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Re: [Unbearable] JWS

Mike Jones <Michael.Jones@microsoft.com> Fri, 24 July 2015 18:58 UTC

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From: Mike Jones <Michael.Jones@microsoft.com>
To: Tony Arcieri <bascule@gmail.com>, Jim Fenton <fenton@bluepopcorn.net>
Thread-Topic: [Unbearable] JWS
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Date: Fri, 24 Jul 2015 18:58:24 +0000
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Archived-At: <http://mailarchive.ietf.org/arch/msg/unbearable/lLq-CDpFNsi5qpJuTI2e_du7jf8>
Cc: "unbearable@ietf.org" <unbearable@ietf.org>, Martin Thomson <martin.thomson@gmail.com>
Subject: Re: [Unbearable] JWS
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When using the JWS Compact Serialization http://tools.ietf.org/html/rfc7515#section-3.1, which is what Token Binding would use, attackers can’t choose algorithms because the algorithm identifier is included in the integrity-protected information.

                                                            -- Mike

From: Unbearable [mailto:unbearable-bounces@ietf.org] On Behalf Of Tony Arcieri
Sent: Friday, July 24, 2015 11:54 AM
To: Jim Fenton
Cc: unbearable@ietf.org; Martin Thomson
Subject: Re: [Unbearable] JWS

On Fri, Jul 24, 2015 at 11:46 AM, Jim Fenton <fenton@bluepopcorn.net<mailto:fenton@bluepopcorn.net>> wrote:
I would point to the fact that we (now) have libraries that have been vetted as a reason to use JWS, not the other way around.

(or maybe that's what you meant)

If finding a single vuln meant that a library is "vetted", OpenSSL would be an impenetrable fortress by now.

This particular vulnerability speaks to what I think is a design flaw that adds sharp edges to the format, namely that an attacker can choose algorithms as part of a malleable portion of the message which the verifier needs to interpret in order to verify the messages.

It's not an intractable problem, but it is one that leads to implementation difficulties, and is one of the reasons I've avoided JWT and instead used bearer token formats where these sort of problems are impossible by design.

I would like to leverage tokbind, but it saddens me that to do so I might have to add this attack surface back.

--
Tony Arcieri

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