An issue was discovered in XListExtensions in ListExt.c in libX11 through 1.6.5. A malicious server can send a reply in which the first string overflows, causing a variable to be set to NULL that will be freed later on, leading to DoS (segmentation fault).
https://usn.ubuntu.com/3758-2/
https://usn.ubuntu.com/3758-1/
https://security.gentoo.org/glsa/201811-01
https://lists.x.org/archives/xorg-announce/2018-August/002916.html
https://lists.debian.org/debian-lts-announce/2018/08/msg00030.html
https://cgit.freedesktop.org/xorg/lib/libX11/commit/?id=e83722768fd5c467ef61fa159e8c6278770b45c2
https://bugzilla.suse.com/show_bug.cgi?id=1102073
https://access.redhat.com/errata/RHSA-2019:2079
http://www.securitytracker.com/id/1041543