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.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.