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Internet Security Systems Security Advisory 
April 30, 2002 

Remote Denial of Service Vulnerability in RealSecure Network Sensor 

Synopsis: 

ISS X-Force has learned of a denial of service (DoS) vulnerability that 
affects Internet Security Systems RealSecure Network Sensor. This 
vulnerability may allow remote attackers to crash RealSecure by sending 
specially crafted packets to network segments monitored by RealSecure. 
RealSecure X-Press Update 4.3 contains a fix for the DHCP vulnerability 
and is available for immediate download on the ISS download center. 

Affected Versions: 

RealSecure Network Sensor 5.x, XPU 3.4 and later 
RealSecure Network Sensor 6.0, XPU 3.4 and later 
RealSecure Network Sensor 6.5 

Description: 

RealSecure Network Sensor has three informational signatures associated 
with DHCP (Dynamic Host Configuration Protocol): DHCP_ACK (7131), 
DHCP_Discover (7132), and DHCP_Request (7133). These signatures contain 
a flaw that will result in an illegal attempt to de-reference a null 
memory pointer when RealSecure detects certain types of DHCP traffic. 
This action may generate an exception error or a segmentation fault 
which can cause the RealSecure sensor to crash. This vulnerability was 
introduced in RealSecure Network Sensor 6.5. XPU 3.4 delivered the 
vulnerable DHCP signatures to older RealSecure product lines including 
6.0 and 5.x. 

It may be possible for remote attackers to create specially-crafted DHCP 
traffic to cause the sensor to malfunction or crash entirely. The three 
DHCP signatures were disabled by default in Network Sensor 5.x and 6.0. 
The signatures were enabled by default in Network Sensor 6.5 within the 
"Maximum" policy. However, if these signatures are not enabled, 
RealSecure Network Sensor is not vulnerable to these attacks. 

Recommendations: 

X-Force recommends that all RealSecure customers tune their policies to 
their environments. RealSecure X-Press Update 4.3 contains a fix for the 
DHCP vulnerability. X-Press Update 4.3 is available for download on the 
ISS download center: http://www.iss.net/download/. 

DHCP traffic is commonly blocked at perimeter firewalls. Network 
administrators are advised to assess their network perimeter defenses 
routinely. Exploitation of this vulnerability is blocked by proper 
filtering of DHCP traffic on UDP port 67. 

______ 

About Internet Security Systems (ISS) 
Founded in 1994, Internet Security Systems (ISS) (Nasdaq: ISSX) is a 
pioneer and world leader in software and services that protect critical 
online resources from an ever-changing spectrum of threats and misuse. 
Internet Security Systems is headquartered in Atlanta, GA, with 
additional operations throughout the Americas, Asia, Australia, Europe 
and the Middle East. 

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

Permission is hereby granted for the electronic redistribution of this 
document. It is not to be edited or altered in any way without the 
express written consent of the Internet Security Systems X-Force. If you 
wish to reprint the whole or any part of this document in any other 
medium excluding electronic media, please email xforce@iss.net for 
permission. 

Disclaimer: 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, implied or otherwise, with regard to 
this information or its use. Any use of this information is at the 
user's risk. In no event shall the author/distributor (Internet Security 
Systems X-Force) be held liable for any damages whatsoever arising out 
of or in connection with the use or spread of this information. 

X-Force PGP Key available on MIT's PGP key server and PGP.com's key 
server, 
as well as at http://www.iss.net/security_center/sensitive.php 

Please send suggestions, updates, and comments to: X-Force 

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