Cisco VPN 3000 Concentrator 2.2.x, and 3.x before 3.5.3, allows remote attackers to cause a denial of service (crash) via a long (1) username or (2) password to the HTML login interface.
http://www.securityfocus.com/bid/5617
http://www.iss.net/security_center/static/10025.php
http://www.cisco.com/warp/public/707/vpn3k-multiple-vuln-pub.shtml