Original release date: July 18, 2003<br>
Last revised: --<br>
Source: CERT/CC<br>

<p>A complete revision history can be found at the end of this file.</p>

<a name="affected"></a>
<h3>Systems Affected</h3>

<li>All Cisco devices running Cisco IOS software and configured to
process Internet Protocol version 4 (IPv4) packets </li> </ul>

<a name="overview"></a>

An exploit has been posted publicly for the vulnerability described in
<a href="http://www.kb.cert.org/vuls/id/411332">VU#411332</a>, which
was announced in 

<a href="http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml">http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml</a>

<a name="description"></a>
<h2>I. Description</h2>

<p>An exploit has been posted publicly for <a
href="http://www.kb.cert.org/vuls/id/411332">VU#411332</a>. This
exploit allows an attacker to interrupt the normal operation of a
vulnerable device. We believe it is likely that intruders will begin
using this or other exploits to cause service outages. </p>

<!--#include virtual="/include/vu411332-advice.html" -->

<p>Many large service providers have already taken action or are in
the midst of upgrading. However, if you have not already taken action,
we strongly encourage you to review the advisory provided by Cisco and
take action in accordance with your site's maintenance and change
management procedures. Cisco's advisory can be found at

<a href="http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml">http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml</a>

<p>The CERT/CC will continue to provide information about this
vulnerability through <A
HREF="http://www.kb.cert.org/vuls/id/411332">VU#411332</A>.  </p>

<p>Any information regarding intruder activity related to this
vulnerability will be posted to the CERT/CC Currect Activity page,
available at


<a name="impact"></a>
<h2>II. Impact</h2>

<p>By sending specially crafted IPv4 packets to an interface on a
vulnerable device, an intruder can cause the device to stop processing
packets destined to that interface. Quoting from Cisco's advisory:</p>
<i>A device receiving these specifically crafted IPv4 packets will
force the inbound interface to stop processing traffic. The device may
stop processing packets destined to the router, including routing
protocol packets and ARP packets. No alarms will be triggered, nor
will the router reload to correct itself. This issue can affect all
Cisco devices running Cisco IOS software. This vulnerability may be
exercised repeatedly resulting in loss of availability until a
workaround has been applied or the device has been upgraded to a fixed
version of code.</i> </blockquote>

<a name="solution"></a>
<h2>III. Solution</h2>

<h4>Apply a patch from Cisco</h4>

Upgrade as described in <a

<h4>Restrict access</h4>

<p>Until a patch can be applied, you can mitigate the risks presented
by this vulnerability by judicious use of access control lists (ACLs). The
correct use of ACLs depends on your network topology. Additionally,
ACLs may degrade performance on some systems. We recommend reviewing
the following before applying ACLs:</p>

<a href="http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#workarounds">http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#workarounds</a>

<hr noshade>

<p>The CERT Coordination Center thanks Cisco Systems for notifying
us about this problem and for helping us to construct this advisory.</p>


<hr noshade>

<p>Authors: <a
href="mailto:cert@cert.org?subject=CA-2003-17%20Feedback%20VU%23411332/CERT%2324229">Shawn Hernan and Martin Lindner</a>


<!--#include virtual="/include/footer_nocopyright2.html" -->

<p>Copyright 2003 Carnegie Mellon University.</p>

<p>Revision History
July 18, 2003:  Initial release