of Homeland Security
Cisco IOS Interface Blocked by IPv4 Packet
A complete revision history can be found at the end of this file.
All Cisco devices running Cisco IOS software and configured
process Internet Protocol version 4 (IPv4) packets
vulnerability in many versions of Cisco IOS could allow
to execute a denial-of-service attack against a vulnerable device.
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
CERT/CC is tracking this issue as VU#411332. This reference
corresponds to CVE candidate CAN-2003-0567.
sending specially crafted IPv4 packets to an interface
vulnerable device, an intruder can cause the device to stop processing
packets destined to that interface. Quoting from Cisco's advisory:
device receiving these specifically crafted IPv4 packets
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."
a patch from Cisco
a patch as described in Cisco's Advisory.
a patch can be applied, you can mitigate the risks
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
CERT Coordination Center thanks Cisco Systems of for
about this problem and for helping us to construct this advisory.
about this advisory may be directed to the author,
document is available from:
Phone: +1 888-282-0870 (24-hour toll-free hotline)
Phone: +1 703-375-4220 (24-hour hotline)
Fax: +1 703-326-9461
personnel answer the hotline 24 hours a day, 7 days
strongly urge you to encrypt sensitive information
sent by email.
Our public PGP key is available from
you prefer to use DES, please call the DHS/FedCIRC
hotline for more
publications and other security information are available
from our web site
(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
* "CERT" and "CERT
Coordination Center" are registered in the U.S.
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2003 Carnegie Mellon University.
July 16, 2003: Initial release