Re: [OAUTH-WG] PAR and client metadata

[Brian Campbell <[e-mail]>]

On Wed, Apr 15, 2020 at 1:44 PM Richard Backman, Annabelle <richanna=
[e-mail]> wrote:

> As I think through this, I’m struggling to identify the threats this
> actually helps mitigate.
>
> A client metadata parameter implies that the value may be different for
> different clients, and that any given client won’t already know via other
> means whether or not it needs to use PAR. That means we’re only talking
> about dynamic clients since statically registered clients already have some
> proprietary out-of-band registration mechanism (e.g., a developer console).
>

As I tried to articulate in the original email and Filip also mentioned in
a different fork of this email thread, defining metadata can be beneficial
even when it's not used dynamically at runtime. So we're not only talking
about dynamic clients.



>
> A client metadata parameter also implies that the AS allows some clients
> to make non-PAR requests (otherwise it would be an AS metadata parameter).
>

A per client setting seems necessary for existing ASs to roll out PAR
support over time or just generally in support of diverse client
capabilities and requirements.



> If that’s the case then presumably a malicious party could register their
> own client that doesn’t use PAR.
>
> So it seems to me that the only scenario that this parameter would protect
> against is a malicious party impersonating a dynamically registered client
> that uses PAR. That wouldn’t apply to Mix-Up, since in that attack the
> attacker uses its own client.
>

This client metadata parameter is meant to be something that would prevent
a malicious actor from controlling the content of the authz request
parameters, which could be done by crafting the link and tricking a user
into following it. I mentioned mix-up as an example because the first
variant of it desribed at
https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-4.4.1
does something along those lines.



>
> If a client can do PAR, then it can do authorization code grant and PKCE,
> so we’re further limited to scenarios where the attacker does not need to
> be able to redeem the authorization code themselves. What threats fall into
> this category?
>
> —
> Annabelle Backman (she/her)
> AWS Identity
>
> On Apr 14, 2020, at 2:44 PM, Brian Campbell <bcampbell=
> [e-mail]> wrote:
>
> 
>
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>
> I was hoping to get to a rough consensus in support of the idea before
> coming up with a name that everyone will hate :)
>
> In the meantime, however, name suggestions are of course welcome.
>
>
> On Tue, Apr 14, 2020 at 2:22 PM Vladimir Dzhuvinov <
> [e-mail]> wrote:
>
>> I'm all for that.
>>
>> I suppose you have already thought of a suitable name? :)
>>
>> Vladimir
>> On 14/04/2020 23:08, Brian Campbell wrote:
>>
>> Using PAR can facilitate improved security by giving clients a
>> (relatively) simple means for sending a confidential, integrity protected,
>> and (for confidential clients anyway) authenticated authorization request.
>>
>> It seems like some of that improved security could be undermined by a
>> malicious actor somehow getting a user to navigate to a URL of a regular
>> old parameterized authorization request. One variation of the Mix-Up Attack
>> does this
>> https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-4.4.1
>> <https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-4.4..1>
>> for example and email, social media, online forums, etc., are other ways a
>> user might be tricked into following a maliciously crafted link.
>>
>> Following from that it seems logical that an authorization server might
>> want to restrict some clients from sending a regular parameterized
>> authorization request and instead require use of the PAR endpoint to
>> initiate an authorization request. Something like this could, of course, be
>> implemented as custom policy or configuration in any AS. But I'm thinking
>> it might be common enough to warrant adding a client metadata parameter to
>> the PAR draft specifically for it. The metadata (and registered extensions)
>> defined by Dynamic Client Registration [RFC7591] has come to imply a
>> general data model and expected associated behaviors for clients that is
>> useful for authorization server implementations, even when the Dynamic
>> Client Registration Protocol isn't directly in play. This particular
>> situation seems like a good potential candidate for a new such client
>> metadata parameter that would indicate that the given client can only use a
>> request_uri value obtained from the PAR endpoint to initiate an
>> authorization request. And that a regular old fashioned authorization
>> request from the given client would result in an error.
>>
>> Do the folks of this fine WG think something like this would be a
>> worthwhile addition to the PAR draft?
>>
>>
>>
>>
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