APC Web/SNMP Management Card prior to Firmware 310 only supports one telnet connection, which allows a remote attacker to create a denial of service via repeated failed logon attempts which temporarily locks the card.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6199
https://euvd.enisa.europa.eu/vulnerability/EUVD-2001-0556
http://www.securityfocus.com/bid/2430
http://archives.neohapsis.com/archives/bugtraq/2001-02/0436.html