ISS Advisory: Remote Denial of Service Vulnerability in RealSecure Network Sensor

From: X-Force (
Date: 04/30/02

Date: Tue, 30 Apr 2002 14:59:20 -0400 (EDT)
From: X-Force <>


Internet Security Systems Security Advisory
April 30, 2002

Remote Denial of Service Vulnerability in RealSecure Network Sensor


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


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.


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:

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

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