[OpenID] Abusing Authentication Failure messages

[Andrew Arnott]

I don't consider this a flaw at all.  In fact you can solve it yourself
today, in total compliance with the OpenID spec.

Use the return_to parameter to add whatever unique ID you wish was there.
 Then in your logs, you'll see the ID on the outgoing request (if you log
it) and on the response (via the incoming HTTP indirect message).

Problem solved, right?  It's totally up to the RP (you) to leverage this
hidden "feature" of ways to use return_to.

In particular, DotNetOpenAuth utilizes return_to when its optional feature
is activated that disables "unsolicited assertions", which guarantees that
each authentication response starts with an authentication request that
originated at the RP.  We had a signed nonce from the RP to the return_to,
and verify it when it comes back.  Again, this is only if you wanted to
disable unsolicited assertions deliberately for some reason.

--
Andrew Arnott
"I [may] not agree with what you have to say, but I'll defend to the death
your right to say it." - S. G. Tallentyre


On Fri, Aug 20, 2010 at 2:12 AM, Maarten Billemont <lhunath at gmail.com>wrote:

> It has always bothered me that the OpenID protocol does not assign a unique
> ID to requests that is required to be repeated in the request's response.
>  This makes it annoying to cleanly identify whether a response matches a
> particular request; instead relying on chronology (response Y comes after
> request X?  It must be a response to X!).
>
> This is an annoyance, yet, in the case of successful authentication
> responses, not a disaster: The response can be signed and verified.
>
> In the case of a failure response, however, there is a significant lack of
> useful information passed from the OP to the RP.  This lack of useful
> information makes it impossible for the RP to verify that the response did
> originate from the OP.  Unfortunately for the RP, (and conveniently for an
> attacker), the response is sent via indirect communication (via the User
> Agent); so the RP has no clue whatsoever where the response came from or who
> crafted it.  There is no association handle, no signature, no nonce, no
> nothing.
>
> Does that mean that to sabotage an OpenID site's authentication process,
> all I have to do is craft a website which, when opened by the user in a
> separate tab, continuously makes requests to the RP providing authentication
> failure responses?
>
> Am I missing something here, or is the OpenID protocol really so flawed?
>  And if it is, can I expect anyone to fix the protocol any time soon?
>
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