Cisco IOS 12.2 and earlier generates a "% Login invalid" message instead of prompting for a password when an invalid username is provided, which allows remote attackers to identify valid usernames on the system and conduct brute force password guessing, as reported for the Aironet Bridge.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5824
http://www.vigilante.com/inetsecurity/advisories/VIGILANTE-2003002.htm
http://www.kb.cert.org/vuls/id/886796
http://www.cisco.com/warp/public/707/cisco-sn-20030724-ios-enum.shtml
http://archives.neohapsis.com/archives/vulnwatch/2003-q3/0056.html