The OpenID module in Drupal 6.x before 6.18, and the OpenID module 5.x before 5.x-1.4 for Drupal, violates the OpenID 2.0 protocol by not ensuring that fields are signed, which allows remote attackers to bypass authentication by leveraging an assertion from an OpenID provider.
http://www.securityfocus.com/bid/42388
http://www.debian.org/security/2010/dsa-2113
http://marc.info/?l=oss-security&m=128440896914512&w=2