Re: [OAUTH-WG] draft-bertocci-oauth-access-token-jwt-00

["Schanzenbach, Martin" <[e-mail]>]

Hi,

first time posting here but I have a comment on the "sub" claim:

We use RFC7523 for client authentication in a machine to machine scenario (i.e. client credentials grant).
In this RFC, the JWT used as client assertion is defined that it "MUST contain a "sub" (subject) claim identifying the principal that is the subject of the JWT" (Section 3).

Now, we also issue JWT access tokens which look already very similar to what is proposed in this draft.
As an implementer of an AS, I would find it a bit odd that I need to add the "sub" client with client_id for the self-signed assertion JWT on token request, but it is then not included in the JWT issued by the AS.
Regarding our application-specific claims in the JWT access token issued by the AS, the interpretation of the "sub" claim should not change imho.

BR
Martin

> On 26. Mar 2019, at 00:51, Vittorio Bertocci <Vittorio=[e-mail]> wrote:
> 
> Hey Nov, Dominick, Hans-
> thanks for the comments. That was an area I was expecting to cause more discussion, and I am glad we are having this opportunity to clarify.
> The current language in the draft traces the etymology of sub to OpenID Connect core, hence Dominick observation is on point. However in the description I express something in line with 7519, which was in fact my intent.
> 
> The idea was to provide an identifier of the calling subject that is guaranteed to be present in all cases- this would allow an SDK developer to use the same code for things like lookups and membership checks regardless of the nature of the caller (user in a delegated case, app in app-only grants). The information to discriminate between user and app callers is always available in the token (say, the caller is a user if sub!=client_id, where client_id is always guaranteed to be present as well) hence there's no loss in expressive power, should that difference be relevant for the resource server..
> 
> Dominick, Hans- I probably missed the security issue you guys are thinking of in this case. Of course, if this would introduce a risk I completely agree it should be changed- I'd just like to understand better the problem. Could you expand it in a scenario/use case to clarify the risk?
> Nov- playing this back: is the concern that a user and a client might have the same identifier within an IDP? When using collision resistant IDs, as it is usually the case, that seems to be a remote possibility- did you stumble in such scenario in production?
> 
> Thanks
> V.
> 
> 
> On Mon, Mar 25, 2019 at 7:44 AM Hans Zandbelt <[e-mail]> wrote:
> I believe there are plenty of OAuth 2.0 only use cases out there... but nevertheless I agree with the potential confusion and thus security problems arising from that (though one may argue the semantics are the same).
> 
> Hans.
> 
> On Mon, Mar 25, 2019 at 3:39 PM Dominick Baier <[e-mail]> wrote:
> Yes I know - and I think in hindsight it was a mistake to use the same claim type for multiple semantics.
> 
> All the “this is OIDC not OAuth” arguments are making things more complicated than they need to be - in my experience almost no-one (that I know) does OIDC only - nor OAuth only. They always combine it.
> 
> In reality this leads to potential security problems - this spec has the potential to rectify the situation.
> 
> Dominick
> 
> On 25. March 2019 at 14:58:56, Hans Zandbelt ([e-mail]) wrote:
> 
>> Without agreeing or disagreeing: OIDC does not apply here since it is not OAuth and an access token is not an id_token.
>> The JWT spec says in https://tools.ietf.org/html/rfc7519#section-4.1.2:
>> 
>> "The "sub" (subject) claim identifies the principal that is the
>>    subject of the JWT.  The claims in a JWT are normally statements
>>    about the subject.  The subject value MUST either be scoped to be
>>    locally unique in the context of the issuer or be globally unique.
>>    The processing of this claim is generally application specific"
>> 
>> which kind of spells "client" in case of the client credentials grant but I also do worry about Resource Servers thinking/acting only in terms of users
>> 
>> Hans.
>> 
>> On Mon, Mar 25, 2019 at 2:41 PM Dominick Baier <[e-mail]> wrote:
>> IMHO the sub claim should always refer to the user - and nothing else.
>> 
>> OIDC says:
>> 
>> "Subject - Identifier for the End-User at the Issuer."
>> 
>> client_id should be used to identify clients.
>> 
>> cheers
>> Dominick
>> 
>> On 25.. March 2019 at 05:13:03, Nov Matake ([e-mail]) wrote:
>> 
>>> Hi Vittorio,
>>> 
>>> Thanks for the good starting point of standardizing JWT-ized AT.
>>> 
>>> One feedback.
>>> The “sub” claim can include 2 types of identifier, end-user and client, in this spec.
>>> It requires those 2 types of identifiers to be unique each other in the IdP context.
>>> 
>>> I prefer omitting “sub” claim in 2-legged context, so that no such constraint needed.
>>> 
>>> thanks
>>> 
>>> nov
>>> 
>>>> On Mar 25, 2019, at 8:29, Vittorio Bertocci <vittorio.bertocci=[e-mail]> wrote:
>>>> 
>>>> Dear all,
>>>> I just submitted a draft describing a JWT profile for OAuth 2.0 access tokens. You can find it in https://datatracker.ietf.org/doc/draft-bertocci-oauth-access-token-jwt/.
>>>> I have a slot to discuss this tomorrow at IETF 104 (I'll be presenting remotely). I look forward for your comments!
>>>> 
>>>> Here's just a bit of backstory, in case you are interested in how this doc came to be. The trajectory it followed is somewhat unusual.
>>>> 	• Despite OAuth2 not requiring any specific format for ATs, through the years I have come across multiple proprietary solution using JWT for their access token. The intent and scenarios addressed by those solutions are mostly the same across vendors, but the syntax and interpretations in the implementations are different enough to prevent developers from reusing code and skills when moving from product to product.
>>>> 	• I asked several individuals from key products and services to share with me concrete examples of their JWT access tokens (THANK YOU Dominick Baier (IdentityServer), Brian Campbell (PingIdentity), Daniel Dobalian (Microsoft), Karl Guinness (Okta) for the tokens and explanations!).
>>>> I studied and compared all those instances, identifying commonalities and differences.
>>>> 	• I put together a presentation summarizing my findings and suggesting a rough interoperable profile (slides: https://sec.uni-stuttgart.de/_media/events/osw2019/slides/bertocci_-_a_jwt_profile_for_ats.pptx ) - got early feedback from Filip Skokan on it. Thx Filip!
>>>> 	• The presentation was followed up by 1.5 hours of unconference discussion, which was incredibly valuable to get tight-loop feedback and incorporate new ideas. John Bradley, Brian Campbell Vladimir Dzhuvinov, Torsten Lodderstedt, Nat Sakimura, Hannes Tschofenig were all there and contributed generously to the discussion. Thank you!!!
>>>> Note: if you were at OSW2019, participated in the discussion and didn't get credited in the draft, my apologies: please send me a note and I'll make things right at the next update.
>>>> 	• On my flight back I did my best to incorporate all the ideas and feedback in a draft, which will be discussed at IETF104 tomorrow. Rifaat, Hannes and above all Brian were all super helpful in negotiating the mysterious syntax of the RFC format and submission process.
>>>> I was blown away by the availability, involvement and willingness to invest time to get things right that everyone demonstrated in the process. This is an amazing community.
>>>> V.
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Martin Schanzenbach
Fraunhofer AISEC
Department Service & Application Security
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