LXDM before 0.5.2 did not start X server with -auth parameter. Therefore any user able to connect to it (typically all local users) would have their X connections accepted. The issue was fixed via: http://git.lxde.org/gitweb/?p=lxde/lxdm.git;a=commitdiff;h=e8f387089e241360bdc6955d3e479450722dcea3 Additional information is available in bug 1268900.
This was already fixed in all Fedora versions via bug 1268900.