The Node Access User Reference module 5.x before 5.x-2.0-beta4 and 6.x before 6.x-2.0-beta6, a module for Drupal, interprets an empty CCK user reference as a reference to the anonymous user, which might allow remote attackers to bypass intended access restrictions to read or modify a node.
http://www.vupen.com/english/advisories/2009/1212
http://www.securityfocus.com/bid/34778