Re: [TLS] How ALPN makes the http2-tls-relaxed option less secure, compared to NPN (was Re: ALPN concerns)

Martin Thomson <martin.thomson@gmail.com> Mon, 09 December 2013 21:32 UTC

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Date: Mon, 9 Dec 2013 13:32:34 -0800
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From: Martin Thomson <martin.thomson@gmail.com>
To: Andrei Popov <Andrei.Popov@microsoft.com>
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Cc: Peter Gutmann <p.gutmann@auckland.ac.nz>, Alfredo Pironti <alfredo@pironti.eu>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [TLS] How ALPN makes the http2-tls-relaxed option less secure, compared to NPN (was Re: ALPN concerns)
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On 9 December 2013 12:59, Andrei Popov <Andrei.Popov@microsoft.com> wrote:
> If I understand correctly, an active attacker can prevent http2-tls-relaxed from happening by manipulating Alt-Svc headers, regardless of whether ALPN or NPN is used.

An active attacker can always exploit a willingness on the part of a
client to settle for something less than their best effort at
authenticating the server, yes.  This goes for Alt-Svc, Upgrade, or
any other scheme where the default behaviour is HTTP/1.1 in the clear.
 As long as the client is willing to do anything without
authentication, they are vulnerable to an active attack of this
nature.