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

[Takahiko Kawasaki <[e-mail]>]

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]> 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]> 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]>
>> 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}
>>
>> 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]> 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]
>>> To: Karsten Meyer zu Selhausen <[e-mail]>
>>> <[e-mail]>, Karsten zu Selhausen
>>> <[e-mail]>
>>> <[e-mail]>, Daniel Fett
>>> <[e-mail]> <[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.
>>>
>>> The IETF Secretariat
>>>
>>>
>>> --
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