CVE-2026-50257: Xorg-x11-server: xorg-x11-server-xwayland: xorg-x11-server: use-after-free in misyncdestroyfence()

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-05 · updated 2026-08-05
Weakness: CWE-416

Description

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.

Affected products

Red Hat Enterprise Linux: 7.0, 8.0, 9.0, 10.0
xorg-x11-server: before 21.1.23 — fixed in 21.1.23
X.Org xwayland: before 24.1.12 — fixed in 24.1.12

How to fix

Upgrade to a fixed version: xorg-x11-server 21.1.23; X.Org xwayland 24.1.12. Check the vendor advisory for mitigations.

References

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