The cookie authentication method in WordPress 2.5 relies on a hash of a concatenated string containing USERNAME and EXPIRY_TIME, which allows remote attackers to forge cookies by registering a username that results in the same concatenated string, as demonstrated by registering usernames beginning with "admin" to obtain administrator privileges, aka a "cryptographic splicing" issue.  NOTE: this vulnerability exists because of an incomplete fix for CVE-2007-6013.