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.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=519927
http://secunia.com/advisories/36620
http://www.openwall.com/lists/oss-security/2009/09/08/7
http://www.securityfocus.com/bid/36306