Cisco IOS 12.2T, 12.3 and 12.3T, when using Easy VPN Server XAUTH version 6 authentication, allows remote attackers to bypass authentication via a "malformed packet."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5852
http://www.cisco.com/warp/public/707/cisco-sa-20050406-xauth.shtml