Cisco PIX Firewall 515 and 520 with 5.1.4 OS running aaa authentication to a TACACS+ server allows remote attackers to cause a denial of service via a large number of authentication requests.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6353
https://euvd.enisa.europa.eu/vulnerability/EUVD-2001-0372
http://www.securityfocus.com/bid/2551
http://www.cisco.com/warp/public/707/pixfirewall-authen-flood-pub.shtml