A flaw was found in xorg-x11-server in versions before 1.20.11. An integer underflow can occur in xserver which can lead to a local privilege escalation. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.
https://www.zerodayinitiative.com/advisories/ZDI-21-463/
https://www.tenable.com/plugins/nessus/148701
https://www.debian.org/security/2021/dsa-4893
https://security.gentoo.org/glsa/202104-02
https://seclists.org/oss-sec/2021/q2/20
https://lists.x.org/archives/xorg-announce/2021-April/003080.html
https://lists.debian.org/debian-lts-announce/2021/04/msg00013.html
https://gitlab.freedesktop.org/xorg/xserver/-/commit/7aaf54a1884f71dc363f0b884e57bcb67407a6cd