Exposure of Sensitive Information to an Unauthorized Actor
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.
CWE-200 - Information Exposure
An information exposure vulnerability is categorized as an information flow (IF) weakness, which can potentially allow unauthorized access to otherwise classified information in the application, such as confidential personal information (demographics, financials, health records, etc.), business secrets, and the application's internal environment.