Cisco Security Advisory – A series of TCP packets may cause a denial of service (DoS) condition on Cisco IOS devices that are configured as Easy VPN servers with the Cisco Tunneling Control Protocol (cTCP) encapsulation feature. Cisco has released free software updates that address this vulnerability. No workarounds are available; however, the IPSec NAT traversal (NAT-T) feature can be used as an alternative.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/76060/cisco-sa-20090325-ctcp.txt
Source: https://packetstormsecurity.com/files/76060/Cisco-Security-Advisory-20090325-ctcp.html