I think the complication here is that CSRF issues are multi-site issues where the attacker cross connecting his client with a victims resource, or a victims client with the attackers resource.
So while an individual site (e.g. Facebook) may presume little or no risk - there is a network effect here. A CSRF attacker could be using facebook to attack another site. See Yaron's original text about Plaxo/Live at the start of this thread. Would it be reasonable to assess whether a resource site could make it mandatory based on a pre-registered client? IOW, would Facebook want to make state mandatory for Confidential clients, but not public clients? Would it be acceptable to change status from OPTIONAL to RECOMMENDED? Phil @independentid www.independentid.com phil.h...@oracle.com On 2011-08-21, at 6:10 PM, David Recordon wrote: > So far Facebook has used `state` in examples within our documentation > and strongly recommend it but have not gone so far as to mandate it. > > Quoting https://developers.facebook.com/docs/authentication/: >> Cross site request forgery is an attack in which an trusted (authenticated >> and authorized) user unknowingly performs an action on website. To prevent >> this attack, you should pass an identifier in the state parameter, and then >> validate the state parameter matches on the response. We strongly recommend >> that any app implementing Facebook user login implement CSRF protection using >> this mechanism. > > I'd rather clearly document this in the spec, strongly recommend a > solution but not mandate this specific parameter. > > --David > > > On Sun, Aug 21, 2011 at 12:02 PM, Eran Hammer-Lahav <e...@hueniverse.com> > wrote: >> I light to the recent discussion, do you still feel that changing ‘state’ >> from optional to required is the best approach? >> >> >> >> EHL >> >> >> >> From: Torsten Lodderstedt [mailto:tors...@lodderstedt.net] >> Sent: Sunday, August 21, 2011 11:04 AM >> To: Eran Hammer-Lahav >> Cc: OAuth WG (oauth@ietf.org) >> >> Subject: Re: [OAUTH-WG] Auth Code Swap Attack >> >> >> >> My intention is to require clients to implement CSRF prevention. I thought >> making the state parameter mandatory would be the straightforward way. >> >> regards, >> Torsten. >> >> Am 18.08.2011 08:04, schrieb Eran Hammer-Lahav: >> >> I would like to hear from the other 3 authors of the proposed change about >> their reasons for changing the use of ‘state’ from recommended to required >> for CSRF prevention. It would also help moving this issue forward if the 4 >> authors can provide answers or clarifications on the issues raised below. >> >> >> >> Assuming we can count all 4 authors are in favor of making the change, I >> believe we have a tie (4:4) and therefore no consensus for making it (as of >> this point). However, we did identify issues with the section’s language and >> clarity which we should address either way. >> >> >> >> To clarify – I am not proposing we close this issue just yet. >> >> >> >> EHL >> >> >> >> From: oauth-boun...@ietf.org [mailto:oauth-boun...@ietf.org] On Behalf Of >> Eran Hammer-Lahav >> Sent: Monday, August 15, 2011 9:35 AM >> To: OAuth WG (oauth@ietf.org) >> Subject: Re: [OAUTH-WG] Auth Code Swap Attack >> >> >> >> To demonstrate why making state required as proposed isn’t very helpful, >> here is an incomplete list of other requirements needed to make an effective >> CSRF: >> >> >> >> * State value must not be empty (a common bug in many implementations using >> simple value comparison). >> >> >> >> * ‘Non-guessable’ isn’t sufficient as most developers will simply use a hash >> of the session cookie, with or without salt which isn’t sufficient. We use >> “cannot be generated, modified, or guessed to produce valid values” >> elsewhere in the document, but this is much easier to get right for access >> tokens and refresh tokens than CSRF tokens which are often just some >> algorithm on top of the session cookie. >> >> >> >> * State CSRF value should be short-lived or based on a short-lived session >> cookie to prevent the use of a leaked state value in multiple attacks on the >> same user session once the leak is no longer viable. >> >> >> >> In addition, this is not what “state” was originally intended for. If the >> working group decides to mandate a CSRF parameter, it should probably be a >> new parameter with a more appropriate name (e.g. ‘csrf’). By forcing clients >> to use “state” for this purpose, developers will need to use dynamic queries >> for other state information which further reduces the security of the >> protocol (as the draft recommends not using dynamic callback query >> components). Encoding both CSRF tokens and other state information can be >> non-intuitive or complicated for some developers/platforms. >> >> >> >> EHL >> >> >> >> >> >> >> >> >> >> From: Eran Hammer-Lahav >> Sent: Friday, August 12, 2011 2:53 PM >> To: Anthony Nadalin; OAuth WG (oauth@ietf.org) >> Subject: Re: [OAUTH-WG] Auth Code Swap Attack >> >> >> >> This is really just a flavor of CSRF attacks. I have no objections to better >> documenting it (though I feel the current text is already sufficient), but >> we can't realistically expect to identify and close every possible >> browser-based attack. A new one is invented every other week. >> >> >> >> The problem with this text is that developers who do no understand CSRF >> attacks are not likely to implement it correctly with this information. >> Those who understand it do not need the extra verbiage which is more >> confusing than helpful. >> >> >> >> As for the new requirements, they are insufficient to actually accomplish >> what the authors propose without additional requirements on state local >> storage and verification to complete the flow. Also, the proposed text needs >> clarifications as noted below. >> >> >> >> >> >> From: Anthony Nadalin <tony...@microsoft.com> >> Date: Fri, 12 Aug 2011 12:06:36 -0700 >> To: "OAuth WG (oauth@ietf.org)" <oauth@ietf.org> >> Subject: [OAUTH-WG] Auth Code Swap Attack >> >> >> >> >> >> >> >> Recommended Changes to draft-ietf-oauth-v2 >> >> >> >> In section 4, request options (e.g. 4.1.1) featuring "state" should change >> from: >> >> >> >> state >> >> OPTIONAL. An opaque value used by the client to maintain state between the >> request and callback. The authorization server includes this value when >> redirecting the user-agent back to the client. >> >> >> >> to: >> >> >> >> state >> >> REQUIRED. An opaque value used by the client to maintain state between the >> request and callback. The authorization server includes this value when >> redirecting the user-agent back to the client. The encoded value SHOULD >> enable the client application to determine the user-context that was active >> at the time of the request (see section 10.12). The value MUST NOT be >> guessable or predictable, and MUST be kept confidential. >> >> >> >> >> >> Making the parameter required without making its usage required (I.e. "value >> SHOULD enable") accomplishes nothing. Also, what does "MUST be kept >> confidential" mean? Confidential from what? Why specify an "encoded value"? >> >> >> >> >> >> Section 10.12 Cross-Site Request Forgery >> >> >> >> Change to: >> >> >> >> Cross-site request forgery (CSRF) is a web-based attack whereby HTTP >> requests are transmitted from the user-agent of an end-user the server >> trusts or has authenticated. CSRF attacks enable the attacker to intermix >> the attacker's security context with that of the resource owner resulting in >> a compromise of either the resource server or of the client application >> itself. In the OAuth context, such attacks allow an attacker to inject their >> own authorization code or access token into a client, which can result in >> the client using an access token associated with the attacker's account >> rather than the victim's. Depending on the nature of the client and the >> protected resources, this can have undesirable and damaging effects. >> >> In order to prevent such attacks, the client application MUST encode a >> non-guessable, confidential end-user artifact and submit as the "state" >> parameter to authorization and access token requests to the authorization >> server. The client MUST keep the state value in a location accessible only >> by the client or the user-agent (i.e., protected by same-origin policy), for >> example, using a DOM variable, HTTP cookie, or HTML5 client-side storage. >> >> The authorization server includes the value of the "state" parameter when >> redirecting the user-agent back to the client. Upon receiving a redirect, >> the client application MUST confirm that returned value of "state" >> corresponds to the state value of the user-agent's user session. If the >> end-user session represents an authenticated user-identity, the client MUST >> ensure that the user-identity has NOT changed. >> >> >> >> >> >> The above text uses 'user-context' and this 'user-identity'. Neither term is >> defined. >> >> >> >> EHL >> >> >> _______________________________________________ >> >> OAuth mailing list >> >> OAuth@ietf.org >> >> https://www.ietf.org/mailman/listinfo/oauth >> >> _______________________________________________ >> OAuth mailing list >> OAuth@ietf.org >> https://www.ietf.org/mailman/listinfo/oauth >> >> > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth _______________________________________________ OAuth mailing list OAuth@ietf.org https://www.ietf.org/mailman/listinfo/oauth
