gnome screensaver before 2.14, when running on an X server with AllowDeactivateGrabs and AllowClosedownGrabs enabled, allows attackers with physical access to cause the screensaver to crash and access the session via the Ctl+Alt+Keypad-Multiply keyboard sequence, which removes the grab from gnome.
https://exchange.xforce.ibmcloud.com/vulnerabilities/25340