Cisco VPN 3000 series concentrators before 2.5.2(F) allow remote attackers to cause a denial of service via a flood of invalid login requests to (1) the SSL service, or (2) the telnet service, which do not properly disconnect the user after several failed login attempts.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6298
http://www.cisco.com/warp/public/707/vpn3k-telnet-vuln-pub.shtml