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Cisco high-end routers in DoS peril

12000 series flaw is causing ISP pain

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Cisco has issued a fix for a flaw which potentially makes its high-end routers susceptible to denial of service attacks.

The performance of Cisco 12000 series routers can be degraded when they have to send a large number of Internet Control Message Protocol (ICMPs) unreachable packets, something that usually happens as a result of heavy network scanning.

In an advisory on the issue, Cisco spells out the potential gravity of the problem and also admits some ISPs "have (been) experiencing difficulties" due to the vulnerability.

"Exploitation of this vulnerability may lead to Denial-of-Service. The router's performance will degrade and, in the worst case scenario, the router will stop forwarding packets," the notice states.

The problem is restricted to certain line cards in Cisco high-end 12000 series routers.

Cisco has suggested workarounds which involve either preventing a router from sending ICMP unreachable packets at all, or rate-limiting them.

It has also provided a schedule for updates to its IOS software that will provide a more complete solution to the problem. Since this availability is dependant on the underlying technology (or engine) an individual line card is based upon, service provider users will have to first determine this before looking up what they need to do.

A less serious problem means that one of four 'engines' that underpin line cards used in the 12000 series has a series of six vulnerabilities involving the implementation of Access Control Lists. Cisco has published an advisory which informs users how to deal with the problem here. ®

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