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Multiple Vendor Snort IP Fragment TTL Evasion Vulnerability

21.05.08

BACKGROUND

Snort is an open source network intrusion detection (IDS) and prevention system (IPS). In addition to being available as a package for most Unix operating system distributions, various commercial hardware devices also use Snort as an IDS/IPS. For more information, see the vendor's website found at the following URL.

http://www.snort.org/

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 discarded. 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. 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

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2008-1804 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org/), which standardizes names for security problems.

DISCLOSURE TIMELINE

02/26/2008 Initial vendor notification
02/26/2008 Initial vendor response
05/21/2008 Coordinated public disclosure

CREDIT

This vulnerability was reported to iDefense by Silvio Cesare.

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LEGAL NOTICES

Copyright © 2008 Verisign, Inc.

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