xorg-x11-server before 1.19.5 was vulnerable to integer overflow in (S)ProcXIBarrierReleasePointer functions allowing malicious X client to cause X server to crash or possibly execute arbitrary code.
https://www.debian.org/security/2017/dsa-4000
https://security.gentoo.org/glsa/201711-05
https://cgit.freedesktop.org/xorg/xserver/commit/?id=d088e3c1286b548a58e62afdc70bb40981cdb9e8