Pakistan Computer Emergency Response Team

The Security Standard


Copyright | Disclaimer




CA-2003-13 Multiple Vulnerabilities in Snort Preprocessors


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:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from

If you prefer to use DES, please call the CERT hotline for more

Getting security information

CERT publications and other security information are available from
our web site

To subscribe to the CERT mailing list for advisories and bulletins,
send email to Please include in the body of your

subscribe cert-advisory

* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.

Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.

Conditions for use, disclaimers, and sponsorship information

Copyright 2003 Carnegie Mellon University.

Revision History
April 17, 2003: Initial release

Version: PGP 6.5.8


All rights reserved. Copyright© PakCERT 2000-2017