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.