"Yesterday a vulnerability was discovered: a specially crafted URL could be requested that would allow an attacker to bypass a security check to verify a user requested a password reset. As a result, the first account without a key in the database (usually the admin account) would have its password reset and a new password would be emailed to the account owner." - Wordpress.org
User's are urged to upgrade to 2.8.4 immediately. Additional details can be found below.
Vendor Blog: http://wordpress.org/development/2009/08/2-8-4-security-release/
Vulnerability Details: http://lists.grok.org.uk/pipermail/full-disclosure/2009-August/070137.html