[Openid-specs-ab] OpenID Connect Logout using HTTP GET

Thomas Broyer t.broyer at gmail.com
Sat Feb 21 12:52:08 UTC 2015

Could you please add "Security Considerations" section?

IMO, "sid" should be required, and RPs MUST validate the "sid" claim and
not only rely on the sent credentials (generally a cookie or set of
cookies). Failing to do that leaves the door open to CSRF logging out users
from third-party RPs (possibly a competitor). Sure the impact is "limited",
in the sense those RPs could probably log the user in transparently the
next time they come in (that depends on the OP though, possibly some user
configuration at the OP; or the RP if they use the 'prompt' parameter), but
it's still a threat (the attacker could log you out every few minutes, and
you' likely never blame them, but rather the victim RP).

Also, quite obviously the "sid" claim should be kept as private as possible
(see my previous mails in this thread); but as it's returned in the
id_token and that one is sent as id_token_hint to authorization_endpoints
and end_session_endpoints, or even right to the redirect_uri… Or maybe the
OP SHOULD "revoke" the "sid" in this case (and issue a new one to the
redirect_uri when received at the authorization_endpoint). But there's
still a "risk" if the RP doesn't use HTTPS (redirection to the OP with the
id_token_hint could be easily intercepted; or to the RP when using an
id_token response_type)
It should also be "sufficiently unique" as to not be discoverable (or
subject to brute force; see CSRF attack scenario above).

IMO, the logout_uri should receive some crypto material (signature or
encryption) he can verify to validate that the caller is the OP, and
that parameter
should have a short validity timeframe and/or include some sort of "nonce"
to prevent replay attacks. It could just be a JWT (JWS/JWE) including the
"sid" you propose as a claim, along with "iat", "exp", and/or a "jti".
If this has been rejected for any reason, please make it public somewhere
(and ideally even add it in an appendix). Explaining the "why" is almost as
important as describing the "how" of the settled-on solution in any spec
(why that thing is a MUST, or a SHOULD, why has this been designed that way
and not another, etc.)

On Sat Feb 21 2015 at 01:38:02 Mike Jones <Michael.Jones at microsoft.com>

>  A third iteration of the proposed OpenID Connect spec on logout using
> HTTP GET is attached.  (It’s now a two-pager.) This incorporates the
> results of the useful discussion on Thursday’s call.  Keep those cards and
> letters coming!
> Changes were:
> ·         Replaced the optional id_token parameter with an optional sid
> (Session ID) parameter.
> ·         Enabled the use of iframes with nested images or iframes to
> achieve downstream logouts.
>                                                             -- Mike
>  _______________________________________________
> Openid-specs-ab mailing list
> Openid-specs-ab at lists.openid.net
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