Re: [OAUTH-WG] Auth Code Swap Attack

Eran Hammer-Lahav <> Thu, 25 August 2011 17:01 UTC

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From: Eran Hammer-Lahav <>
To: Phil Hunt <>, Anthony Nadalin <>
Date: Thu, 25 Aug 2011 10:02:15 -0700
Thread-Topic: [OAUTH-WG] Auth Code Swap Attack
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Subject: Re: [OAUTH-WG] Auth Code Swap Attack
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We already have one recommended implementation with 'state'. It is clearly described in the security section which is the right place to define it. I will further emphasize it in the 'state' parameter definition as previously proposed.

BTW, as stated repeatedly before, making the parameter required does not provide *ANY* CSRF protection. It just forces it to be included. To guarantee CSRF protection, you need a LOT more normative language in how to bind the state value with some form of session information on the user-agent – which is clearly outside the scope of this working group. Requiring CSRF protection accomplishes that, and if a developer doesn't know how to do that, unfortunately, we will not be able to help within the scope of this document – which doesn't exclude it from being discussed in details in the thread model document as advised by the chairs.

CSRF is a serious attack vector, but our correct course is to highlight it, suggest a path, and leave it to developers to implement proper security like the other handful of possible attacks listed.


From: Phil Hunt <<>>
Date: Thu, 25 Aug 2011 09:25:30 -0700
To: Anthony Nadalin <<>>
Cc: Eran Hammer-lahav <<>>, Torsten Lodderstedt <<>>, "OAuth WG (<>)" <<>>
Subject: Re: [OAUTH-WG] Auth Code Swap Attack

I agree. I think at least one implementation must be included in the spec.  However, I'd like to see it left that more methods could be implemented in the future.

I can live with "SHOULD" given the definition in 2119.  Or phrased "MUST implement at least the following method..." for stronger language.

Tony is correct, I think it is a serious issue given the dependence on grants being passed by URLs through redirects.



On 2011-08-25, at 8:11 AM, Anthony Nadalin wrote:

I have not seen any updated text, so I don’t believe we have consensus. Also we have a flawed protocol and we are not providing a fix, suggest that MUST be on the state also unless someone has a better fix

From:<> [] On Behalf Of Eran Hammer-Lahav
Sent: Wednesday, August 24, 2011 7:54 AM
To: Torsten Lodderstedt
Cc: OAuth WG (<>)
Subject: Re: [OAUTH-WG] Auth Code Swap Attack

I believe we have full consensus on this approach.


From: Torsten Lodderstedt []<mailto:[]>
Sent: Tuesday, August 23, 2011 11:06 PM
To: Eran Hammer-Lahav
Cc: OAuth WG (<>)
Subject: Re: [OAUTH-WG] Auth Code Swap Attack

making CSRF prevention a MUST and recommending the state parameter as implementation pattern is ok with me.


Am 21.08.2011 21:02, schrieb Eran Hammer-Lahav:
I light to the recent discussion, do you still feel that changing ‘state’ from optional to required is the best approach?


From: Torsten Lodderstedt []
Sent: Sunday, August 21, 2011 11:04 AM
To: Eran Hammer-Lahav
Cc: OAuth WG (<>)
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.


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.


From:<> [] On Behalf Of Eran Hammer-Lahav
Sent: Monday, August 15, 2011 9:35 AM
To: OAuth WG (<>)
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.


From: Eran Hammer-Lahav
Sent: Friday, August 12, 2011 2:53 PM
To: Anthony Nadalin; OAuth WG (<>)
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 <<>>
Date: Fri, 12 Aug 2011 12:06:36 -0700
To: "OAuth WG (<>)" <<>>
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:

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.


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.



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