Multiple Vulnerabilities in Snort Preprocessors
Last revised: April 23, 2003
A complete revision history can be found at the end of this file.
- For VU#139129: Snort versions 1.8.x, 1.9.x, and 2.0 prior to RC1.
- For VU#916785: Snort versions 1.8.x through 1.9.0 and 2.0 Beta.
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.
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 preprocessor.
For additional information regarding Snort, please see http://www.snort.org/.
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, including Snort 1.9.1. Snort has published an advisory regarding this vulnerability; it is available at http://www.snort.org/advisories/snort-2003-04-16-1.txt.
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
Both VU#139129 and VU#916785 allow remote attackers to execute
arbitrary code with the privileges of the user running Snort, typically
root. Please note that 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.
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:
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 effects 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 comments.
Apple Computer, Inc.
Ingrian Networks products are not susceptible to VU#139129 and VU#916785 since they do not use Snort.
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.
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.
Copyright 2003 Carnegie Mellon University.
Apr 17, 2003: Initial release
Apr 17, 2003: Fixed CVE candidate reference for VU#139129; now reads "CAN-2003-0209"
Apr 17, 2003: Minor grammar changes in Impact section
Apr 22, 2003: Fixed spelling error in Solution section
Apr 23, 2003: Revised Systems Affected section
Apr 23, 2003: Revised systems affected information in Description section