The login redirection mechanism in the Drupal 4.7 Pubcookie module before 1.2.2.4 2006/09/06 and the Drupal 4.6 Pubcookie module before 1.6.2.1 2006/09/07 allows remote attackers to bypass authentication requirements and spoof identities of arbitrary users via unspecified vectors.
http://www.vupen.com/english/advisories/2006/3530
http://www.securityfocus.com/bid/19920