Cisco switches that support 802.1x security allow remote attackers to bypass port security and gain access to the VLAN via spoofed Cisco Discovery Protocol (CDP) messages.
https://exchange.xforce.ibmcloud.com/vulnerabilities/20939
http://www.securitytracker.com/alerts/2005/Jun/1014135.html
http://www.fishnetsecurity.com/csirt/disclosure/cisco/Cisco+802.1x+Advisory.pdf
http://www.cisco.com/warp/public/707/cisco-sn-20050608-8021x.shtml