Drupal 6.x before 6.38, 7.x before 7.43, and 8.x before 8.0.4 might allow remote attackers to conduct open redirect attacks by leveraging (1) custom code or (2) a form shown on a 404 error page, related to path manipulation.
https://www.drupal.org/SA-CORE-2016-001
http://www.openwall.com/lists/oss-security/2016/03/15/10