X.Org Security Advisory: CVE-2023-1393: X.Org Server Overlay Window Use-After-Free

[Olivier Fourdan]

X.Org Security Advisory: March 29, 2023

X.Org Server Overlay Window Use-After-Free
==========================================

This issue can lead to local privileges elevation on systems where the X
server is running privileged and remote code execution for ssh X forwarding
sessions.

ZDI-CAN-19866/CVE-2023-1393: X.Org Server Overlay Window Use-After-Free
Local Privilege Escalation Vulnerability

If a client explicitly destroys the compositor overlay window (aka COW),
the Xserver would leave a dangling pointer to that window in the CompScreen
structure, which will trigger a use-after-free later.

Patches
-------
Patch for this issue have been committed to the xorg server git repository.
xorg-server 21.1.8 will be released shortly and will include this patch.

- commit 26ef545b3 - composite: Fix use-after-free of the COW
   (https://gitlab.freedesktop.org/xorg/xserver/-/commit/26ef545b3)

ZDI-CAN-19866/CVE-2023-1393

If a client explicitly destroys the compositor overlay window (aka COW),
we would leave a dangling pointer to that window in the CompScreen
structure, which will trigger a use-after-free later.

Make sure to clear the CompScreen pointer to the COW when the latter gets
destroyed explicitly by the client.

Thanks
======

The vulnerabilities have been discovered by Jan-Niklas Sohn working with
Trend Micro Zero Day Initiative.
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