In X.Org Server (aka xserver and xorg-server) before 1.19.4, a local attacker authenticated to the X server could overflow a global buffer, causing crashes of the X server or potentially other problems by injecting large or malformed XKB related atoms and accessing them via xkbcomp.
www.debian.org/security/2017/dsa-4000
www.openwall.com/lists/oss-security/2017/10/04/10
www.securityfocus.com/bid/101253
cgit.freedesktop.org/xorg/xserver/commit/?id=94f11ca5cf011ef123bd222cabeaef6f424d76ac
lists.debian.org/debian-lts-announce/2017/11/msg00032.html
lists.x.org/archives/xorg-announce/2017-October/002808.html
security.gentoo.org/glsa/201710-30