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                     National Cyber Alert System

               Technical Cyber Security Alert TA05-291A


Snort Back Orifice Preprocessor Buffer Overflow

   Original release date: October 18, 2005
   Last revised: --
   Source: US-CERT


Systems Affected

     * Snort versions 2.4.0 to 2.4.2
     * Sourcefire Intrusion Sensors

   Other products that use Snort or Snort components may be affected.


Overview

   The Snort Back Orifice preprocessor contains a buffer overflow that
   could allow a remote attacker to execute arbitrary code on a
   vulnerable system.


I. Description

   Snort is a widely-deployed, open-source network intrusion detection
   system (IDS). Snort and its components are used in other IDS
   products, notably Sourcefire Intrusion Sensors, and Snort is
   included with a number of operating system distributions.

   Snort preprocessors are modular plugins that extend functionality
   by operating on packets before the detection engine is run. The
   Back Orifice preprocessor decodes packets to determine if they
   contain Back Orifice ping messages. The ping detection code does
   not adequately limit the amount of data that is read from the
   packet into a fixed-length buffer, thus creating the potential for
   a buffer overflow.

   The vulnerable code will process any UDP packet that is not
   destined to or sourced from the default Back Orifice port
   (31337/udp). An attacker could exploit this vulnerability by
   sending a specially crafted UDP packet to a host or network
   monitored by Snort.

   US-CERT is tracking this vulnerability as VU#175500. Further
   information is available in an advisory from Internet Security
   Systems (ISS).


II. Impact

   A remote attacker who can send UDP packets to a Snort sensor may be
   able to execute arbitrary code. Snort typically runs with root or
   SYSTEM privileges, so an attacker could take complete control of a
   vulnerable system. An attacker does not need to target a Snort
   sensor directly; the attacker can target any host or network
   monitored by Snort.


III. Solution

Upgrade

   Sourcefire has released Snort 2.4.3 which is available from the
   Snort download site. For information about other vendors, please
   see the Systems Affected section of VU#175500.

Disable Back Orifice Preprocessor

   To disable the Back Orifice preprocessor, comment out the line that
   loads the preprocessor in the Snort configuration file (typically
   /etc/snort.conf on UNIX and Linux systems):

     [/etc/snort.conf]
     ...
     #preprocessor bo
     ...
   
   Restart Snort for the change to take effect.

Restrict Outbound Traffic

   Consider preventing Snort sensors from initiating outbound
   connections and restricting outbound traffic to only those hosts
   and networks that have legitimate requirements to communicate with
   the sensors. While this will not prevent exploitation of the
   vulnerability, it may make it more difficult for an attacker to
   access a compromised system or reconnoiter other systems.


Appendix A. References

     * US-CERT Vulnerability Note VU#175500 -
       <http://www.kb.cert.org/vuls/id/177500>

     * Fixes and Mitigation Instructions Available for Snort Back
       Orifice Vulnerability -
       <http://www.snort.org/pub-bin/snortnews.cgi#99>

     * Snort downloads - <http://www.snort.org/dl/>

     * Snort 2.4.3 Changelog -
       <http://www.snort.org/docs/change_logs/2.4.3/Changelog.txt>

     * Preprocessors -
       <http://www.snort.org/docs/snort_htmanuals/htmanual_2.4/
       node11.html#SECTION00310000000000000000>

     * Snort Back Orifice Parsing Remote Code Execution -
       <http://xforce.iss.net/xforce/alerts/id/207>


 ____________________________________________________________________

   This vulnerability was researched and reported by Internet Security
   Systems (ISS).
 ____________________________________________________________________

   The most recent version of this document can be found at:

     <http://www.us-cert.gov/cas/techalerts/TA05-291A.html>
 ____________________________________________________________________

   Feedback can be directed to US-CERT Technical Staff. Please send
   email to <cert@cert.org> with "TA05-291A Feedback VU#175500" in the
   subject.
 ____________________________________________________________________

   For instructions on subscribing to or unsubscribing from this
   mailing list, visit <http://www.us-cert.gov/cas/signup.html>.
 ____________________________________________________________________

   Produced 2005 by US-CERT, a government organization.

   Terms of use:

     <http://www.us-cert.gov/legal.html>
 ____________________________________________________________________


Revision History

   Oct 18, 2005: Initial release


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