In X.Org Server (aka xserver and xorg-server) before 1.19.4, an attacker authenticated to an X server with the X shared memory extension enabled can cause aborts of the X server or replace shared memory segments of other X clients in the same session.
https://security.gentoo.org/glsa/201710-30
https://lists.x.org/archives/xorg-announce/2017-October/002808.html
https://cgit.freedesktop.org/xorg/xserver/commit/?id=b95f25af141d33a65f6f821ea9c003f66a01e1f1
http://www.securityfocus.com/bid/101238