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NIPC Seal National Infrastructure Protection Center
NIPC Seal CyberNotes

Department of Homeland Security


Cisco IOS Interface Blocked by IPv4 Packet

July 16, 2003

A complete revision history can be found at the end of this file.

Systems Affected

* All Cisco devices running Cisco IOS software and configured to
process Internet Protocol version 4 (IPv4) packets


A vulnerability in many versions of Cisco IOS could allow an intruder
to execute a denial-of-service attack against a vulnerable device.

I. Description

Cisco IOS is a very widely deployed network operating system.
A vulnerability in IOS could allow an intruder to execute a
denial-of-service attack against an affected device. Cisco has
published an advisory on this topic, available at
We strongly encourage sites using IOS to read this document and take
appropriate action.

The CERT/CC is tracking this issue as VU#411332. This reference number
corresponds to CVE candidate CAN-2003-0567.

II. Impact

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:

"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."

III. Solution

Apply a patch from Cisco

Apply a patch as described in Cisco's Advisory.

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 of your network topology.
Additionally, ACLs may degrade performace on some systems. We
recommend reviewing the following before applying ACLs


The CERT Coordination Center thanks Cisco Systems of for notifying us
about this problem and for helping us to construct this advisory.

Feedback about this advisory may be directed to the author, Shawn

This document is available from:

DHS/FedCIRC Contact Information

Email: fedcirc@fedcirc.gov
Phone: +1 888-282-0870 (24-hour toll-free hotline)
Phone: +1 703-375-4220 (24-hour hotline)
Fax: +1 703-326-9461

DHS/FedCIRC personnel answer the hotline 24 hours a day, 7 days a

Using encryption

We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from


If you prefer to use DES, please call the DHS/FedCIRC hotline for more

Getting security information

DHS/FedCIRC publications and other security information are available
from our web site


DHS/FedCIRC (Federal Computer Incident Response Center) provides
security services to U.S. Federal civilian agencies. DHS/FedCIRC is a
component of the Department of Homeland Security (DHS) Information
Assurance and Infrastructure Protection Directorate. The CERT
Coordination Center performs incident and vulnerability analysis and
issues advisories.

* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.


Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
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does not make any warranty of any kind with respect to freedom from
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Copyright 2003 Carnegie Mellon University.

Revision History
July 16, 2003: Initial release