The auth_via_key function in pam_ssh.c in pam_ssh before 1.92, when the allow_blank_passphrase option is disabled, allows remote attackers to bypass authentication restrictions and use private encryption keys requiring a blank passphrase by entering a non-blank passphrase.
http://www.vupen.com/english/advisories/2007/0524
http://www.securityfocus.com/bid/22461
http://sourceforge.net/project/shownotes.php?release_id=484376