Re: [OAUTH-WG] Conflicting definitions in JWT Response for OAuth Token Introspection

[Takahiko Kawasaki <[e-mail]>]

>From RFC 7662, Section 2.1

*To prevent token scanning attacks, the endpoint MUST also require some
form of authorization to access this endpoint, such as client
authentication as described in OAuth 2.0 [RFC6749] or a separate OAuth 2.0
access token such as the bearer token described in OAuth 2.0 Bearer Token
Usage [RFC6750].  The methods of managing and validating these
authentication credentials are out of scope of this specification.*


"Some form of authorization" may not be able to distinguish API callers.
For example, if one pair of API key and API secret is shared by multiple
resource servers.

Even if the number of "resource" request parameters is one, its value is
not necessarily the identifier of a resource server. What if "
https://rs.example.com/resource1" is given as "resource"? Which value
should "aud" hold, "https://rs.example.com/resource1" or "
https://rs.example.com"?

If the format of access tokens is JWT, when an authorization request
includes "resource=https://host1.example.com/resource1" and "resource=
https://host2.example.com/resource2", the resultant access token will have
"aud" like below.

"aud" : [
  "https://host1.example.com/resource1",
  "https://host2.example.com/resource2"
]

However, if your logic applies, the introspection response for the access
token will include a different value for "aud". This behavior is confusing.
Because "some form of authorization" is performed when an introspection
call is made, I don't think it's necessary to modify the value of "aud" per
API caller.

Anyway, the root cause is that (a) "aud" in RFC 8707 and (b) "aud"
in draft-ietf-oauth-jwt-introspection-response (in general "aud" in JWT)
are different concepts but they share the same name. Any workaround without
solving the root cause will bring about unhappy future.

Taka


On Tue, Mar 3, 2020 at 1:44 AM Torsten Lodderstedt <[e-mail]>
wrote:

> Hi Taka,
>
> I see, the audience is multi value. In my impression both specs assume a
> single value audience.
>
> But I think we can handle this as follows: Since the caller of the
> introspection respect is a single RS, the AS first needs to map & check the
> resources associated with the underlying access token against that caller.
> The RS is authenticated at the introspection endpoint using a client_id
> (and some credential), so the AS needs to map the client_id to resource
> identifier. If the result is in the set of the resources associated with
> the underlying access token, the AS can create an Introspection Response,
> which only contains this particular resource.
>
> What do you think?
>
> best regards,
> Torsten.
>
> > On 2. Mar 2020, at 17:31, Takahiko Kawasaki <[e-mail]> wrote:
> >
> > Hi Torsten,
> >
> > For example, if an authorization request includes two "resource" request
> parameters like below,
> >
> > resource=https://host1.example.com/resource1
> > resource=https://host2.example.com/resource2
> >
> > RFC 8707 expects that the value of "aud" in an introspection response
> look like the following.
> >
> > "aud" : [
> >   "https://host1.example.com/resource1",
> >   "https://host2.example.com/resource2",
> > ]
> >
> > How does the implementation of the introspection endpoint insert the
> identifier of the resource server (the API caller?) into the "aud" array
> above? In other words, what is the expected resultant value of the "aud"
> array in this case?
> >
> > Taka
> >
> >
> > On Mon, Mar 2, 2020 at 10:54 PM Torsten Lodderstedt <
> [e-mail]> wrote:
> > Hi Taka,
> >
> > > On 1. Mar 2020, at 08:10, Takahiko Kawasaki <[e-mail]> wrote:
> > >
> > > Hello,
> > >
> > > I'm wondering if the following conflicts in "JWT Response for OAuth
> Token Introspection" (draft 8) have already been pointed out.
> > >
> > > RFC 8707 (Resource Indicators for OAuth 2.0) requires that 'aud' in an
> introspection response hold the values of the 'resource' request
> parameters, whereas "JWT Response for OAuth Token Introspection" says that
> 'aud' MUST identify the resource server receiving the token introspection
> response. The definitions conflict.
> >
> > RFC 8707 states
> >
> > The authorization server may use
> >    the exact "resource" value as the audience or it may map from that
> >    value to a more general URI or abstract identifier for the given
> >    resource.
> >
> > draft-ietf-oauth-jwt-introspection-response-08 states
> >
> > The value of the "aud" claims MUST identify the resource server
> >    receiving the token introspection response.
> >
> > So RFC 8707 gives choices of how the resource server might be identified
> and draft-ietf-oauth-jwt-introspection-response-08 says the AS must
> identify the RS without prescribing any particular way. So basically you
> can use the advice given by  RFC 8707 to implement the requirement stated
> by draft-ietf-oauth-jwt-introspection-response-08.
> >
> > I don’t see a conflict.
> >
> > >
> > > RFC 7662 (OAuth 2.0 Token Introspection) requires that 'iat' in an
> introspection response indicate when the access/refresh token was issued,
> whereas "JWT Response for OAuth Token Introspection" says that 'iat'
> indicates when the introspection response in JWT format was issued. The
> definitions conflict.
> >
> > I will come back to this issue in an answer to Filip’s post.
> >
> > best regards,
> > Torsten.
> >
> > >
> > > Best Regards,
> > > Takahiko Kawasaki
> > > Authlete, Inc.
> > >
> > >
> > >
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>
>