PAM-MySQL is a PAM module used to authenticate users against a MySQL backend.
A flaw in handling the result of pam_get_item() as well as further unspecified flaws were discovered in PAM-MySQL.
By exploiting the mentioned flaws an attacker can cause a Denial of Service and thus prevent users that authenticate against PAM-MySQL from logging into a machine. There is also a possible additional attack vector with more malicious impact that has not been confirmed yet.
There is no known workaround at this time.
All PAM-MySQL users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=sys-auth/pam_mysql-0.7_rc1"