Re: [OAUTH-WG] OAuth 2.0 Introspection RFC Issues

Hi Michael,

Thanks for the comments. First off, the text of an RFC is fixed and cannot be changed. The spec can only be altered with a new document that obsoletes RFC7662, which would have to go through the working group process again from the very beginning.

Authentication is required but we’re open to how it happens. The introspection spec is targeted at resource servers, not clients, since a client can always just *use* the token in order to see if it’s valid or not. Why bother with the extra round trip to check if the token is good before trying it, since trying it will also tell you if the token’s good? The recovery process for a failed token is the same in either state. We initially had this written for either clients or protected resources, but that’s not how people were using it and restricting its focus helped clarify the spec immensely.

ID tokens are a slightly different case as they’re an assertion targeted at the client itself, but in those cases an ID token is really meant to be self-contained as a JWT, and generally has a short validity period. If you’re looking to introspect an old ID token to check an OIDC session status, then you might be better served implementing one of the OIDC session spec components for that purpose.

Also, we didn’t mandate a specific authentication technology. Most people are going to use either client authentication or a specific OAuth token designed for this purpose. The latter of these is technically available for public clients, but then you have the question of if you want to check the status of *that* token too. Still, I think that for any clients (including public ones) the right thing to do is just use the token and see if it works. 

To the other comment, HTTP POST is the only defined verb and we’re silent on the use of other verbs. That said, many HTTP implementation frameworks don’t differentiate between verbs for requests and so a POST with body form parameters and a GET with query parameters would come out the same. We don’t outright prohibit this behavior, but it’s off-label and there are security considerations for its use.

Hope this helps,
 — Justin

> On Nov 2, 2015, at 3:17 AM, Michael Ciarlillo <michael.ciarlillo@gmail.com> wrote:
> 
> Hello all,
> 
> I have a couple of comments/issues with the RFC at https://tools.ietf.org/html/rfc7662 <https://tools.ietf.org/html/rfc7662>.
> 
> According to Section 2.1 (Introspection Request) says that "To prevent token scanning attacks, the endpoint MUST also require some form of authorization to access this endpoint..." This might make sense for token introspection requests only coming from Resource Servers, but to many implementers it makes sense to allow Clients access to the introspection endpoint since it would serve the same purpose (token validity checking, particularly for refresh tokens, but also for access tokens, or identity tokens in OpenID Connect, or possibly other token types in UMA and other specs). Unfortunately, this means that public clients should be accounted for in some way, as refresh tokens don't have an explicit requirement in the OAuth 2.0 spec that they be issued only to Confidential Clients. As such, many implementers don't feel like the Introspection spec fits well for Client consumption because of the "MUST" requirement on authentication, when public clients won't have a Client Secret to authenticate with. Further, token scanning attacks would not be mitigated for authenticated callers (be they Resource Servers or Clients).
> 
> An example of a real-world implementer discussion talking about this topic is one I had with Kévin Chalet: https://github.com/aspnet-contrib/AspNet.Security.OpenIdConnect.Server/pull/159 <https://github.com/aspnet-contrib/AspNet.Security.OpenIdConnect.Server/pull/159>
> 
> We are not the only ones who see this as a problem with the spec, and there do not seem to be many concrete reasons for this endpoint ONLY being for Resource Servers themselves. Can some insight into why this is the case? Can the MUST be changed to a SHOULD with the Security Considerations section expanded to talk about the issue? If not, is there another spec or draft out there somewhere for OAuth token validity for which the intent is explicitly client consumption? Any and all feedback on this would be greatly appreciated as we develop the ASOS middleware project.
> 
> Another (small) comment on the spec: Section 2.1 (Introspection Request) currently says that requests be HTTP POST method requests, however Section 4 (Security Considerations) mentions Authorization Servers explicitly disallowing the GET method due to server logs. What is the requirement here? POST with optional GET or explicitly only allowing POST requests?
> 
> Sincerely,
> Michael Ciarlillo
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