Re: [OAUTH-WG] A Scope Attack against OAuth 2.0

[Wenjie Lin <[e-mail]>]

We assume that the authorization server is trustworthy and won’t do
anything that violates the spec. We want to prevent attacks by the client
on the user, as well as the attacks by the user on the client.****


The scope attack is by the user on the client. From the point view of the
user it is not an attack. However, the client takes it as an attack; the
user violates the service agreement by the client who may have no knowledge
about it, unless the authorization server always sends him the authorized
scope information when granting an access token, as we've proposed, and the
client can figure out the difference/violation and decide how to deal with
it.****

** **

When the client identifies the difference between his specified scope and
that in the access token, he can choose to abort or continue the service,
or take other actions. This is an implementation issue and is beyond the
scope of OAuth. There are other possible anomalies from implementations,
such as revocation of the scope by the user. However, they are not well
specified in the current spec and we take them as an implementation rather
than a protocol issue.****

** **

There might be applications where the client would not perceive scope
attack as an attack if he could allow any changes of the service agreement
scope by the user. If OAuth were aimed for such clients only, it would
significantly limit the applicability of the protocol.

We appreciate the insightful discussions on how the client could check and
handle the scope differences and beyond. They are extremely helpful for the
implementers.


-W. Lin and D. Lee

On Tue, Feb 21, 2012 at 4:59 PM, John Bradley <[e-mail]> wrote:

> Yes,  OpenID Connect deals with the issue by using a signed request
> extension in the cases where the client needs to be certain the request is
> only from the legitimate client and not tampered with.
>
> As you observe anyone can send a request the client_id and redirect_uri
> are not secret in any way.
>
> Though a well behaved client should be using state to check for XSRF
> attacks. (that is a real attack)
>
> Signed requests have uses,  but are not core to OAuth.
>
> John B.
> On 2012-02-21, at 9:38 PM, André DeMarre wrote:
>
> > The consensus seems to be, and I agree, that this shouldn't be
> > considered an "attack," but that's really just nomenclature. I do
> > concede that there is a spec issue here that I failed to appreciate at
> > first. Where draft-ietf-oauth-v2-23 section 3.3 says "If the issued
> > access token scope is different from the one requested by the client,"
> > it suggests that the authorization server knows what was requested by
> > the client. That isn't strictly true; it only knows what was in the
> > authorization request, not who put it there. For the client to truly
> > know what the auth server wants, (1) it would need to communicate
> > directly with the client, (2) the client would need to preregister its
> > desired scope, or (3) the client would need to sign a message that the
> > user passes to the authorization endpoint. 1 and 3 are clearly not
> > compatible with the spec, 2 might be; but that's not the point.
> >
> > A more correct statement in section 3.3 would be "If the issued access
> > token scope is different from the one in the authorization request..."
> >
> > Regards,
> > Andre DeMarre
> >
> > On Tue, Feb 21, 2012 at 3:01 PM, John Bradley <[e-mail]> wrote:
> >>
> >> On 2012-02-21, at 7:32 PM, Nicholas Devenish wrote:
> >>
> >>>
> >>> On 21 Feb 2012, at 21:59, John Bradley wrote:
> >>>> This 'attack'  is one that only works with badly designed clients
> that are making unwarranted assumptions about the behaviour of the
> Authorization server.
> >>>
> >>> Unwarranted assumptions? The spec in 3.3 says:
> >>>
> >>> "If the issued access token scope is different from the one requested
> by the client, the authorization server MUST include the "scope" response
> parameter to inform the client of the actual scope granted."
> >>>
> >>> - It says MUST; therefore any server that doesn't do this is
> non-compliant?
> >>> - It says scope different from the one requested by the *client*. The
> possibility that the resource owner intercepts this request and changes it
> doesn't seem to be considered here (it is not strictly the clients request
> if that happens)
> >>> - The purpose seems to be that the client should be told if the scope
> changes; this would be important if the client requires a certain scope
> level to work (though this could be solved more generally in many other
> ways that wouldn't be in the scope of the oauth spec)
> >>>
> >>> Thus, assuming that the server is stating compliance, isn't the
> assumption completely warranted?
> >>
> >> No the authorization server may at any time for any reason remove a
> scope from a granted access_token or refresh_token.
> >>
> >> Reporting back changes in scopes granted along with the access_token is
> a convenience not a security feature.
> >>
> >> Assuming it is a security feature and those scopes will continue to be
> valid for the token after granting is a bad design given the OAuth 2 spec.
> >>>
> >>>> The only way for a client to know if a token has a scope it to try
> it, or use a introspection endpoint.  End of story.
> >>>
> >>> An introspection endpoint obviously isn't part of the specification,
> so isn't relevant to the discussion (though it solves the discussed
> facebook issue).
> >>>
> >>> You are right though, that the only way for a client to know for sure
> is to try to use it; Even if the spec mandated always returning the scope
> to the client, the user could always intercept the return redirection
> (assuming a non-confidential client) and change the scope there.
> >>>
> >>> Perhaps MUST should change to SHOULD, given that this essentially
> seems unenforceable?
> >>
> >> A SHOULD may lead people to the conclusion that it is secure.   I am
> happy with saying it is not secure the only want to know is to have the
> client be prepared to deal with tokens that do not contain the desired
> scope when used.   That is the only 100% solution.
> >>
> >> John B.
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >> OAuth mailing list
> >> [e-mail]
> >> https://www.ietf.org/mailman/listinfo/oauth
> >>
>
>


-- 
Wenjie Lin