Re: [OAUTH-WG] [EXTERNAL] Re: Mix-Up Revisited

[Karsten Meyer zu Selhausen <[e-mail]>]

Hi all,


I think we all agree that proper countermeasures of mix-up attacks
should definitely be part of the BCP and 2.1 due to the severe impact
successful mix-up attacks have.

Theoretically speaking every client which supports multiple AS is
potentially vulnerable to mix-up attacks. While in practice it might
seem unlikely it is possible that one of the honest AS the client
supports gets compromised and can be utilized for a mix-up attack. 


In a previous thread of last November
(https://mailarchive.ietf.org/arch/msg/oauth/A7F5xSEa8DdfxHKWHW3Mqol_a4A/)
I discussed why “use distinct redirect URIs for each AS” is not an
effective countermeasure against mix-up attacks (if dynamic client
registration is used). I would argue that this is especially important
for mobile applications, SPAs, and other native applications because it
is very common for them to use dynamic client registration. Many iOS
applications now must support multiple AS since Apple forces the support
for “Sign in with Apple” if any single sign-on provider is supported.
This policy makes mix-up attacks applicable.


I fully agree with Daniel, Brian, and Mike that it is important to
define and register the “iss” parameter properly to ensure that both
clients and AS understand and use it in the correct way. I understand
that OAuth 2.1 is not meant to introduce and define new parameters but I
strongly suggest that the “iss” parameter should be introduced and
defined elsewhere so that it can be natively included in 2.1. In other
words the “iss” parameter should be integrated in 2.1 the same way PKCE
is: the initial definition of the parameter(s) is in another document
(RFC 7636 in the case of PKCE) and then included in 2.1.


What do you think would be the best place to define and register the
“iss” parameter? Would it be worthwhile to reactivate and update
“draft-ietf-oauth-mix-up-mitigation”?


Best regards,Karsten

On 18.06.2020 22:49, Mike Jones wrote:
>
> I support documenting the use of the issuer to mitigate mix-up
> attacks.  Note that while issuer was first defined by OpenID Connect,
> it became art of OAuth 2.0 in RFC 8414 - OAuth 2.0 Authorization
> Server Metadata.
>
>  
>
>                                                        -- Mike
>
>  
>
> *From:* OAuth <[e-mail]> *On Behalf Of * Brian Campbell
> *Sent:* Thursday, June 18, 2020 1:32 PM
> *To:* Daniel Fett <[e-mail]>
> *Cc:* oauth <[e-mail]>
> *Subject:* [EXTERNAL] Re: [OAUTH-WG] Mix-Up Revisited
>
>  
>
> 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 <[e-mail]
> <mailto:[e-mail]>> 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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>
>      
>
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-- 
Karsten Meyer zu Selhausen
IT Security Consultant
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