It was discovered that the gnome-shell lock screen since version 3.15.91 did not properly restrict all contextual actions. An attacker with physical access to a locked workstation could invoke certain keyboard shortcuts, and potentially other actions.
https://usn.ubuntu.com/3966-1/
https://gitlab.gnome.org/GNOME/gnome-shell/issues/851
http://lists.opensuse.org/opensuse-security-announce/2019-06/msg00049.html
http://lists.opensuse.org/opensuse-security-announce/2019-06/msg00023.html