[OAUTH-WG] Client assertions to endpoints other than the token endpoint

[Dave Tonge <[e-mail]>]

Dear OAuth WG

We have an issue that we are discussing in the OIDF MODRNA work group
relating to the Client Initiated Back Authentication spec (which is an
OAuth 2 extension). As the issue affects the wider OAuth ecosystem we
wanted to post it here and gain feedback from the OAuth Working Group.

Full details of the issue are here:
https://bitbucket.org/openid/mobile/issues/155/aud-to-use-in-client_assertion-passed-to
(including
a helpful context setting by Brian), but the summary is:

*What audience value should a Client use when using a client assertion
(RFC7521) to authenticate at an endpoint other than the token endpoint?*

The three options we have are:
1. the token endpoint (as RFC7521 says)
2. the endpoint the assertion is being sent to (e.g. revocation,
backchannel)
3. the issuer

We are leaning towards requiring the Authorization Server to accept any of
the above values, but recommending that the Client use the issuer value.

The reasons for this are:
1. All of the above values are arguably valid, so in the interest of
interoperability the AS should accept them all.
2. We see no clear security benefit to requiring the audience to be the
value of the endpoint the assertion is being sent to, and therefore think
that the issuer value is the one we should recommend that clients use.

We would be grateful for your feedback on this issue and believe it would
be in the best interest of the ecosystem for there to be a consistent
approach to this across OAuth 2 extensions and profiles.

Thanks

Dave Tonge