A new version of WordPress is available for download. Version 2.3.3 fixes a security issue with the way XML-RPC has been implemented. The fixed version of the xmlrpc.php file is also available as a separate download for those who don’t want to do a full upgrade.
XML-RPC is a protocol that allows bloggers to remotely post without having to login to their WordPress installation. The issue here is that by using a special XML-RPC request, one user could edit another users posts.