Re: [TLS] Another ClientHello length intolerance bug?

David Benjamin <davidben@chromium.org> Wed, 12 September 2018 16:38 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 12 Sep 2018 11:38:28 -0500
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To: "David A. Cooper" <david.cooper@nist.gov>
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Subject: Re: [TLS] Another ClientHello length intolerance bug?
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Wow! That's a bizarre one. I don't think we've run into this one before,
but, from your description, any given implementation would only have a
1/256 chance of hitting it on every ClientHello change.

10 is a newline, so perhaps some implementation is doing a terrible job
detecting TLS vs. some plaintext protocol. No idea about 14. (This kind of
broken and unsound protocol-guessing behavior appears to be sadly common.
Some buggy middlebox started misidentifying our ClientHellos as SIP when we
removed the pre-standard ChaCha20 cipher suites!)

Do you have an example server? It would be good to get the problematic
implementation fixed.

David

On Wed, Sep 12, 2018 at 9:24 AM David A. Cooper <david.cooper@nist.gov>
wrote:

> According to RFC 7685 there was at least one TLS implementation that would
> hang the connection if it received a ClientHello record with a
> TLSCiphertext.length between 256 and 511 bytes.
>
> During some recent testing I believe that I have come across a similar
> length intolerance bug. A number of servers seem to hang or close the
> connection if sent a ClientHello record with a TLSCiphertext.length of 266,
> 522, 778, ... (i.e., if TLSCiphertext.length *mod* 256 = 10). I have also
> encountered one server that will also hang the connection if sent a
> ClientHello record with a TLSCiphertext.length of 270, 526, 782 ... (i.e.,
> if TLSCiphertext.length *mod* 256 = 14).
>
> A test for this was just added to the development branch of testssl.sh (
> https://github.com/drwetter/testssl.sh) -- run testssl.sh with the
> "--grease" option.
>
> As the server banner being returned by the servers that seem to have this
> problem are not all the same it is my guess that it is actually some
> middlebox that the is the source of the problem.
>
> Has anyone else encountered this problem? We are trying to validate that
> this is a real bug (https://github.com/drwetter/testssl.sh/pull/1113).
>
> Thanks,
>
> David
>
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