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Subject: CERT Advisory CA-2003-13 Multiple Vulnerabilities in Snort Preprocessors
From: CERT Advisory <This email address is being protected from spambots. You need JavaScript enabled to view it.>
Date: Thu, 17 Apr 2003 11:29:29 -0400


CERT Advisory CA-2003-13 Multiple Vulnerabilities in Snort Preprocessors

   Original release date: April 17, 2003
   Last revised: --
   Source: CERT/CC

   A complete revision history can be found at the end of this file.

Systems Affected

     * Snort IDS, versions 1.8 through 2.0 RC1


   There are two vulnerabilities in the Snort Intrusion Detection System,
   each  in  a  separate  preprocessor module. Both vulnerabilities allow
   remote  attackers to execute arbitrary code with the privileges of the
   user running Snort, typically root.

I. Description

   The   Snort  intrusion  detection  system  ships  with  a  variety  of
   preprocessor  modules  that  allow  the  user  to  selectively include
   additional    functionality.    Researchers   from   two   independent
   organizations have discovered vulnerabilities in two of these modules,
   the  RPC  preprocessor  and  the  "stream4"  TCP  fragment  reassembly

   For additional information regarding Snort, please see

   VU#139129 - Heap overflow in Snort "stream4" preprocessor (CAN-2003-0029)

   Researchers  at  CORE Security Technologies have discovered a remotely
   exploitable  heap overflow in the Snort "stream4" preprocessor module.
   This  module  allows  Snort  to  reassemble  TCP  packet fragments for
   further analysis.

   To  exploit  this  vulnerability,  an  attacker must disrupt the state
   tracking  mechanism  of the preprocessor module by sending a series of
   packets  with  crafted  sequence  numbers.  This  causes the module to
   bypass a check for buffer overflow attempts and allows the attacker to
   insert arbitrary code into the heap.

   For additional information, please read the Core Security Technologies
   Advisory located at

   This  vulnerability affects Snort versions 1.8.x, 1.9.x, and 2.0 prior
   to  RC1. Snort has published an advisory regarding this vulnerability;
   it is available at

   VU#916785 - Buffer overflow in Snort RPC preprocessor (CAN-2003-0033)

   Researchers  at  Internet  Security  Systems  (ISS)  have discovered a
   remotely  exploitable  buffer  overflow  in the Snort RPC preprocessor
   module.  Martin  Roesch,  primary  developer  for Snort, described the
   vulnerability as follows:

     When the RPC decoder normalizes fragmented RPC records, it
     incorrectly checks the lengths of what is being normalized against
     the current packet size, leading to an overflow condition. The RPC
     preprocessor is enabled by default.

   For  additional  information,  please  read  the  ISS X-Force advisory
   located at

   This  vulnerability  affects  Snort  versions  1.8.x through 1.9.1 and
   version 2.0 Beta.

II. Impact

   Both  VU#139129  and  VU#916785  allow  remote  attackers  to  execute
   arbitrary  code  with  the  privileges  of  the  user  running  Snort,
   typically  root.  In addition, it is not necessary for the attacker to
   know  the  IP  address of the Snort device they wish to attack; merely
   sending  malicious  traffic  where  it  can be observed by an affected
   Snort sensor is sufficient to exploit these vulnerabilities.

III. Solution

Upgrade to Snort 2.0

   Both VU#139129 and VU#916785 are addressed in Snort version 2.0, which
   is available at

   Binary-only versions of Snort are available from

   For  information  from  other  vendors  that ship affected versions of
   Snort, please see Appendix A of this document.

Disable affected preprocessor modules

   Sites  that  are  unable to immediately upgrade affected Snort sensors
   may  prevent  exploitation of this vulnerability by commenting out the
   affected preprocessor modules in the "snort.conf" configuration file.

   To prevent exploitation of VU#139129, comment out the following line:

     preprocessor stream4_reassemble

   To prevent exploitation of VU#916785, comment out the following line:

     preprocessor rpc_decode: 111 32771

   After commenting out the affected modules, send a SIGHUP signal to the
   affected   Snort  process  to  update  the  configuration.  Note  that
   disabling these modules may have adverse affects on a sensor's ability
   to correctly process RPC record fragments and TCP packet fragments. In
   particular,  disabling  the "stream4" preprocessor module will prevent
   the Snort sensor from detecting a variety of IDS evasion attacks.

Block outbound packets from Snort IDS systems

   You  may  be  able  limit  an attacker's capabilities if the system is
   compromised  by  blocking  all outbound traffic from the Snort sensor.
   While   this   workaround   will   not  prevent  exploitation  of  the
   vulnerability,  it  may  make  it  more  difficult for the attacker to
   create a useful exploit.

Appendix A. - Vendor Information

   This  appendix  contains  information  provided  by  vendors  for this
   advisory.  As  vendors  report new information to the CERT/CC, we will
   update this section and note the changes in our revision history. If a
   particular  vendor  is  not  listed  below, we have not received their

Apple Computer, Inc.

   Snort is not shipped with Mac OS X or Mac OS X Server.

Ingrian Networks

   Ingrian  Networks  products  are  not  susceptible  to  VU#139129  and
   VU#916785 since they do not use Snort.

   Ingrian  customers  who  are  using the IDS Extender Service Engine to
   mirror  cleartext  data  to a Snort-based IDS should upgrade their IDS


   NetBSD does not include snort in the base system.

   Snort  is  available from the 3rd party software system, pkgsrc. Users
   who  have  installed  net/snort,  net/snort-mysql  or  net/snort-pgsql
   should  update  to a fixed version. pkgsrc/security/audit-packages can
   be used to keep up to date with these types of issues.

Red Hat Inc.

   Not  vulnerable.  Red  Hat does not ship Snort in any of our supported


   SGI does not ship snort as part of IRIX.


   Snort  2.0 has undergone an external third party professional security
   audit funded by Sourcefire.

   The  CERT/CC  acknowledges  Bruce Leidl, Juan Pablo Martinez Kuhn, and
   Alejandro David Weil of Core Security Technologies for their discovery
   of  VU#139129.  We  also  acknowledge  Mark Dowd and Neel Mehta of ISS
   X-Force for their discovery of VU#916785.

   Authors: Jeffrey P. Lanza and Cory F. Cohen.

   This document is available from:

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   Revision History
April 17, 2003:  Initial release

Version: PGP 6.5.8


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