Drupal 6 and Drupal 7 include an XML-RPC endpoint which is publicly available (xmlrpc.php). The PHP XML parser used by this XML-RPC endpoint is vulnerable to an XML entity expansion attack and other related XML payload attacks which can cause CPU and memory exhaustion and the site's database to reach the maximum number of open connections. Any of these may lead to the site becoming unavailable or unresponsive (denial of service).
All Drupal sites are vulnerable to this attack whether XML-RPC is used or not.
In addition, a similar vulnerability exists in the core OpenID module (for sites that have this module enabled).
This is a joint release as the XML-RPC vulnerability also affects WordPress (see the announcement).
Install the latest version:
If you are unable to install the latest version of Drupal immediately, you can alternatively remove the xmlrpc.php file from the root of Drupal core (or add a rule to .htaccess to prevent access to xmlrpc.php) and disable the OpenID module. These steps are sufficient to mitigate the vulnerability in Drupal core if your site does not require the use of XML-RPC or OpenID functionality. However, this mitigation will not be effective if you are using a contributed module that exposes Drupal's XML-RPC API at a different URL (for example, the Services module); updating Drupal core is therefore strongly recommended.
Also see the Drupal core project page.