Note: This page is part of the archive.

This document is part of the US-CERT website archive. These documents are no longer updated and may contain outdated information. Links may also no longer function. Please contact if you have any questions about the US-CERT website archive.

Alert (TA05-026A)

Multiple Denial of Service Vulnerablities in Cisco IOS

Systems Affected

  • Cisco routers and switches running IOS in various configurations


Several denial-of-service vulnerabilities have been discovered in Cisco's Internet Operating System (IOS). A remote attacker may be able to cause an affected device to reload the operating system.


Cisco has published three advisories describing flaws in IOS that could allow a remote attacker to cause an affected device to reload. Further details are available in the following vulnerability notes:

VU#583638 - Cisco IOS contains DoS vulnerability in MPLS packet processing

The IOS implementation of Multi Protocol Label Switching (MPLS) contains a vulnerability that allows malformed MPLS packets to cause an affected device to reload. An unauthenticated attacker can send these malformed packets on a local network segment that is connected to a vulnerable device interface.

VU#472582 - Cisco IOS IPv6 denial-of-service vulnerability

A vulnerability in the way that IOS handles a sequence of specially crafted IPv6 packets could cause an affected device to reload, resulting in a denial of service. The vulnerability is exposed on both physical interfaces (i.e., hardware interfaces), and logical interfaces (i.e., software defined interfaces such as tunnels) that are configured for IPv6.

VU#689326 - Cisco IOS vulnerable to DoS via malformed BGP packet

An IOS device that is enabled for Border Gateway Protocol (BGP) and set up with the bgp log-neighbor-changes option is vulnerable to a denial-of-service attack via a malformed BGP packet.


Although the underlying causes of these three vulnerabilities is different, in each case a remote attacker could cause an affected device to reload the operating system. This creates a denial-of-service condition since packets are not forwarded through the affected device while it is reloading. Repeated exploitation of these vulnerabilites would result in a sustained denial-of-service condition.

Since devices running IOS may transit traffic for a number of other networks, the secondary impacts of a denial of service may be severe.


Upgrade to a fixed version of IOS

Cisco has updated versions of its IOS software to address these vulnerabilities. Please refer to the "Software Versions and Fixes" sections of the Cisco Security Advisories listed in Appendix A for more information on upgrading.


Cisco has also published practical workarounds for VU#689326 and VU#583638. Please refer to the "Workarounds" section of each Cisco Security Advisory listed in Appendix A for more information.

Sites that are unable to install an upgraded version of IOS are encouraged to implement these workarounds.

Appendix A. References

  • Cisco Security Advisory: Crafted Packet Causes Reload on Cisco Routers ->
  • Cisco Security Advisory: Multiple Crafted IPv6 Packets Cause Reload ->
  • Cisco Security Advisory: Cisco IOS Malformed BGP Packet Causes Reload Updated ->
  • US-CERT Vulnerability Note VU#583638 ->
  • US-CERT Vulnerability Note VU#472582 ->
  • US-CERT Vulnerability Note VU#689326 ->

Revision History

  • January 26, 2005: Initial release

    Last updated

This product is provided subject to this Notification and this Privacy & Use policy.

Was this document helpful?  Yes  |  Somewhat  |  No