Gutmann Soundwave Therapy

[Daniel Carosone]

Others have made similar points and suggestions, not picking on this
instance in particular:

On Mon, Feb 04, 2008 at 02:48:08PM -0700, Martin James Cochran wrote:
> Additionally, in order to conserve bandwidth you might want to make a 
> trade-off where some packets may be forged with small probability (in the 
> VOIP case, that means an attacker gets to select a fraction of a second of 
> sound, which is probably harmless)

This is ok, if you consider the only threat to be against the final
endpoint: a human listening to a short-term, disposable conversation.
I can think of some counter-examples where these assumptions don't
hold:

 - A data-driven exploit against an implementation vulnerability in
   your codec of choice.  Always a possibility, but a risk you might
   rate differently (or a patch you might deploy on a different
   schedule) for conversations with known and trusted peers than you
   would for arbitrary peers, let alone maliciously-inserted traffic.
   How many image decoding vulnerabilities have we seen lately, again?

 - People have invented and do use such horribly-wrong things as
   fax-over-voip; while they seem to have some belief in their own
   business case, I may not have as much faith in their implementation
   robustness.
   
 - Where it's audio, but the audience is different such that the
   impact of short bursts of malicious sound is different: larger
   teleconferences, live interviews or reporting by journalists, and
   other occasions, particularly where the credibility of the speaker
   is important.  Fractions of seconds of sound is all I might need to
   insert to .. er .. emulate tourette's syndrome. Fractions of
   seconds of soundwave therapy could still be highly unpleasant or
   embarassing.

Particularly for the first point, early validation for packet
integrity in general can be a useful defensive tool against unknown
potential implementation vulnerabilities.  I've used similar arguments
before around the use of keyed authentication of other protocols, such
as SNMPv3 and NTP.

It also reminds me of examples where cryptographic protections have
only covered certain fields in a header or message.  Attackers may
find novel ways to use the unprotected space, plus it just makes the
whole job of risk analysis at deployment orders of magnitude more
complex.

Without dismissing the rest of the economic arguments, when it comes
to these kinds of vulnerabilities, be very wary of giving an attacker
this inch, they may take a mile.  

--
Dan.
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