CVE-2026-50257: A use-after-free flaw was found in the X.Org X server and Xwayland in miSyncDestroyFence()

Q: How do I fix CVE-2026-50257?

x server: upgrade to 21.1.23; xwayland: upgrade to 24.1.12; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2026-50257 being exploited?

CVE-2026-50257 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-50257 affect?

CVE-2026-50257 affects x server, xwayland, enterprise linux. Severity is high (CVSS 7.8). 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 u

Q: How urgent is CVE-2026-50257?

Priority 33/100. High severity; On your watchlist

Routine

HIGH 7.8WATCHLISTUPGRADEUPGRADEMITIGATE

CVE-2026-50257 is a high-severity flaw (CVSS 7.8) affecting x server, xwayland, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.8 (High)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 97%)
CISA exploited list	No
Weakness	CWE-416
Published	2026-06-05
Last modified	2026-08-05

Publisher (CNA): Red Hat, Inc. · Vendor: redhat · x.org

Last updated 2026-08-05 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-05 12:16 UTCPublished CVE record published
2026-08-05 13:23 UTCUpdated Record last modified

x.org x server UPGRADE

Affected: < 21.1.23

Fixed in: 21.1.23

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy x-server || rpm -q x-server || x-server --version

2. UPGRADE to 21.1.23 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade x-server

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade x-server

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

x.org xwayland UPGRADE

Affected: < 24.1.12

Fixed in: 24.1.12

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy xwayland || rpm -q xwayland || xwayland --version

2. UPGRADE to 24.1.12 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade xwayland

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade xwayland

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

redhat enterprise linux MITIGATE

Affected: 7.0 · 8.0 · 9.0 · 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 7.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:26590] Important: xorg-x11-server-Xwayland security, bug fix, and enhancement update 2026-07-02
AlmaLinux Errata: [ALSA-2026:26610] Important: xorg-x11-server security, bug fix, and enhancement update 2026-07-02
AlmaLinux Errata: [ALSA-2026:29844] Important: tigervnc security update 2026-06-29

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-50257?

x server: upgrade to 21.1.23; xwayland: upgrade to 24.1.12; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-50257 being exploited?

CVE-2026-50257 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-50257 affect?

CVE-2026-50257 affects x server, xwayland, enterprise linux. Severity is high (CVSS 7.8). 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 u

How urgent is CVE-2026-50257?

Priority 33/100. High severity; On your watchlist

Related CVEs

CVE-2026-34000 MEDIUM 6.1 MITIGATE MITIGATE A flaw was found in the X.Org X server
CVE-2026-34002 MEDIUM 6.1 MITIGATE MITIGATE A flaw was found in the X.Org X server
CVE-2026-50261 HIGH 7.8 UPGRADE UPGRADE A use-after-free flaw was found in the X.Org X server and Xwayland in SyncChangeCounter()
CVE-2026-50260 HIGH 7.8 UPGRADE UPGRADE A use-after-free flaw was found in the X.Org X server and Xwayland in FreeCounter()
CVE-2026-50259 HIGH 7.8 UPGRADE UPGRADE A stack-based buffer overflow flaw was found in the X.Org X server and Xwayland

References

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