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Date: Fri, 27 Feb 2015 12:59:57 +0100
From: Florent Daigniere <florent.daigniere@...stmatta.com>
To: oss-security@...ts.openwall.com
Subject: CVE request: RFC 4253 section 8 wooes

Hi,

RFC 4253 section 8 describes how the DiffieHellman exchange is done in
SSH... It mandates a few sanity bound-checks (for both the values of
exponents and exponentials) that some implementations are not doing...

Can you please assign three CVEs for the following bugs?

MATTA-2015-002 PuTTY
will be fixed in the upcoming release (0.64 I think)
- The exponential is not checked for trivial values

MATTA-2015-001 Dropbox
fixed in: https://secure.ucc.asn.au/hg/dropbear/rev/a1e79ffa5862
- The exponential is not checked for all trivial values (it just does
what the RFC mandates, which is clearly not enough!)
- The exponent picked might be a trivial value (this is theoretical more
than anything else assuming the CSPRNG is working). It's a regression
from 0.49
(https://secure.ucc.asn.au/hg/dropbear/diff/00703f1df67a/random.c)

Further details and a full advisory will be published at 
https://www.trustmatta.com/advisories/MATTA-2015-001.txt
https://www.trustmatta.com/advisories/MATTA-2015-002.txt
when the patches are in a released build. Our current understanding is
that no third party can take advantage of those bugs unless both the
client and the server are vulnerable AND either side picks a weak
exponent. The likelihood of that happening in practice is almost nil and
the impact limited in any case.

Regards,
	Florent

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