Cisco VPN 3000 Concentrator before 2.5.2(F), with encryption enabled, allows remote attackers to cause a denial of service (reload) via a Windows-based PPTP client with the "No Encryption" option set.
http://www.securityfocus.com/bid/5625
http://www.iss.net/security_center/static/10021.php
http://www.cisco.com/warp/public/707/vpn3k-multiple-vuln-pub.shtml