http://blog.sucuri.net/2014/08/wordpress-and-drupal-denial-of-service-vulnerability.html

Both WordPress and Drupal are affected by a DoS (denial of service) vulnerability on the PHP XML parser used by their XMLRPC implementations. The issue lies in the XML entity expansion parser that can cause CPU and memory exhaustion and the site’s database to reach the maximum number of open connections. That will cause the vulnerable site (and server) to go down for a period of time, hence affecting Availability of your website.