Re: [OAUTH-WG] Auth Code Swap Attack

[David Recordon <[e-mail]>]

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-mail]> 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:[e-mail]]
> Sent: Sunday, August 21, 2011 11:04 AM
> To: Eran Hammer-Lahav
> Cc: OAuth WG ([e-mail])
>
> 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: [e-mail] [mailto:[e-mail]] On Behalf Of
> Eran Hammer-Lahav
> Sent: Monday, August 15, 2011 9:35 AM
> To: OAuth WG ([e-mail])
> 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 ([e-mail])
> 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 <[e-mail]>
> Date: Fri, 12 Aug 2011 12:06:36 -0700
> To: "OAuth WG ([e-mail])" <[e-mail]>
> 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
>
>
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