In my (probably simplistic) understanding of things, the root underlying
issue that allows for mix-up in its variations is the lack of anything
identifying the AS in the authorization response. Following from that,
introducing and using an `iss` authorization response parameter has always
seemed like the most straightforward approach for mitigating the issue
(which was part of the draft-ietf-oauth-mix-up-mitigation but other
parameters were also included and, for reasons I'm not sure about, interest
in that work faded in favor of telling clients to use per AS redirect URIs)
.. Though for the `iss` authorization response parameter to be effective,
all parties involved need to know about it and act on it. So I think it'd
need to be something more than a passing recommendation in the BCP. It
should be defined, registered, explained, etc.. Actually introducing a new
parameter is maybe going beyond the expected scope of the BCP (or 2.1). But
maybe that's ok, if we're at least more intentional about it.
On Sun, Jun 7, 2020 at 7:53 AM Daniel Fett wrote:
> Hi all,
> I was wondering if we should move towards introducing and (more
> explicitly) recommending the iss parameter in the security BCP, for the
> reasons laid out below and in the article (which is now at
> https://danielfett.de/2020/05/04/mix-up-revisited/).
>
> Any thoughts on this?
>
> -Daniel
>
> Am 04.05.20 um 19:34 schrieb Daniel Fett:
>
> 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
>
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