xorg-x11-server before 1.19.5 was missing length validation in XFIXES extension allowing malicious X client to cause X server to crash or possibly execute arbitrary code.
https://bugzilla.redhat.com/show_bug.cgi?id=1509224
https://cgit.freedesktop.org/xorg/xserver/commit/?id=55caa8b08c84af2b50fbc936cf334a5a93dd7db5
https://lists.debian.org/debian-lts-announce/2017/11/msg00032.html