The XML-RPC system in Drupal 6.x before 6.38 and 7.x before 7.43 might make it easier for remote attackers to conduct brute-force attacks via a large number of calls made at once to the same method.
https://www.drupal.org/SA-CORE-2016-001
http://www.openwall.com/lists/oss-security/2016/03/15/10