Vulnerability Note VU#411332

Cisco IOS Interface Blocked by IPv4 Packet

Original Release date: 17 Jul 2003 | Last revised: 17 Jul 2003


A denial-of-service vulnerability exists in Cisco's Internetwork Operating System (IOS). This vulnerability may allow remote attackers to conduct denial-of-service attacks on an affected device.


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.


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."


Apply a patch as described in the Cisco 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

It is important to understand your network requirements before making changes.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
Cisco Systems Inc.Affected-17 Jul 2003
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



Thanks to Cisco Systems for reporting this vulnerability.

This document was written by Shawn V Hernan.

Other Information

  • CVE IDs: CAN-2003-0567
  • CERT Advisory: CA-2003-15
  • Date Public: 16 Jul 2003
  • Date First Published: 17 Jul 2003
  • Date Last Updated: 17 Jul 2003
  • Severity Metric: 73.50
  • Document Revision: 24


If you have feedback, comments, or additional information about this vulnerability, please send us email.