CVE-2026-50257

A use-after-free flaw was found in the X.Org X server and Xwayland in miSyncDestroyFence(). A client that sets up multiple fence triggers can trigger a use-after-free function pointer call. An attacker would connect to the X server to set up a fence and await that fence, then a second X connection destroys the fence, causing the use-after-free. This may be used to crash the server, or for privilege escalation if the X server runs as root.

Red Hat rates this issue as Important impact. In xorg-x11-server and xorg-x11-server-Xwayland, a local X client can trigger a use-after-free function pointer call in miSyncDestroyFence() by setting up a fence trigger on one connection and destroying the fence from a second connection. Any local user who can connect to the X server display can attempt exploitation. This may crash the display server or, where the X server runs with elevated privileges, could contribute to local privilege escalation. Upstream fixed this in xorg-server 21.1.23 and xwayland 24.1.12.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.