The following flaw was reported in Drupal:

Password reset URLs can be forged under certain circumstances, allowing an attacker to gain access to another user's account without knowing the account's password.

In Drupal 7, this vulnerability is mitigated by the fact that it can only be exploited on sites where accounts have been imported or programmatically edited in a way that results in the password hash in the database being the same for multiple user accounts. In Drupal 6, it can additionally be exploited on sites where administrators have created multiple new user accounts with the same password via the administrative interface, or where accounts have been imported or programmatically edited in a way that results in the password hash in the database being empty for at least one user account.

Drupal 6 sites that have empty password hashes, or a password field with a guessable string in the database, are especially prone to this vulnerability. This could apply to sites that use external authentication so that the password field is set to a fixed, invalid value.

Upstream advisory:

https://www.drupal.org/SA-CORE-2015-001
https://www.drupal.org/node/2455005

Upstream patch:

http://cgit.drupalcode.org/drupal/commit/?id=8e54eca05a65c6231b02510e1917af0c9191e549

CVE assignment:

http://seclists.org/oss-sec/2015/q1/891