Re: [OAUTH-WG] Second OAuth 2.0 Mix-Up Mitigation Draft

David Waite <> Fri, 22 January 2016 04:22 UTC

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From: David Waite <>
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Date: Thu, 21 Jan 2016 21:22:39 -0700
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To: John Bradley <>
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Subject: Re: [OAUTH-WG] Second OAuth 2.0 Mix-Up Mitigation Draft
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> On Jan 21, 2016, at 2:50 PM, John Bradley <> wrote:
> In that case you probably would put a hash of the state in the code to manage size.  The alg would be up to the AS, as long as it used the same hash both places it would work.
Yes, true. 
> Sending the state to the token endpoint is like having nonce and c_hash in the id_token, it binds the issued code to the browser instance.
I think I understand what you are saying. Someone won’t be able to frankenstein up a state and a token from two different responses from an AS, and have a client successfully fetch an access token based on the amalgamation.
> This protects against codes that leak via redirect uri pattern matching. failures etc.  It prevents an attacker from being able to replay a code from a different browser.
Yes, if a party intercepts the redirect_url, or the AS fails to enforce one time use (which even for a compliant implementation could just mean the attacker was faster than the state propagated within the AS)

Makes sense. Thanks John.


> If the client implements the other mitigations on the authorization endpoint, then it wouldn't be leaking the code via the token endpoint. 
> The two mitigations are for different attacks, however some of the attacks combined both vulnerabilities.
> Sending the iss and client_id is enough to stop the confused client attacks, but sending state on its own would not have stopped all of them.
> We discussed having them in separate drafts, and may still do that.   However for discussion having them in one document is I think better in the short run.
> John B.
>> On Jan 21, 2016, at 4:48 PM, David Waite < <>> wrote:
>> Question: 
>> I understand how “iss" helps mitigate this attack (client knows response was from the appropriate issuer and not an attack where the request was answered by another issuer). 
>> However, how does passing “state” on the authorization_code grant token request help once you have the above in place? Is this against some alternate flow of this attack I don’t see, or is it meant to mitigate some entirely separate attack?
>> If one is attempting to work statelessly (e.g. your “state” parameter is actual state and not just a randomly generated value), a client would have always needed some way to differentiate which issuer the authorization_code grant token request would be sent to.
>> However, if an AS was treating “code” as a token (for instance, encoding: client, user, consent time and approved scopes), the AS now has to include the client’s state as well. This would effectively double (likely more with encoding) the state sent in the authorization response back to the client redirect URL, adding more pressure against maximum URL sizes.
>> -DW