The default configuration in gnome-screensaver 3.5.4 through 3.6.0 sets the AutostartCondition line to fallback mode in the .desktop file, which prevents the program from starting automatically after login and allows physically proximate attackers to bypass screen locking and access an unattended workstation.
https://git.gnome.org/browse/gnome-screensaver/commit/?id=1940dc6bc8ad5ee2c029714efb1276c05ca80bd4
https://bugzilla.gnome.org/show_bug.cgi?id=683060
https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/1120126