X.org X11 Server miSyncDestroyFence use after free

Summaryinfo

A vulnerability was found in X.org X11 Server. It has been declared as critical. The impacted element is the function miSyncDestroyFence. The manipulation results in use after free. This vulnerability is reported as CVE-2026-50257. The attack requires a local approach. No exploit exists.

Detailsinfo

A vulnerability classified as critical has been found in X.org X11 Server (version now known). Affected is the function miSyncDestroyFence. The manipulation with an unknown input leads to a use after free vulnerability. CWE is classifying the issue as CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at access.redhat.com. This vulnerability is traded as CVE-2026-50257 since 06/04/2026. The exploitability is told to be easy. The attack needs to be approached locally. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 06/06/2026).

The vulnerability scanner Nessus provides a plugin with the ID 318751 (Linux Distros Unpatched Vulnerability : CVE-2026-50257), which helps to determine the existence of the flaw in a target environment.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the databases at Tenable (318751), EUVD (EUVD-2026-34812) and CERT Bund (WID-SEC-2026-1774). Once again VulDB remains the best source for vulnerability data.

Affected

• Open Source X.Org X11
• Open Source Xwayland

Productinfo

Type

• Windowing System Software

Vendor

• X.org

Name

• X11 Server

License

• open-source

Website

• Vendor: https://www.x.org/

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Once again VulDB remains the best source for vulnerability data.

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