
Endpoint Authentication Methods" registry [IANA.OAuth.Parameters].
self_signed_tls_client_auth
Indicates that client authentication to the authorization server
will occur using mutual TLS with the client utilizing a self-
signed certificate.
2.2.2. Client Registration Metadata
For the Self-Signed Certificate method of binding a certificate with
a client using mutual-TLS client authentication, the existing
"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
[RFC7517]. The "jwks" metadata parameter is a JWK Set containing the
client's public keys as an array of JWKs, while the "jwks_uri"
parameter is a URL that references a client's JWK Set. A certificate
is represented with the "x5c" parameter of an individual JWK within
the set. Note that the members of the JWK representing the public
key (e.g., "n" and "e" for RSA, "x" and "y" for Elliptic Curve (EC))
are required parameters per [RFC7518] so will be present even though
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"
parameter match the public key represented by those other members of
the JWK.
3. Mutual-TLS Client Certificate-Bound Access Tokens
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
token to the client certificate. Such a binding is accomplished by
associating the certificate with the token in a way that can be
accessed by the protected resource, such as embedding the certificate
hash in the issued access token directly, using the syntax described
in Section 3.1, or through token introspection as described in
Section 3.2. Binding the access token to the client certificate in
that fashion has the benefit of decoupling that binding from the
client's authentication with the authorization server, which enables
mutual TLS during protected resource access to serve purely as a
proof-of-possession mechanism. Other methods of associating a
certificate with an access token are possible, per agreement by the
authorization server and the protected resource, but are beyond the
scope of this specification.
In order for a resource server to use certificate-bound access
tokens, it must have advance knowledge that mutual TLS is to be used
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
request mutual TLS because (from the TLS perspective) it is
"Application Data", only exchanged after the TLS handshake has been
completed, and the initial CertificateRequest occurs during the
handshake, before the Application Data is available. Although
subsequent opportunities for a TLS client to present a certificate
may be available, e.g., via TLS 1.2 renegotiation [RFC5246] or TLS
1.3 post-handshake authentication [RFC8446], this document makes no
provision for their usage. It is expected to be common that a
mutual-TLS-using resource server will require mutual TLS for all
resources hosted thereupon or will serve mutual-TLS-protected and
regular resources on separate hostname and port combinations, though
other workflows are possible. How resource server policy is
synchronized with the authorization server (AS) is out of scope for
this document.
Within the scope of a mutual-TLS-protected resource-access flow, the
client makes protected resource requests, as described in [RFC6750],
however, those requests MUST be made over a mutually authenticated
TLS connection using the same certificate that was used for mutual
TLS at the token endpoint.
The protected resource MUST obtain, from its TLS implementation
layer, the client certificate used for mutual TLS and MUST verify
that the certificate matches the certificate associated with the
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 and the "invalid_token" error code.
Metadata to convey server and client capabilities for mutual-TLS
client certificate-bound access tokens is defined in Sections 3.3 and
3.4, respectively.
3.1. JWT Certificate Thumbprint Confirmation Method
When access tokens are represented as JSON Web Tokens (JWT)
[RFC7519], the certificate hash information SHOULD be represented
using the "x5t#S256" confirmation method member defined herein.
To represent the hash of a certificate in a JWT, this specification
defines the new JWT Confirmation Method [RFC7800] member "x5t#S256"
for the X.509 Certificate SHA-256 Thumbprint. The value of the
"x5t#S256" member is a base64url-encoded [RFC4648] SHA-256 [SHS] hash
(a.k.a., thumbprint, fingerprint, or digest) of the DER encoding
[X690] of the X.509 certificate [RFC5280]. The base64url-encoded
value MUST omit all trailing pad '=' characters and MUST NOT include
any line breaks, whitespace, or other additional characters.
The following is an example of a JWT payload containing an "x5t#S256"
certificate thumbprint confirmation method. The new JWT content
introduced by this specification is the "cnf" confirmation method
claim at the bottom of the example that has the "x5t#S256"
confirmation method member containing the value that is the hash of
the client certificate to which the access token is bound.
{
"iss": "https://server.example.com",
"sub": "ty.webb@example.com",
"exp": 1493726400,
"nbf": 1493722800,
"cnf":{
"x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
}
}
Figure 2: Example JWT Claims Set with an X.509 Certificate Thumbprint
Confirmation Method
3.2. Confirmation Method for Token Introspection
OAuth 2.0 Token Introspection [RFC7662] defines a method for a
protected resource to query an authorization server about the active
state of an access token as well as to determine metainformation
about the token.
For a mutual-TLS client certificate-bound access token, the hash of
the certificate to which the token is bound is conveyed to the
protected resource as metainformation in a token introspection
response. The hash is conveyed using the same "cnf" with "x5t#S256"
member structure as the certificate SHA-256 thumbprint confirmation
method, described in Section 3.1, as a top-level member of the
introspection response JSON. The protected resource compares that
certificate hash to a hash of the client certificate used for mutual-
TLS authentication and rejects the request if they do not match.
The following is an example of an introspection response for an
active token with an "x5t#S256" certificate thumbprint confirmation
method. The new introspection response content introduced by this
specification is the "cnf" confirmation method at the bottom of the
example that has the "x5t#S256" confirmation method member containing
the value that is the hash of the client certificate to which the
access token is bound.
HTTP/1.1 200 OK
Content-Type: application/json
{
"active": true,
"iss": "https://server.example.com",
"sub": "ty.webb@example.com",
"exp": 1493726400,
"nbf": 1493722800,
"cnf":{
"x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
}
}
Figure 3: Example Introspection Response for a Certificate-Bound
Access Token
3.3. Authorization Server Metadata
This document introduces the following new authorization server
metadata [RFC8414] parameter to signal the server's capability to
issue certificate-bound access tokens:
tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value indicating server support for mutual-TLS
client certificate-bound access tokens. If omitted, the default
value is "false".
3.4. Client Registration Metadata
The following new client metadata parameter is introduced to convey
the client's intention to use certificate-bound access tokens:
tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value used to indicate the client's intention
to use mutual-TLS client certificate-bound access tokens. If
omitted, the default value is "false".
Note that if a client that has indicated the intention to use mutual-
TLS client certificate-bound tokens makes a request to the token
endpoint over a non-mutual-TLS connection, it is at the authorization
server's discretion as to whether to return an error or issue an
unbound token.
4. Public Clients and Certificate-Bound Tokens
Mutual-TLS OAuth client authentication and certificate-bound access
tokens can be used independently of each other. Use of certificate-
bound access tokens without mutual-TLS OAuth client authentication,
for example, is possible in support of binding access tokens to a TLS
client certificate for public clients (those without authentication
credentials associated with the "client_id"). The authorization
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
certificate presented by the client during the handshake is signed by
a trusted CA. Individual instances of a client would create a self-
signed certificate for mutual TLS with both the authorization server
and resource server. The authorization server would not use the
mutual-TLS certificate to authenticate the client at the OAuth layer
but would bind the issued access token to the certificate for which
the client has proven possession of the corresponding private key.
The access token is then bound to the certificate and can only be
used by the client possessing the certificate and corresponding
private key and utilizing them to negotiate mutual TLS on connections
to the resource server. When the authorization server issues a
refresh token to such a client, it SHOULD also bind the refresh token
to the respective certificate and check the binding when the refresh
token is presented to get new access tokens. The implementation
details of the binding of the refresh token are at the discretion of
the authorization server.
5. Metadata for Mutual-TLS Endpoint Aliases
The process of negotiating client certificate-based mutual TLS
involves a TLS server requesting a certificate from the TLS client
(the client does not provide one unsolicited). Although a server can
be configured such that client certificates are optional, meaning
that the connection is allowed to continue when the client does not
provide a certificate, the act of a server requesting a certificate
can result in undesirable behavior from some clients. This is
particularly true of web browsers as TLS clients, which will
typically present the end user with an intrusive certificate
selection interface when the server requests a certificate.
Authorization servers supporting both clients using mutual TLS and
conventional clients MAY chose to isolate the server side mutual-TLS
behavior to only clients intending to do mutual TLS, thus avoiding
any undesirable effects it might have on conventional clients. The
following authorization server metadata parameter is introduced to
facilitate such separation:
mtls_endpoint_aliases
OPTIONAL. A JSON object containing alternative authorization
server endpoints that, when present, an OAuth client intending to
do mutual TLS uses in preference to the conventional endpoints.
The parameter value itself consists of one or more endpoint
parameters, such as "token_endpoint", "revocation_endpoint",
"introspection_endpoint", etc., conventionally defined for the top
level of authorization server metadata. An OAuth client intending
to do mutual TLS (for OAuth client authentication and/or to
acquire or use certificate-bound tokens) when making a request
directly to the authorization server MUST use the alias URL of the
endpoint within the "mtls_endpoint_aliases", when present, in
preference to the endpoint URL of the same name at the top level
of metadata. When an endpoint is not present in
"mtls_endpoint_aliases", then the client uses the conventional
endpoint URL defined at the top level of the authorization server
metadata. Metadata parameters within "mtls_endpoint_aliases" that
do not define endpoints to which an OAuth client makes a direct
request have no meaning and SHOULD be ignored.
Below is an example of an authorization server metadata document with
the "mtls_endpoint_aliases" parameter, which indicates aliases for
the token, revocation, and introspection endpoints that an OAuth
client intending to do mutual TLS would use in preference to the
conventional token, revocation, and introspection endpoints. Note
that the endpoints in "mtls_endpoint_aliases" use a different host
than their conventional counterparts, which allows the authorization
server (via TLS "server_name" extension [RFC6066] or actual distinct
hosts) to differentiate its TLS behavior as appropriate.
{
"issuer": "https://server.example.com",
"authorization_endpoint": "https://server.example.com/authz",
"token_endpoint": "https://server.example.com/token",
"introspection_endpoint": "https://server.example.com/introspect",
"revocation_endpoint": "https://server.example.com/revo",
"jwks_uri": "https://server.example.com/jwks",
"response_types_supported": ["code"],
"response_modes_supported": ["fragment","query","form_post"],
"grant_types_supported": ["authorization_code", "refresh_token"],
"token_endpoint_auth_methods_supported":
["tls_client_auth","client_secret_basic","none"],
"tls_client_certificate_bound_access_tokens": true true,
"mtls_endpoint_aliases": {
"token_endpoint": "https://mtls.example.com/token",
"revocation_endpoint": "https://mtls.example.com/revo",
"introspection_endpoint": "https://mtls.example.com/introspect"
}
}
Figure 4: Example Authorization Server Metadata with Mutual-TLS
Endpoint Aliases
6. Implementation Considerations
6.1. Authorization Server
The authorization server needs to set up its TLS configuration
appropriately for the OAuth client authentication methods it
supports.
An authorization server that supports mutual-TLS client
authentication and other client authentication methods or public
clients in parallel would make mutual TLS optional (i.e., allowing a
handshake to continue after the server requests a client certificate
but the client does not send one).
In order to support the Self-Signed Certificate method alone, the
authorization server would configure the TLS stack in such a way that
it does not verify whether the certificate presented by the client
during the handshake is signed by a trusted CA certificate.
As described in Section 3, the authorization server binds the issued
access token to the TLS client certificate, which means that it will
only issue certificate-bound tokens for a certificate that the client
has proven possession of the corresponding private key.
The authorization server may also consider hosting the token endpoint
and other endpoints requiring client authentication on a separate
host name or port in order to prevent unintended impact on the TLS
behavior of its other endpoints, e.g., the authorization endpoint.
As described in Section 5, it may further isolate any potential
impact of the server requesting client certificates by offering a
distinct set of endpoints on a separate host or port, which are
aliases for the originals that a client intending to do mutual TLS
will use in preference to the conventional endpoints.
6.2. Resource Server
OAuth divides the roles and responsibilities such that the resource
server relies on the authorization server to perform client
authentication and obtain resource-owner (end-user) authorization.
The resource server makes authorization decisions based on the access
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 certificate and how a protected resource verifies the proof-
of-possession decouples that from the specific method that the client
used to authenticate with the authorization server. Mutual TLS
during protected resource access can, therefore, serve purely as a
proof-of-possession mechanism. As such, it is not necessary for the
resource server to validate the trust chain of the client's
certificate in any of the methods defined in this document. The
resource server would, therefore, configure the TLS stack in a way
that it does not verify whether the certificate presented by the
client during the handshake is signed by a trusted CA certificate.
6.3. Certificate Expiration and Bound Access Tokens
As described in Section 3, an access token is bound to a specific
client certificate, which means that the same certificate must be
used for mutual TLS on protected resource access. It also implies
that access tokens are invalidated when a client updates the
certificate, which can be handled similarly to expired access tokens
where the client requests a new access token (typically with a
refresh token) and retries the protected resource request.
6.4. Implicit Grant Unsupported
This document describes binding an access token to the client
certificate presented on the TLS connection from the client to the
authorization server's token endpoint, however, such binding of
access tokens issued directly from the authorization endpoint via the
implicit grant flow is explicitly out of scope. End users interact
directly with the authorization endpoint using a web browser, and the
use of client certificates in user's browsers bring operational and
usability issues that make it undesirable to support certificate-
bound access tokens issued in the implicit grant flow.
Implementations wanting to employ certificate-bound access tokens
should utilize grant types that involve the client making an access
token request directly to the token endpoint (e.g., the authorization
code and refresh token grant types).
6.5. TLS Termination
An authorization server or resource server MAY choose to terminate
TLS connections at a load balancer, reverse proxy, or other network
intermediary. How the client certificate metadata is securely
communicated between the intermediary and the application server, in
this case, is out of scope of this specification.
7. Security Considerations
7.1. Certificate-Bound Refresh Tokens
The OAuth 2.0 Authorization Framework [RFC6749] requires that an
authorization server (AS) bind refresh tokens to the client to which
they were issued and that confidential clients (those having
established authentication credentials with the AS) authenticate to
the AS when presenting a refresh token. As a result, refresh tokens
are indirectly certificate-bound by way of the client ID and the
associated requirement for (certificate-based) authentication to the
AS when issued to clients utilizing the "tls_client_auth" or
"self_signed_tls_client_auth" methods of client authentication.
Section 4 describes certificate-bound refresh tokens issued to public
clients (those without authentication credentials associated with the
"client_id").
7.2. Certificate Thumbprint Binding
The binding between the certificate and access token specified in
Section 3.1 uses a cryptographic hash of the certificate. It relies
on the hash function having sufficient second-preimage resistance so
as to make it computationally infeasible to find or create another
certificate that produces to the same hash output value. The SHA-256
hash function was used because it meets the aforementioned
requirement while being widely available. If, in the future,
certificate thumbprints need to be computed using hash function(s)
other than SHA-256, it is suggested that, for additional related JWT
confirmation methods, members be defined for that purpose and
registered in the IANA "JWT Confirmation Methods" registry
[IANA.JWT.Claims] for JWT "cnf" member values.
Community knowledge about the strength of various algorithms and
feasible attacks can change suddenly, and experience shows that a
document about security is a point-in-time statement. Readers are
advised to seek out any errata or updates that apply to this
document.
7.3. TLS Versions and Best Practices
This document is applicable with any TLS version supporting
certificate-based client authentication. Both TLS 1.3 [RFC8446] and
TLS 1.2 [RFC5246] are cited herein, because, at the time of writing,
1.3 is the newest version, while 1.2 is the most widely deployed.
General implementation and security considerations for TLS, including
version recommendations, can be found in [BCP195].
TLS certificate validation (for both client and server certificates)
requires a local database of trusted certificate authorities (CAs).
Decisions about what CAs to trust and how to make such a
determination of trust are out of scope for this document.
7.4. X.509 Certificate Spoofing
If the PKI method of client authentication is used, an attacker could
try to impersonate a client using a certificate with the same subject
(DN or SAN) but issued by a different CA that the authorization
server trusts. To cope with that threat, the authorization server
SHOULD only accept, as trust anchors, a limited number of CAs whose
certificate issuance policy meets its security requirements. There
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 certificate chain. Without this assumption the use of a subject
to identify the client certificate would open the server up to
certificate spoofing attacks.
7.5. X.509 Certificate Parsing and Validation Complexity
Parsing and validation of X.509 certificates and certificate chains
is complex, and implementation mistakes have previously exposed
security vulnerabilities. Complexities of validation include (but
are not limited to) [CX5P] [DCW] [RFC5280]:
* checking of basic constraints, basic and extended key usage
constraints, validity periods, and critical extensions;
* handling of embedded NUL bytes in ASN.1 counted-length strings and
non-canonical or non-normalized string representations in subject
names;
* handling of wildcard patterns in subject names;
* recursive verification of certificate chains and checking
certificate revocation.
For these reasons, implementors SHOULD use an established and well-
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
write their own X.509 certificate validation procedures.
8. Privacy Considerations
In TLS versions prior to 1.3, the client's certificate is sent
unencrypted in the initial handshake and can potentially be used by
third parties to monitor, track, and correlate client activity. This
is likely of little concern for clients that act on behalf of a
significant number of end users because individual user activity will
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
application on a mobile device, should use TLS version 1.3 whenever
possible or consider the potential privacy implications of using
mutual TLS on earlier versions.
9. IANA Considerations
9.1. JWT Confirmation Methods Registration
Per this specification, the following value has been registered in
the IANA "JWT Confirmation Methods" registry [IANA.JWT.Claims] for
JWT "cnf" member values established by [RFC7800].
Confirmation Method Value: "x5t#S256"
Confirmation Method Description: X.509 Certificate SHA-256
Thumbprint
Change Controller: IESG
Specification Document(s): Section 3.1 of RFC 8705
9.2. Authorization Server Metadata Registration
Per this specification, the following values have been registered in
the IANA "OAuth Authorization Server Metadata" registry
[IANA.OAuth.Parameters] established by [RFC8414].
Metadata Name: "tls_client_certificate_bound_access_tokens"
Metadata Description: Indicates authorization server support for
mutual-TLS client certificate-bound access tokens.
Change Controller: IESG
Specification Document(s): Section 3.3 of RFC 8705
Metadata Name: "mtls_endpoint_aliases"
Metadata Description: JSON object containing alternative
authorization server endpoints, which a client intending to do
mutual TLS will use in preference to the conventional endpoints.
Change Controller: IESG
Specification Document(s): Section 5 of RFC 8705
9.3. Token Endpoint Authentication Method Registration
Per this specification, the following values have been registered in
the IANA "OAuth Token Endpoint Authentication Methods" registry
[IANA.OAuth.Parameters] established by [RFC7591].
Token Endpoint Authentication Method Name: "tls_client_auth"
Change Controller: IESG
Specification Document(s): Section 2.1.1 of RFC 8705
Token Endpoint Authentication Method Name: "self_signed_tls_client_
auth"
Change Controller: IESG
Specification Document(s): Section 2.2.1 of RFC 8705
9.4. Token Introspection Response Registration
"Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)"
[RFC7800] defined the "cnf" (confirmation) claim that enables
confirmation key information to be carried in a JWT. However, the
same proof-of-possession semantics are also useful for introspected
access tokens whereby the protected resource obtains the confirmation
key data as metainformation of a token introspection response and
uses that information in verifying proof-of-possession. Therefore,
this specification defines and registers proof-of-possession
semantics for OAuth 2.0 Token Introspection [RFC7662] using the "cnf"
structure. When included as a top-level member of an OAuth token
introspection response, "cnf" has the same semantics and format as
the claim of the same name defined in [RFC7800]. While this
specification only explicitly uses the "x5t#S256" confirmation method
member (see Section 3.2), it needs to define and register the higher-
level "cnf" structure as an introspection response member in order to
define and use the more specific certificate thumbprint confirmation
method.
As such, the following values have been registered in the IANA "OAuth
Token Introspection Response" registry [IANA.OAuth.Parameters]
established by [RFC7662].
Claim Name: "cnf"
Claim Description: Confirmation
Change Controller: IESG
Specification Document(s): [RFC7800] and RFC 8705
9.5. Dynamic Client Registration Metadata Registration
Per this specification, the following client metadata definitions
have been registered in the IANA "OAuth Dynamic Client Registration
Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591]:
Client Metadata Name: "tls_client_certificate_bound_access_tokens"
Client Metadata Description: Indicates the client's intention to use
mutual-TLS client certificate-bound access tokens.
Change Controller: IESG
Specification Document(s): Section 3.4 of RFC 8705
Client Metadata Name: "tls_client_auth_subject_dn"
Client Metadata Description: String value specifying the expected
subject DN of the client certificate.
Change Controller: IESG
Specification Document(s): Section 2.1.2 of RFC 8705
Client Metadata Name: "tls_client_auth_san_dns"
Client Metadata Description: String value specifying the expected
dNSName SAN entry in the client certificate.
Change Controller: IESG
Specification Document(s): Section 2.1.2 of RFC 8705
Client Metadata Name: "tls_client_auth_san_uri"
Client Metadata Description: String value specifying the expected
uniformResourceIdentifier SAN entry in the client certificate.
Change Controller: IESG
Specification Document(s): Section 2.1.2 of RFC 8705
Client Metadata Name: "tls_client_auth_san_ip"
Client Metadata Description: String value specifying the expected
iPAddress SAN entry in the client certificate.
Change Controller: IESG
Specification Document(s): Section 2.1.2 of RFC 8705
Client Metadata Name: "tls_client_auth_san_email"
Client Metadata Description: String value specifying the expected
rfc822Name SAN entry in the client certificate.
Change Controller: IESG
Specification Document(s): Section 2.1.2 of RFC 8705
10. References
10.1. Normative References
[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<https://www.rfc-editor.org/info/bcp195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://www.rfc-editor.org/info/rfc4514>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[SHS] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[X690] ITU-T, "Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, August 2015.
10.2. Informative References
[CX5P] Wong, D., "Common x509 certificate validation/creation
pitfalls", September 2016,
<https://www.cryptologie.net/article/374/common-x509-
certificate-validationcreation-pitfalls>.
[DCW] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
D., and V. Shmatikov, "The Most Dangerous Code in the
World: Validating SSL Certificates in Non-Browser
Software", DOI 10.1145/2382196.2382204, October 2012,
<http://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf>.
[IANA.JWT.Claims]
IANA, "JSON Web Token Claims",
<https://www.iana.org/assignments/jwt>.
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.
[OpenID.CIBA]
Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B.
Campbell, "OpenID Connect Client Initiated Backchannel
Authentication Flow - Core 1.0", 16 January 2019,
<https://openid.net/specs/openid-client-initiated-
backchannel-authentication-core-1_0.html>.
[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517,
DOI 10.17487/RFC4517, June 2006,
<https://www.rfc-editor.org/info/rfc4517>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
August 2013, <https://www.rfc-editor.org/info/rfc7009>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[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>.
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,
Figures 5 and 6, respectively. A JWK representation of the
certificate's public key along with the "x5c" member is also provided
in Figure 7.
"cnf":{"x5t#S256":"A4DtL2JmUMhAsvJj5tKyn64SqzmuXbMrJa0n761y5v0"}
Figure 5: x5t#S256 Confirmation Claim
-----BEGIN CERTIFICATE-----
MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTAx
ODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGByqG
SM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZjjJ
/Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0EAwID
SQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2eXZOV
bUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY=
-----END CERTIFICATE-----
Figure 6: PEM Encoded Self-Signed Certificate
{
"kty":"EC",
"x":"1yfLHCpXqFjxCeHHHMVDTcLscpb07KUxudBmOMn8C7Q",
"y":"8_coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8",
"crv":"P-256",
"x5c":[
"MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTA
xODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGBy
qGSM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZ
jjJ/Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0E
AwIDSQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2
eXZOVbUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY="
]
}
Figure 7: JSON Web Key
Appendix B. Relationship to Token Binding
OAuth 2.0 Token Binding [TOKEN] enables the application of Token
Binding to the various artifacts and tokens employed throughout
OAuth. That includes binding of an access token to a Token Binding
key, which bears some similarities in motivation and design to the
mutual-TLS client certificate-bound access tokens defined in this
document. Both documents define what is often called a proof-of-
possession security mechanism for access tokens, whereby a client
must demonstrate possession of cryptographic keying material when
accessing a protected resource. The details differ somewhat between
the two documents but both have the authorization server bind 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
that pair with respect to the TLS connection over which the protected
resource is accessed.
Token Binding uses bare keys that are generated on the client, which
avoids many of the difficulties of creating, distributing, and
managing certificates used in this specification. However, at the
time of writing, Token Binding is fairly new, and there is relatively
little support for it in available application development platforms
and tooling. Until better support for the underlying core Token
Binding specifications exists, practical implementations of OAuth 2.0
Token Binding are infeasible. Mutual TLS, on the other hand, has
been around for some time and enjoys widespread support in web
servers and development platforms. As a consequence, OAuth 2.0
Mutual-TLS Client Authentication and Certificate-Bound Access Tokens
can be built and deployed now using existing platforms and tools. In
the future, the two specifications are likely to be deployed in
parallel for solving similar problems in different environments.
Authorization servers may even support both specifications
simultaneously using different proof-of-possession mechanisms for
tokens issued to different clients.
Acknowledgements
Scott "not Tomlinson" Tomilson and Matt Peterson were involved in
design and development work on a mutual-TLS OAuth client
authentication implementation that predates this document.
Experience and learning from that work informed some of the content
of this document.
This specification was developed within the OAuth Working Group under
the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with
Eric Rescorla, Benjamin Kaduk, and Roman Danyliw serving as Security
Area Directors. Additionally, the following individuals contributed
ideas, feedback, and wording that helped shape this specification:
Vittorio Bertocci, Sergey Beryozkin, Ralph Bragg, Sophie Bremer,
Roman Danyliw, Vladimir Dzhuvinov, Samuel Erdtman, Evan Gilman, Leif
Johansson, Michael Jones, Phil Hunt, Benjamin Kaduk, Takahiko
Kawasaki, Sean Leonard, Kepeng Li, Neil Madden, James Manger, Jim
Manico, Nov Matake, Sascha Preibisch, Eric Rescorla, Justin Richer,
Vincent Roca, Filip Skokan, Dave Tonge, and Hannes Tschofenig.
Authors' Addresses
Brian Campbell
Ping Identity
Email: brian.d.campbell@gmail.com
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Nat Sakimura
Nomura Research Institute
Email: n-sakimura@nri.co.jp
URI: https://nat.sakimura.org/
Torsten Lodderstedt
YES.com AG
Email: torsten@lodderstedt.net