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

William Denniss <wdenniss@google.com> Fri, 22 January 2016 13:11 UTC

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From: William Denniss <wdenniss@google.com>
Date: Fri, 22 Jan 2016 13:11:24 +0000
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To: John Bradley <ve7jtb@ve7jtb.com>, David Waite <david@alkaline-solutions.com>
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Cc: IETF oauth WG <oauth@ietf.org>
Subject: Re: [OAUTH-WG] Second OAuth 2.0 Mix-Up Mitigation Draft
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+1 ;)
On Fri, Jan 22, 2016 at 8:45 PM John Bradley <ve7jtb@ve7jtb.com> wrote:

> Perhaps Frankenstein response is a better name than cut and paste attack.
>
> John B.
> On Jan 22, 2016 1:22 AM, "David Waite" <david@alkaline-solutions.com>
> wrote:
>
>> On Jan 21, 2016, at 2:50 PM, John Bradley <ve7jtb@ve7jtb.com> 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.
>>
>> -DW
>>
>> 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 <david@alkaline-solutions.com>
>> 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
>>
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