[Openid-specs-ab] FW: OpenID Connection session management: determining which RP is initiating the logout

Mike Jones Michael.Jones at microsoft.com
Wed Nov 20 02:32:48 UTC 2013

A Microsoft developer asked this question.  Thoughts?

                I've been reviewing our implementation of OpenID Connect session management.  In the RP-initiated logout mechanism, the STS is required to validate that, if the post_logout_redirect_uri is supplied, that it match one of the values registered for that RP.  However, what's not clear from the OpenID Connect Session Management spec is how the STS can determine which RP sent the logout request (so the STS can determine which RP's info to use to validate the post_logout_redirect_uri).  There does not seem to be any required parameter in the logout request that would specify the RP sending it.  The closest is the id_token_hint (i.e., the STS can look inside the token to see its audience), but the id_token_hint is recommended, not required (and really seems intended for determining the user identity, not the RP identity).
                What is the expected means by which an STS can determine which RP is initiating the logout?

5.  RP-Initiated Logout
An RP can notify the OP that the End-User has logged out of the site, and might want to log out of the OP as well. In this case, the RP, after having logged the End-User out of the RP, redirects the End-User's User-Agent to the OP's logout endpoint URL. This URL is normally obtained via the end_session_endpoint element of the OP's Discovery response, or MAY be learned via other mechanisms.
This specification also defines the following parameters that are passed as query parameters in the logout request:
RECOMMENDED. Previously issued ID Token passed to the logout endpoint as a hint about the End-User's current authenticated session with the Client. This is used as an indication of the identity of the End-User that the RP is requesting be logged out by the OP. The OP need not be listed as an audience of the ID Token when it is used as an id_token_hint value.
OPTIONAL. URL to which the RP is requesting that the End-User's User-Agent be redirected after a logout has been performed. The value MUST have been previously registered with the OP, either using the post_logout_redirect_uris Registration parameter or via another mechanism. If supplied, the OP SHOULD honor this request following the logout.

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