pam-auth-update for PAM, as used in Ubuntu 8.10 and 9.4, and Debian GNU/Linux, does not properly handle an "empty selection" for system authentication modules in certain rare configurations, which causes any attempt to be successful and allows remote attackers to bypass authentication.
https://launchpad.net/bugs/410171
https://euvd.enisa.europa.eu/vulnerability/EUVD-2009-3215
http://www.securityfocus.com/bid/36306
http://www.openwall.com/lists/oss-security/2009/09/08/7