AUSCERT External Security Bulletin Redistribution

                    ESB-2002.051 -- ISS Security Alert
            Remote Denial of Service Vulnerability in Snort IDS
                              30 January 2002


        AusCERT Security Bulletin Summary

Product:                Snort IDS
Impact:                 Denial of Service
Access Required:        Remote

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Internet Security Systems Security Alert
January 28, 2002

Remote Denial of Service Vulnerability in Snort IDS


Internet Security Systems (ISS) X-Force is aware of a remote Denial of
service (DoS) vulnerability present in Marty Roesch’s Snort Intrusion
Detection System (IDS). It may be possible for remote attackers to send
specially crafted ICMP packets to the program, resulting in a
segmentation fault that would crash the Snort engine. This attack can be
launched from any routable address, and if launched successfully against
a Snort-protected network, all IDS functionality may be disabled until
Snort is manually restarted.

Affected Versions:

Marty Roesch Snort Version 1.8.3 and earlier for all supported platforms


Snort is an open-source Intrusion Detection System designed to be simple
and lightweight.  Snort has packet logging, protocol analysis, attack
signature matching and recognition capabilities and is maintained by
Marty Roesch of Snort.org.

An exploit has been published that demonstrates a flaw in the ICMP
protocol handling functionality. Snort incorrectly handles ICMP "Echo"
and ICMP "Echo-Reply" packets that contain less than 5 bytes of ICMP
data. If Snort encounters such a packet, it will crash and exit. Packets
that are used to exploit this vulnerability can be sent with the "ping"
command that is present on most operating systems.

This exploit technique has been publicly documented, and attackers do
not need to have access to the target network or possess knowledge of
its configuration.


ISS X-Force recommends that all Snort users install the vendor-supplied
patch immediately or upgrade to the latest version of Snort.

To apply a source code patch to your Snort package:

1. Locate the "decode.h" file in your source distribution.
2. Enter the directory containing decode.h.
3. To update your decode.h file, create a file named "decode.diff",
   containing the following text:
- - --- olddecode.h Thu Jan 10 15:47:48 2002
+++ decode.h    Thu Jan 10 12:15:33 2002
@@ -105,7 +105,7 @@
 #define IP_HEADER_LEN           20
 #define TCP_HEADER_LEN          20
 #define UDP_HEADER_LEN          8
- - -#define ICMP_HEADER_LEN         8
+#define ICMP_HEADER_LEN         4

 #define TH_FIN  0x01
 #define TH_SYN  0x02
4. Apply the source code update using the "patch" command, or a similar
5. Build new binaries and reinstall.

To upgrade to the latest version of Snort:

Use a CVS client to access the Snort CVS server at
"cvs.snort.sourceforge.net" with the following command:

cvs -d:pserver:anonymous@cvs.snort.sourceforge.net:/cvsroot/snort login

Use a blank password when prompted.

cvs -z3 -d:pserver:anonymous@cvs.snort.sourceforge.net:/cvsroot/snort co

Snort'’s default configuration does not have the ability to restat
when it crashes. ISS X-Force advises all Snort users to develop this
functionality using freely available watchdog process monitors,
cronjobs, or shell scripts.

For more information about applying source code patches or upgrading
Snort, please refer to the "SNORT FAQ" document available at:

Additional Information:

ISS X-Force Database,

Marty Roesch Snort,


This vulnerability was discovered by Sinbad <securitymail@263.net>, and
reported to the BugTraq mailing list.


About Internet Security Systems (ISS)
Internet Security Systems is a leading global provider of security
management solutions for the Internet, protecting digital assets and
ensuring safe and uninterrupted e-business.  With its industry-leading
intrusion detection and vulnerability assessment, remote managed
security services, and strategic consulting and education offerings, ISS
is a trusted security provider to more than 9,000 customers worldwide
including 21 of the 25 largest U.S. commercial banks, the top 10 U.S.
telecommunications companies, and all major branches of the U.S. Federal
Government.  Founded in 1994, ISS is headquartered in Atlanta, GA, with
additional offices throughout North America and international operations
in Asia, Australia, Europe, Latin America and the Middle East.  For more
information, visit the Internet Security Systems web site at www.iss.net
or call 888-901-7477.

Copyright (c) 2002 Internet Security Systems, Inc. All rights reserved

Permission is hereby granted for the redistribution of this Alert
electronically. It is not to be edited in any way without express
consent of the X-Force. If you wish to reprint the whole or any part
of this Alert in any other medium excluding electronic medium, please
e-mail xforce@iss.net for permission.


The information within this paper may change without notice. Use of
this information constitutes acceptance for use in an AS IS condition.
There are NO warranties with regard to this information. In no event
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X-Force PGP Key available at: http://xforce.iss.net/sensitive.php
as well as on MIT's PGP key server and PGP.com's key server.

Please send suggestions, updates, and comments to: X-Force
xforce@iss.net of Internet Security Systems, Inc.

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