Re: AD review of draft-ietf-httpbis-alt-svc-10

Mark Nottingham <mnot@mnot.net> Sun, 03 January 2016 21:04 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Mon, 04 Jan 2016 08:01:01 +1100
Cc: "Julian F. Reschke" <julian.reschke@gmx.de>, HTTP Working Group <ietf-http-wg@w3.org>
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Subject: Re: AD review of draft-ietf-httpbis-alt-svc-10
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On 1 Jan 2016, at 3:38 am, Barry Leiba <barryleiba@computer.org> wrote:

>    Furthermore, if the connection to the alternative service fails or is
>    unresponsive, the client MAY fall back to using the origin or another
>    alternative service.  Note, however, that this could be the basis of
>    a downgrade attack, thus losing any enhanced security properties of
>    the alternative service.  If the connection to the alternative
>    service does not negotiate the expected protocol (for example, ALPN
>    fails to negotiate h2, or an Upgrade request to h2c is not accepted),
>    the connection to the alternative service MUST be considered to have
>    failed.
> 
> I don't understand how this stops a downgrade attack if the alternative
> service has better security than the existing connection.  The attacker
> prevents me from establishing the better security, so I consider the
> alternative service to have failed and fall back to the existing
> connection... and the attack has succeeded, blocking me from upgrading
> the security.  No?

IIRC that last sentence is NOT being offered as a mitigation of the downgrade, but instead was something added later. Perhaps just some editorial rearranging would help here.

Cheers,

--
Mark Nottingham   https://www.mnot.net/