Bug 447870 (CVE-2008-1804)

iDefense released a security advisory for snort:

http://labs.idefense.com/intelligence/vulnerabilities/display.php?id=701

DESCRIPTION
Remote exploitation of a design error vulnerability in Snort, as included in
various vendors' operating system distributions, could allow an attacker to
bypass filter rules.

Due to a design error vulnerability, Snort does not properly reassemble
fragmented IP packets. When receiving incoming fragments, Snort checks the Time
To Live (TTL) value of the fragment, and compares it to the TTL of the initial
fragment. If the difference between the initial fragment and the following
fragments is more than a configured amount, the fragments will be silently
discard. This results in valid traffic not being examined and/or filtered by Snort.

ANALYSIS
Exploitation of this vulnerability allows an attacker to bypass all Snort rules.
In order to exploit this vulnerability, an attacker would have to fragment IP
packets destined for a targeted host, ensuring that the TTL difference is
greater than the configured maximum. By default, the maximum difference is 5.

If an attacker is successful, all fragments with invalid TTL differences will be
dropped. No rules will be applied to them.

DETECTION
iDefense has confirmed the existence of this vulnerability in Snort 2.8 and 2.6.
Snort 2.4 is not vulnerable.

WORKAROUND
In the snort.conf file, set the ttl_limit configuration value to 255 as shown below.

  preprocessor frag3_engine: ttl_limit 255

This will set the allowable difference to the maximum possible value, and
prevent fragments from being dropped.

VENDOR RESPONSE
Sourcefire has addressed this vulnerability by releasing version 2.8.1 of Snort. 

Upstream patches:
http://cvs.snort.org/viewcvs.cgi/snort/ChangeLog.diff?r1=1.544&r2=1.545 (part)
http://cvs.snort.org/viewcvs.cgi/snort/src/preprocessors/spp_frag3.c.diff?r1=1.50&r2=1.51
http://cvs.snort.org/viewcvs.cgi/snort/src/generators.h.diff?r1=1.63&r2=1.64
http://cvs.snort.org/viewcvs.cgi/snort/etc/gen-msg.map.diff?r1=1.43&r2=1.44
http://cvs.snort.org/viewcvs.cgi/snort/doc/README.frag3.diff?r1=1.7&r2=1.8
http://cvs.snort.org/viewcvs.cgi/snort/doc/snort_manual.tex.diff?r1=1.98&r2=1.99
+ updated version of snort_manual.pdf

Based on Detection part of the advisory, this should affect snort packages in F7
- F9, rawhide already has 2.8.1 which has changes above included.