Cisco VPN 3000 Concentrator 2.2.x, 3.6(Rel), and 3.x before 3.5.5, allows remote attackers to cause a denial of service via (1) malformed or (2) large ISAKMP packets.
http://www.kb.cert.org/vuls/id/761651
http://www.cisco.com/warp/public/707/vpn3k-multiple-vuln-pub.shtml