The Node access user reference module 6.x-3.x before 6.x-3.5 and 7.x-3.x before 7.x-3.10 for Drupal does not properly restrict access to content containing a user reference field when the author update/delete grants are enabled and the author's user account is deleted, which allows remote attackers to modify the content via unspecified vectors.
https://drupal.org/node/2007122
https://drupal.org/node/2007078