xorg-x11-server before 1.19.5 was missing length validation in MIT-SCREEN-SAVER extension allowing malicious X client to cause X server to crash or possibly execute arbitrary code.
https://bugzilla.redhat.com/show_bug.cgi?id=1509215
https://cgit.freedesktop.org/xorg/xserver/commit/?id=cad5a1050b7184d828aef9c1dd151c3ab649d37e
https://lists.debian.org/debian-lts-announce/2017/11/msg00032.html