An issue was discovered in Cinnamon 1.9.2 through 3.8.6. The cinnamon-settings-users.py GUI runs as root and allows configuration of (for example) other users' icon files in _on_face_browse_menuitem_activated and _on_face_menuitem_activated. These icon files are written to the respective user's $HOME/.face location. If an unprivileged user prepares a symlink pointing to an arbitrary location, then this location will be overwritten with the icon content.
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.