I think this advice isn't a bad idea, though it's of course up to the AS what an "untrusted" client really is. In practice, this is something that was registered by a non-sysadmin type person, either a dynamically registered client or something available through self-service registration of some type. It's also reasonable that a client, even dynamically registered, would be considered "trusted" if enough time has passed and enough users have used it without things blowing up.
-- Justin On Sep 4, 2014, at 1:26 AM, Antonio Sanso <asa...@adobe.com<mailto:asa...@adobe.com>> wrote: hi again *, after thinking a bit further IMHO an alternative for the untrusted clients can also be to always present the consent screen (at least once) before any redirect. Namely all providers I have seen show the consent screen if all the request parameters are correct and if the user accepts the redirect happens. If one of the parameter (with the exclusion of the client id and redirect uri that are handled differently as for spec) is wrong though the redirect happens without the consent screen being shown.. WDYT? regards antonio On Sep 3, 2014, at 7:54 PM, Antonio Sanso <asa...@adobe.com<mailto:asa...@adobe.com>> wrote: Well, I do not know if this is only dynamic registration... let’s talk about real use cases, namely e.g. Google , Facebook , etc.. is that dynamic client registration? I do not know… :) Said that what the other guys think? :) Would this deserve some “spec adjustment” ? I mean there is a reason if Google is by choice “violating” the spec right? (I assume to avoid open redirect…) But other implementers do follow the spec hence they have this open redirector… and this is not nice IMHO... On Sep 3, 2014, at 7:40 PM, Hans Zandbelt <hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com>> wrote: On 9/3/14, 7:14 PM, Antonio Sanso wrote: On Sep 3, 2014, at 7:10 PM, Hans Zandbelt <hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com>> wrote: Is your concern clients that were registered using dynamic client registration? yes I think your issue is then with the trust model of dynamic client registration; that is left out of scope of the dynreg spec (and the concept is not even part of the core spec), but unless you want everything to be open (which typically would not be the case), then it would involve approval somewhere in the process before the client is registered. Without dynamic client registration that approval is admin based or resource owner based, depending on use case. Otherwise the positive case is returning a response to a valid URL that belongs to a client that was registered explicitly by the resource owner well AFAIK the resource owner doesn’t register clients… roles can collapse in use cases especially when using dynamic client registration and the negative case is returning an error to that same URL. the difference is the consent screen… in the positive case you need to approve an app.. for the error case no approval is needed,,, I fail to see the open redirect. why? because the client and thus the fixed URL are explicitly approved at some point Hans. Hans. On 9/3/14, 6:56 PM, Antonio Sanso wrote: On Sep 3, 2014, at 6:51 PM, Hans Zandbelt <hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com> <mailto:hzandb...@pingidentity.com>> wrote: Let me try and approach this from a different angle: why would you call it an open redirect when an invalid scope is provided and call it correct protocol behavior (hopefully) when a valid scope is provided? as specified below in the positive case (namely when the correct scope is provided) the resource owner MUST approve the app via the consent screen (at least once). Hans. On 9/3/14, 6:46 PM, Antonio Sanso wrote: hi John, On Sep 3, 2014, at 6:14 PM, John Bradley <ve7...@ve7jtb.com<mailto:ve7...@ve7jtb.com> <mailto:ve7...@ve7jtb.com> <mailto:ve7...@ve7jtb.com>> wrote: In the example the redirect_uri is vlid for the attacker. The issue is that the AS may be allowing client registrations with arbitrary redirect_uri. In the spec it is unspecified how a AS validates that a client controls the redirect_uri it is registering. I think that if anything it may be the registration step that needs the security consideration. (this is the first time :p) but I do disagree with you. It would be pretty unpractical to block this at registration time…. IMHO the best approach is the one taken from Google, namely returning 400 with the cause of the error.. *400.* That’s an error. *Error: invalid_scope* Some requested scopes were invalid. {invalid=[l]} said that I hope you all agree this is an issue in the spec so far…. regards antonio John B. On Sep 3, 2014, at 12:10 PM, Bill Burke <bbu...@redhat.com<mailto:bbu...@redhat.com> <mailto:bbu...@redhat.com> <mailto:bbu...@redhat.com>> wrote: I don't understand. The redirect uri has to be valid in order for a redirect to happen. The spec explicitly states this. On 9/3/2014 11:43 AM, Antonio Sanso wrote: hi *, IMHO providers that strictly follow rfc6749 are vulnerable to open redirect. Let me explain, reading [0] If the request fails due to a missing, invalid, or mismatching redirection URI, or if the client identifier is missing or invalid, the authorization server SHOULD inform the resource owner of the error and MUST NOT automatically redirect the user-agent to the invalid redirection URI. If the resource owner denies the access request or if the request fails for reasons other than a missing or invalid redirection URI, the authorization server informs the client by adding the following parameters to the query component of the redirection URI using the "application/x-www-form-urlencoded" format, perAppendix B <https://tools.ietf.org/html/rfc6749#appendix-B>: Now let’s assume this. I am registering a new client to thevictim.com<http://thevictim.com/> <http://victim.com/><http://victim.com<http://victim.com/> <http://victim.com/>> <http://victim.com<http://victim.com/> <http://victim.com/>> provider. I register redirect uriattacker.com<http://uriattacker.com/> <http://attacker.com/><http://attacker.com<http://attacker.com/> <http://attacker.com/>> <http://attacker.com<http://attacker.com/> <http://attacker.com/>>. According to [0] if I pass e.g. the wrong scope I am redirected back to attacker.com<http://attacker.com/> <http://attacker.com/><http://attacker.com<http://attacker.com/> <http://attacker.com/>> <http://attacker.com<http://attacker.com/> <http://attacker.com/>>. Namely I prepare a url that is in this form: http://victim.com/authorize?response_type=code&client_id=bc88FitX1298KPj2WS259BBMa9_KCfL3&scope=WRONG_SCOPE&redirect_uri=http://attacker.com and this is works as an open redirector. Of course in the positive case if all the parameters are fine this doesn’t apply since the resource owner MUST approve the app via the consent screen (at least once). A solution would be to return error 400 rather than redirect to the redirect URI (as some provider e.g. Google do) WDYT? regards antonio [0] https://tools.ietf.org/html/rfc6749#section-4.1.2.1 _______________________________________________ OAuth mailing list OAuth@ietf.org<mailto:OAuth@ietf.org> https://www.ietf.org/mailman/listinfo/oauth -- Bill Burke JBoss, a division of Red Hat http://bill.burkecentral.com<http://bill.burkecentral.com/> _______________________________________________ OAuth mailing list OAuth@ietf.org<mailto:OAuth@ietf.org> <mailto:OAuth@ietf.org> https://www.ietf.org/mailman/listinfo/oauth _______________________________________________ OAuth mailing list OAuth@ietf.org<mailto:OAuth@ietf.org> <mailto:OAuth@ietf.org><mailto:OAuth@ietf.org> https://www.ietf.org/mailman/listinfo/oauth _______________________________________________ OAuth mailing list OAuth@ietf.org<mailto:OAuth@ietf.org> <mailto:OAuth@ietf.org> https://www.ietf.org/mailman/listinfo/oauth -- Hans Zandbelt | Sr. Technical Architect hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com> <mailto:hzandb...@pingidentity.com>| Ping Identity -- Hans Zandbelt | Sr. Technical Architect hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com> | Ping Identity -- Hans Zandbelt | Sr. Technical Architect hzandb...@pingidentity.com<mailto:hzandb...@pingidentity.com> | Ping Identity _______________________________________________ OAuth mailing list OAuth@ietf.org<mailto:OAuth@ietf.org> https://www.ietf.org/mailman/listinfo/oauth
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