Open redirect vulnerability in URL-related API functions in Drupal 6.x before 6.35 and 7.x before 7.35 allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via vectors involving the "//" initial sequence.
https://www.drupal.org/SA-CORE-2015-001
http://www.securityfocus.com/bid/73219
http://www.openwall.com/lists/oss-security/2015/03/26/4