Cisco IOS 12.2(15) and earlier allows remote attackers to cause a denial of service (refused VTY (virtual terminal) connections), via a crafted TCP connection to the Telnet or reverse Telnet port.
https://exchange.xforce.ibmcloud.com/vulnerabilities/17131
http://www.kb.cert.org/vuls/id/384230
http://www.cisco.com/warp/public/707/cisco-sa-20040827-telnet.shtml