Software Engineering Institute

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 http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml. 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

http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#workarounds
http://www.cisco.com/warp/public/707/racl.html
http://www.cisco.com/warp/public/707/iacl.html

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