The "have you forgotten your password" links in the User module in Drupal 7.x before 7.43 and 8.x before 8.0.4 allow remote attackers to obtain sensitive username information by leveraging a configuration that permits using an email address to login and a module that permits logging in.
https://www.drupal.org/SA-CORE-2016-001
http://www.openwall.com/lists/oss-security/2016/03/15/10