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Snort IP Fragment TTL Evasion Vulnerability
iDefense Security Advisory 05.21.08: Multiple Vendor Snort IP Fragment TTL Evasion Vulnerability
iDefense Security Advisory 05.21.08: Multiple Vendor Snort IP Fragment TTL Evasion Vulnerability

iDefense Security Advisory 05.21.08 
May 21, 2008


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. 


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.


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.


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


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.


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. 


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 (, which standardizes names for 
security problems.


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


This vulnerability was reported to iDefense by Silvio Cesare.

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Copyright =A9 2008 iDefense, Inc.

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