An issue was discovered in libX11 through 1.6.5. The function XListExtensions in ListExt.c interprets a variable as signed instead of unsigned, resulting in an out-of-bounds write (of up to 128 bytes), leading to DoS or remote code execution.
https://access.redhat.com/errata/RHSA-2019:2079
https://bugzilla.suse.com/show_bug.cgi?id=1102068
https://cgit.freedesktop.org/xorg/lib/libX11/commit/?id=dbf72805fd9d7b1846fe9a11b46f3994bfc27fea
https://lists.debian.org/debian-lts-announce/2018/08/msg00030.html
https://security.gentoo.org/glsa/201811-01
https://usn.ubuntu.com/3758-1/
https://usn.ubuntu.com/3758-2/