A xorg-server security update has been released for Debian GNU/Linux 9 LTS to address privilege escalation for authorized clients.

DLA 2627-1: xorg-server security update

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Debian LTS Advisory DLA-2627-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Chris Lamb
April 15, 2021   https://wiki.debian.org/LTS
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Package : xorg-server
Version : 2:1.19.2-1+deb9u8
CVE ID : CVE-2021-3472

Jan-Niklas Sohn discovered that there was an input validation failure
in the X.Org display server.

Insufficient checks on the lengths of the XInput extension's
ChangeFeedbackControl request could have lead to out of bounds memory
accesses in the X server. These issues can lead to privilege
escalation for authorised clients, particularly on systems where the
X server is running as a privileged user.

For Debian 9 "Stretch", this problem has been fixed in version
2:1.19.2-1+deb9u8.

We recommend that you upgrade your xorg-server packages.

For the detailed security status of xorg-server please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/xorg-server

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS