GTK+ 3.10.9 and earlier, as used in cinnamon-screensaver, gnome-screensaver, and other applications, allows physically proximate attackers to bypass the lock screen by pressing the menu button.
http://advisories.mageia.org/MGASA-2014-0374.html
http://seclists.org/oss-sec/2014/q1/327
http://seclists.org/oss-sec/2014/q1/331
http://www.mandriva.com/security/advisories?name=MDVSA-2015:162
http://www.ubuntu.com/usn/USN-2475-1
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=759145