Software Engineering Institute

The Cisco Internetwork Operating System (IOS) includes support for processing Internet Protocol version 6 (IPv6) packets.

Per Cisco Advisory cisco-sa-20080326-IPv4IPv6:
A device running Cisco IOS software that has Internet Protocol version 6 (IPv6) enabled may be subject to a denial of service (DoS) attack. For the device to be affected by this vulnerability the device also has to have certain Internet Protocol version 4 (IPv4) User Datagram Protocol (UDP) services enabled. To exploit this vulnerability an offending IPv6 packet must be targeted to the device. Packets that are routed throughout the router can not trigger this vulnerability. Successful exploitation will prevent the interface from receiving any additional traffic. The only exception is Resource Reservation Protocol (RSVP) service, which if exploited, will cause the device to crash. Only the interface on which the vulnerability was exploited will be affected.

A remote, unauthenticated attacker could cause a vulnerable system to crash or stop forwarding network traffic.

Upgrade

Cisco has made updates available to address this issue.

Disable IPv6

If it is not needed, disabling IPv6 access will mitigate this vulnerability. See the workarounds section of cisco-sa-20080326-IPv4IPv6 for more information.

Disable IPv4 UDP

If it is not needed, disabling UDP on IPv4 enabled interfaces will mitigate this vulnerability. See the workarounds section of cisco-sa-20080326-IPv4IPv6 for more information.