Looks like WordPress has a bug!
Make sure you update your installation to provide maximum security against hacks.
Discovered only days ago, this vulnerability allows attackers to hijack your website.
Don’t forget to back up your files before updating.
If you have a regular backup service scheduled, consider using the following code in your wp-config.php file to enable automatic updates of your wordpress installation:
define( ‘WP_AUTO_UPDATE_CORE’, true );
Be careful, as update can BREAK the functionality of your website. When in doubt, ask a professional : )