Subject: [OAUTH-WG] Mix-Up Revisited

From: Daniel Fett <[email-masked]>
Date: Mon, 4 May 2020 19:34:56 +0200

Hi all,

to make substantiated recommendations for FAPI 2.0, the security
considerations for PAR, and the security BCP, I did another analysis on
the threats that arise from mix-up attacks. I was interested in
particular in two questions:

  * Does PAR help preventing mix-up attacks?
  * Do we need JARM to prevent mix-up attacks?

I wrote down several attack variants and configurations in the following
document: https://danielfett.github.io/notes/oauth/Mix-Up%20Revisited.html

The key takeaways are:

 1. The security BCP needs to make clear that per-*AS* redirect URIs are
    only sufficient if OAuth Metadata is not used to resolve multiple
    issuers. Otherwise, per-*Issuer* redirect URIs or the iss parameter
    MUST be used.
 2. PAR-enabled authorization servers can protect the integrity better
    and protect against Mix-Up Attacks better if they ONLY accept PAR
    requests.
 3. We should emphasize the importance of the iss parameter (or issuer)
    in the authorization response. Maybe introduce this parameter in the
    security BCP or another document?
 4. Sender-constrained access tokens help against mix-up attacks when
    the access token is targeted.
 5. Sender-constraining the authorization code (PAR + PAR-DPoP?) might
    be worth looking into.

I would like to hear your thoughts!

-Daniel