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Date: Tue, 23 Aug 2011 09:51:46 +0200
From: Petr Matousek <>
Cc: "Steven M. Christey" <>
Subject: Re: CVE request: kernel: change in how tcp seq
 numbers are generated

On Tue, Aug 23, 2011 at 03:47:37PM +0800, Eugene Teo wrote:
> Dan Kaminsky pointed out that using partial MD4 and using that to
> generate a sequence number, of which only 24-bits are truly unguessable,
> seriously undermine the goals of random sequence number generation.
> In particular, with only 24-bits being truly unguessable, packet
> injection into a session using even something like brute force is a real
> potential possibility.
> We only use 24-bits because we regenerate the random number every 5
> minutes "just in case."  But what does is trade a "we don't know" kind
> of theoretical issue for a provably real one (brute force attack).
> Therefore [Dave Miller] moving us more in line with RFC1948 (as well as
> OpenBSD and Solaris), to use MD5 and a full 32-bit result in the
> generated sequence number.
> MD5 was selected as a compromise between performance loss and
> theoretical ability to be compromised.  Willy Tarreau did extensive
> testing and SHA1 was found to harm performance too much to be considered
> seriously at this time.
> We may later add a sysctl for various modes (ie. a "super secure" mode
> that uses SHA1 if people want that, and an "insecure" mode that doesn't
> use cryptographic hashing at all for people in protected environments
> where that might be safe to do).
> [Dave Miller] also moved the sequence number generators out of random.c
> (they never really belonged there, and are only there due to historical
> artifacts), and fixed a bug in DCCP sequence number generation (on ipv6
> the 43-bit sequence number was truncated to 32-bits).
> Upstream commits:
> crypto: Move md5_transform to lib/md5.c
> net: Compute protocol sequence numbers and fragment IDs using MD5
> Thanks, Eugene

Please use CVE-2011-3188.

Petr Matousek / Red Hat Security Response Team

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