Re: [TLS] Suspicious behaviour of TLS server implementations

Martin Thomson <> Sat, 10 September 2016 10:38 UTC

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From: Martin Thomson <>
Date: Sat, 10 Sep 2016 20:38:52 +1000
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To: Andreas Walz <>
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Subject: Re: [TLS] Suspicious behaviour of TLS server implementations
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I think that Martin (R) provided you with the answers I would have.

Have you filed bugs against the servers in question for the issues
that you have seen?

On 10 September 2016 at 00:23, Andreas Walz
<> wrote:
> Dear all,
> we are working on an approach/framework for testing TLS implementations
> (currently only servers, but clients are planned for the future as well).
> While running our tests against a bunch of different TLS (server)
> implementations, we found several types of suspicious behaviour (see below).
> As the TLS specification left me with doubts on what the correct behaviour
> should be, I'd like to raise this questions here (please let me know if this
> is not the appropriate place or this has been answered before).
> (1) Several server implementations seem to ignore the list of proposed
> compression methods in a ClientHello and simply select null compression even
> if that has not been in the ClientHello's list. The specification is rather
> clear that null compression MUST be part of the list. However, I'm not aware
> of any clear statement about what a compliant server should do in case it
> receives a ClientHello without null compression. My best guess would have
> been that in such cases the server should abort the handshake (at least if
> it does not support whatever the client proposed).
> (2) In a ClientHello several server implementations don't ignore data
> following the extension list. That is, they somehow seem to ignore the
> length field of the extension list and simply consider everything following
> the list of compression methods as extensions. Aside from this certainly
> being a deviation from the specification, I was wondering whether a server
> should silently ignore data following the extension list (e.g. for the sake
> of upward compatibility) or (as one could infer from RFC5246, p. 42) send
> e.g. a "decode_error" alert.
> (3) If a ClientHello contains multiple extensions of the same type, several
> server implementations proceed with the handshake (even if they parse these
> specific extensions). The specification again is clear that "there MUST NOT
> be more than one extension of the same type". However, what should a server
> do in case there are? Again, my guess would be that it should abort the
> handshake. Should this also be the case for extensions that a server simply
> ignores (as it e.g. doesn't know them)?
> Thank you very much.
> Cheers,
> Andi
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