Re: [OAUTH-WG] Auth Code Swap Attack

Eran Hammer-Lahav <> Mon, 15 August 2011 16:35 UTC

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From: Eran Hammer-Lahav <>
To: "OAuth WG (" <>
Date: Mon, 15 Aug 2011 09:35:13 -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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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.