Description
WordPress is prone to multiple username enumeration weaknesses because it displays different responses to requests depending on whether or not the username exists. Attackers may exploit these weaknesses to discern valid usernames, which may aid them in brute-force password cracking or other attacks. WordPress versions prior to 2.8.1 are vulnerable.
Remediation
Update to WordPress version 2.8.1 or latest
References
http://www.coresecurity.com/content/WordPress-Privileges-Unchecked