Cisco 6400 Access Concentrator Node Route Processor 2 (NRP2) 12.1DC card does not properly disable access when a password has not been set for vtys, which allows remote attackers to obtain access via telnet.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6691
http://www.kb.cert.org/vuls/id/516659
http://www.cisco.com/warp/public/707/6400-nrp2-telnet-vuln-pub.shtml