The User module in Drupal 6.x before 6.38 and 7.x before 7.43 allows remote attackers to gain privileges by leveraging contributed or custom code that calls the user_save function with an explicit category and loads all roles into the array.
https://www.drupal.org/SA-CORE-2016-001
http://www.openwall.com/lists/oss-security/2016/03/15/10