Jan-Niklas Sohn discovered that a use-after-free flaw in the Composite extension of the X.org X server may result in privilege escalation if the X server is running under the root user.
For Debian 10 buster, this problem has been fixed in version 2:1.20.4-1+deb10u9.
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