Re: [OAUTH-WG] New Version Notification for draft-meyerzuselhausen-oauth-iss-auth-resp-01.txt

[Vladimir Dzhuvinov <[e-mail]>]

Re Case 1: When JARM is used:

A colleague pointed me to the following statement in the JARMs spec, so
I'd suggest to say the "iss" MUST NOT be included when JARM is used:

https://openid.net//specs/openid-financial-api-jarm.html#jwt-based-response-mode

> All response parameters defined for a given response type are conveyed
> in a JWT

Now, there isn't a proper normative keyword in this JARM spec sentence,
so I guess some may interpret this as a strong check for no other query
params, while others may not. Hence the MUST NOT to prevent potential
unintended errors.

What are your thoughts on this?

Vladimir

On 06/11/2020 23:34, Takahiko Kawasaki wrote:
> I implemented the draft quickly and found no big hurdle for
> authorization server implementations. The current snapshot of my
> implementation does not add the `iss` parameter when JARM is used.
> However, for interoperability, I feel that the spec should describe
> expected behaviors when a JWT is included in an authorization
> response. The following is an implementer's comment for some cases.
>
> Case 1: When JARM is used
>
> An `iss` claim is included in the response JWT as one of top-level
> entries together with response parameters. It is not so unnatural to
> regard the `iss` claim as a response parameter. Conclusion would be
> "When JARM is used, the `iss` parameter is not necessary."
>
> Case 2: When an ID token is issued
>
> It is unnatural to regard the `iss` claim in an ID token as a response
> parameter. However, because FAPI Part 2 has already been using an ID
> token as detached signature for integrity protection, it would be
> difficult to find a convincing reason to prohibit using the `iss`
> claim in an ID token as a countermeasure to mix-up attacks. Conclusion
> would be "When an ID token is issued, the `iss` parameter is not
> necessary."
>
> Case 3: When an unencrypted JWT access token is issued
>
> It is technically possible to use the `iss` claim in an unencrypted
> JWT access token as the `iss` parameter. However, requiring the client
> to check the `iss` claim means "The access token is no longer opaque
> to the client." Conclusion would be "Even when an access token is
> issued and its format is JWT, the `iss` parameter is necessary."
>
> BTW, I found that a certain system raises an error when an unknown
> response parameter (that is, the `iss` parameter) is included in error
> authorization responses. To ask the administrator of the system to
> regard the `iss` parameter as a known one, at least the spec draft
> needs to be adopted by the community as a working draft. I hope that
> "call for adoption" for the draft will be conducted soon.
>
> Best Regards,
> Taka
>
> On Wed, Nov 4, 2020 at 4:46 AM Takahiko Kawasaki <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>     It sounds that the Security Considerations section or somewhere
>     appropriate should have a paragraph like below.
>
>     When an authorization response includes a JWT whose `iss` claim
>     represents the issuer identifier of the authorization server, the
>     `iss` claim can be used as a substitute for the `iss` parameter.
>     Therefore, such authorization response does not have to have the
>     `iss` parameter outside the JWT separately. Examples of such JWTs
>     include the value of the `id_token` parameter in OIDC and the
>     value of `response` parameter in JARM.
>
>     Taka
>
>     On Tue, Nov 3, 2020 at 10:46 PM Joseph Heenan <[e-mail]
>     <mailto:[e-mail]>> wrote:
>
>         I agree, it is in redundant in the JARM case.
>
>         I find the text
>         in https://www.ietf.org/archive/id/draft-meyerzuselhausen-oauth-iss-auth-resp-01.html#name-security-considerations
>         (the 4th paragraph where JARM & JWTs) are mentioned a bit
>         confusing - I think it would be good to say something along
>         the lines of:
>
>         Although integrity protection is not necessary to prevent
>         mixup, any authorization response method that includes a JWT
>         with an ‘iss' (for example, JARM or OIDC hybrid flow) will
>         prevent the attack (assuming the client is validating the iss).
>
>
>         I’m not entirely sure I understand what "MUST NOT allow
>         multiple authorization servers to return the same issuer
>         identifier during registration” means as I don’t think
>         https://tools.ietf.org/html/rfc7591 returns the issuer?
>
>         It might be clearer to say something like “When
>         https://tools.ietf.org/html/rfc8414 is used the client MUST
>         implement the validation described in
>         https://tools.ietf.org/html/rfc8414#section-3.3. When
>         authorization server details can be manually configured in the
>         client, the client must verify that all issuer values are
>         unique.” (Or at least something along those lines, I’m sure my
>         wording can be improved. But if the client is correctly
>         implementing rfc8414 or OIDC discovery [and does not have any
>         manually configured authorization servers] then there’s no
>         requirement for any further checks that the issuer is unique.)
>
>         Joseph
>
>
>>         On 3 Nov 2020, at 07:01, Vladimir Dzhuvinov
>>         <[e-mail] <mailto:[e-mail]>> wrote:
>>
>>         This can potentially occur. If JARM is used "iss" becomes
>>         redundant. To me JARM is an "enhanced" iss.
>>
>>         If both are included a sensible client should make sure the
>>         iss and the JARM iss match.
>>
>>         My suggestion is to not require iss when a JARM is present,
>>         but in case both do occur to have the client check both.
>>
>>         Vladimir
>>
>>         On 02/11/2020 22:34, Takahiko Kawasaki wrote:
>>>         Hi Karsten,
>>>
>>>         The specification mentions JARM. Does this specification
>>>         require the iss response parameter even when JARM is used?
>>>         That is, should an authorization response look like below?
>>>
>>>         HTTP/1.1 302 Found
>>>         Location:
>>>         https://client.example.com/cb?response={JWT}&iss={ISSUER}
>>>         <https://client.example.com/cb?response=%7BJWT%7D&iss=%7BISSUER%7D>
>>>
>>>         Or, can the iss response parameter be omitted when JARM is used?
>>>
>>>         A small feedback for the 3rd paragraph in Section 4:
>>>         s/identifes/identifies/
>>>
>>>         Best Regards,
>>>         Taka
>>>          
>>>
>>>         On Tue, Nov 3, 2020 at 3:13 AM Vladimir Dzhuvinov
>>>         <[e-mail] <mailto:[e-mail]>>
>>>         wrote:
>>>
>>>             Thanks Karsten, looks good to me now, no further comments.
>>>
>>>             Vladimir
>>>
>>>             On 02/11/2020 09:54, Karsten Meyer zu Selhausen wrote:
>>>>
>>>>             Hi all,
>>>>
>>>>             Daniel and I published a new version of the "iss"
>>>>             response parameter draft to address the feedback from
>>>>             the WG.
>>>>
>>>>             Changes in -01:
>>>>
>>>>               * Incorporated first WG feedback
>>>>               * Clarifications for use with OIDC
>>>>               * Added note that clients supporting just one AS are
>>>>                 not vulnerable
>>>>               * Renamed metadata parameter
>>>>               * Various editorial changes
>>>>
>>>>
>>>>             We would like to ask you for further feedback and
>>>>             comments on the new draft version.
>>>>
>>>>             Best regards,
>>>>             Karsten
>>>>
>>>>             -------- Forwarded Message --------
>>>>             Subject: 	New Version Notification for
>>>>             draft-meyerzuselhausen-oauth-iss-auth-resp-01.txt
>>>>             Date: 	Sun, 01 Nov 2020 23:31:42 -0800
>>>>             From: 	[e-mail]
>>>>             <mailto:[e-mail]>
>>>>             To: 	Karsten Meyer zu Selhausen
>>>>             <[e-mail]>
>>>>             <mailto:[e-mail]>, Karsten
>>>>             zu Selhausen <[e-mail]>
>>>>             <mailto:[e-mail]>, Daniel
>>>>             Fett <[e-mail]> <mailto:[e-mail]>
>>>>
>>>>
>>>>
>>>>
>>>>             A new version of I-D,
>>>>             draft-meyerzuselhausen-oauth-iss-auth-resp-01.txt
>>>>             has been successfully submitted by Karsten Meyer zu
>>>>             Selhausen and posted to the
>>>>             IETF repository.
>>>>
>>>>             Name: draft-meyerzuselhausen-oauth-iss-auth-resp
>>>>             Revision: 01
>>>>             Title: OAuth 2.0 Authorization Server Issuer Identifier
>>>>             in Authorization Response
>>>>             Document date: 2020-11-01
>>>>             Group: Individual Submission
>>>>             Pages: 10
>>>>             URL:
>>>>             https://www.ietf.org/archive/id/draft-meyerzuselhausen-oauth-iss-auth-resp-01.txt
>>>>             Status:
>>>>             https://datatracker.ietf.org/doc/draft-meyerzuselhausen-oauth-iss-auth-resp/
>>>>             Html:
>>>>             https://www.ietf.org/archive/id/draft-meyerzuselhausen-oauth-iss-auth-resp-01.html
>>>>             Htmlized:
>>>>             https://tools.ietf.org/html/draft-meyerzuselhausen-oauth-iss-auth-resp-01
>>>>             Diff:
>>>>             https://www.ietf.org/rfcdiff?url2=draft-meyerzuselhausen-oauth-iss-auth-resp-01
>>>>
>>>>             Abstract:
>>>>             This document specifies a new parameter "iss" that is
>>>>             used to
>>>>             explicitly include the issuer identifier of the
>>>>             authorization server
>>>>             in the authorization response of an OAuth authorization
>>>>             flow. If
>>>>             implemented correctly, the "iss" parameter serves as an
>>>>             effective
>>>>             countermeasure to "mix-up attacks".
>>>>
>>>>
>>>>
>>>>             Please note that it may take a couple of minutes from
>>>>             the time of submission
>>>>             until the htmlized version and diff are available at
>>>>             tools.ietf.org <http://tools.ietf.org/>.
>>>>
>>>>             The IETF Secretariat
>>>>
>>>>
>>>>             -- 
>>>>             Karsten Meyer zu Selhausen
>>>>             IT Security Consultant
>>>>             Phone:	+49 (0)234 / 54456499
>>>>             Web:	https://hackmanit.de <https://hackmanit.de/> | IT Security Consulting, Penetration Testing, Security Training
>>>>
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>>>>             https://www.hackmanit.de/en/blog-en/123-when-pkce-cannot-protect-your-confidential-oauth-client
>>>>
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-- 
Vladimir Dzhuvinov