The start_authentication function in lightdm-gtk-greeter.c in LightDM GTK+ Greeter before 1.7.1 does not properly handle the return value from the lightdm_greeter_get_authentication_user function, which allows local users to cause a denial of service (NULL pointer dereference) via an empty username.
https://bugzilla.novell.com/show_bug.cgi?id=857303
https://bugs.launchpad.net/lightdm-gtk-greeter/+bug/1266449
http://www.securityfocus.com/bid/64679
http://www.openwall.com/lists/oss-security/2014/01/07/15
http://secunia.com/advisories/56423
http://secunia.com/advisories/56211
http://lists.opensuse.org/opensuse-updates/2014-01/msg00048.html
http://lists.fedoraproject.org/pipermail/package-announce/2014-February/128150.html
http://lists.fedoraproject.org/pipermail/package-announce/2014-February/128117.html