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Credit:
The information has been provided by iDefense Labs Security Advisories.
The original article can be found at: http://labs.idefense.com/intelligence/vulnerabilities/display.php?id=701
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Vulnerable Systems:
* Snort version 2.8
* Snort version 2.6
Immune Systems:
* Snort version 2.4
* Snort version 2.8.1
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.
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. For more information consult their change log and source differences at the following URLs.
http://cvs.snort.org/viewcvs.cgi/snort/ChangeLog?rev=1.534.2.11
http://cvs.snort.org/viewcvs.cgi/snort/src/preprocessors/spp_frag3.c.diff?r1=text&tr1=1.46.2.4&r2=text&tr2=1.46.2.5&diff_format=h
CVE Information:
CVE-2008-1804
Disclosure Timeline:
02/26/2008 - Initial vendor notification
02/26/2008 - Initial vendor response
05/21/2008 - Coordinated public disclosure
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