Subject: [OAUTH-WG] Multi-AS State Re-Use

From: Daniel Fett <[email-masked]>
Date: Fri, 22 Apr 2016 16:23:28 +0200

Hi all,

Besides the state leakage attack we found that another important fact
regarding state is underspecified: Each state value should only be
used for one run of the protocol, in particular, each AS should see a
different state in multi-AS settings. Clients might be tempted to
generate state once and then re-use each time a user wants to
authorize.

If state is re-used, given a setup where one Client allows users to
authorize using two AS, a potentially malicious AS learns the state
value that is valid for authorization at an honest AS. I.e., each AS
can mount a CSRF attack on the user using the other AS.

Just as the attack in the other mail, this is not a big deal in
practice, but should be discussed somewhere.

Cheers,
Daniel, Guido, and Ralf
-- 
Informationssicherheit und Kryptografie
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