Date: Tue, 23 Aug 2011 09:51:46 +0200 From: Petr Matousek <pmatouse@...hat.com> To: oss-security@...ts.openwall.com Cc: "Steven M. Christey" <coley@...us.mitre.org> 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: > http://lwn.net/Articles/455135/ > 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 > http://git.kernel.org/linus/bc0b96b54a21246e377122d54569eef71cec535f > net: Compute protocol sequence numbers and fragment IDs using MD5 > http://git.kernel.org/linus/6e5714eaf77d79ae1c8b47e3e040ff5411b717ec > > Thanks, Eugene Please use CVE-2011-3188. Thanks, -- Petr Matousek / Red Hat Security Response Team
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