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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Recon
    Obtain local session on target host
  2. Delivery
    Open two X client connections with valid auth cookie
  3. Exploit
    Create SyncFence and register trigger on first connection
  4. Install
    Destroy fence via second connection
  5. C2
    Trigger fires on freed object
  6. Execute
    Hijack dangling function pointer for code execution
  7. Impact
    Gain root if Xorg runs privileged

Vulnerability Assessment

Weakness
CWE
CWE-416
ASVS 5.0
V15 - Secure Coding and Architecture Memory lifecycle management
MITRE ATT&CK

2 techniques across 2 tactics: Execution, Privilege Escalation

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Attacker must have a local session capable of opening at least two simultaneous client connections to the target X server - meaning a valid $DISPLAY and MIT-MAGIC-COOKIE authority, which in practice requires a logged-in interactive user, an X-forwarded SSH session, or code execution inside an application that already speaks to the display. The SYNC extension must be enabled (it is on by default in upstream Xorg and Xwayland). Privilege escalation to root specifically requires that the X server process runs as root, which is the legacy default on RHEL 6/7 and on systems without systemd-logind-driven rootless Xorg; on modern rootless Xorg or Wayland-with-Xwayland setups the impact is limited to denial of service or escalation only to the user already running the display server. No remote network exploitation: AV:L excludes attackers without prior local access.
Impact

The CVSS 3.1 vector AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H scores 7.8 and accurately reflects a local, low-complexity, low-privilege flaw with high impact across CIA - the privilege boundary crossed is the X server's, which on traditional setups is root. There is no EPSS score, KEV listing, or POC reference in the provided intelligence, so this is not confirmed actively exploited and there is no public exploit identified at time of analysis; the SSVC posture is therefore best characterized as proof-of-concept-not-public with high technical impact but non-automatable exploitation (a race condition requiring two coordinated client sessions). The realistic priority is moderate-to-high for multi-user systems, kiosks, thin clients, and any host still running Xorg as root, but low for modern desktops running rootless Xorg or pure Wayland sessions where Xwayland runs unprivileged. Note that the description explicitly conditions privilege escalation on 'if the X server runs as root' - defenders should verify their deployment mode rather than assume the worst case.
Exploit ScenarioDerived

A local attacker with an interactive session - for example a logged-in user on a shared workstation, a malicious app sandbox escape, or an SSH user with X forwarding - opens two client connections to the X server, registers a SyncFence and a SyncAwait on the first connection, then destroys the fence from the second connection to free the underlying object while the await still holds a pointer. When the trigger fires it dereferences the freed function pointer, crashing the X server or, if the attacker can heap-spray a controlled object into the freed slot, executing arbitrary code with the X server's privileges (root on legacy deployments). No public exploit identified at time of analysis, but the race is straightforward and similar miSync use-after-free bugs (e.g. CVE-2023-6377) have had working POCs published within weeks.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
No patch listed yet 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Jun 05, 2026

Local privilege escalation in X.Org X server and Xwayland enables authenticated local users to trigger a use-after-free in miSyncDestroyFence() by racing two client connections against a shared fence object. Successful exploitation can crash the display server or escalate privileges to root when the X server runs as root, which remains common on legacy and embedded Linux deployments. No public exploit identified at time of analysis, but an upstream fix has been committed by the X.Org maintainers.

RemediationAI

Apply the upstream fix at commit f5abfb61994471023d8c6470428c8e30c411cc0b from https://gitlab.freedesktop.org/xorg/xserver/-/commit/f5abfb61994471023d8c6470428c8e30c411cc0b once your distribution publishes a tagged release containing it; for Red Hat customers, monitor https://access.redhat.com/security/cve/CVE-2026-50257 and Bugzilla 2485382 for RHSA errata across RHEL 6 through 10 and install the updated xorg-x11-server and xwayland packages. Upstream fix available (commit); a tagged released patched xserver version is not independently confirmed in the provided data, so verify against the X.Org announce list (https://lists.x.org/archives/xorg-announce/2026-June/003702.html) for the corresponding 21.1.x point release. As compensating controls until patches land, run Xorg rootless (already default on modern Fedora/RHEL via systemd-logind seat takeover) to neutralize the privilege-escalation path while still leaving denial of service possible, prefer pure Wayland sessions where Xwayland runs as the unprivileged user, and on multi-user hosts restrict shell access since the attacker must connect to the local X server - note that disabling the SYNC extension entirely will break compositors and many GTK/Qt applications, so it is not a practical mitigation.

Affects your stack

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Technical ContextAI · show details

The vulnerability resides in the MIT-SHM/Sync extension code path of the X.Org xserver, specifically miSyncDestroyFence() within the miSync fence-handling logic shared between native X11 and the Xwayland compatibility server. The root cause is CWE-416 (Use After Free): the fence object's lifetime is not correctly serialized between the client that awaits the fence trigger and a separate client that destroys it, leaving a dangling function pointer that is later invoked when the trigger fires. Because the Sync extension exposes fence primitives over the X protocol, any local client with a session connection to the display server can manipulate these objects. The affected components are shipped as part of Red Hat Enterprise Linux 6 through 10 per the provided CPEs, and the upstream patch is tracked at gitlab.freedesktop.org commit f5abfb6.

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Severity by source

Vendor (redhat) PRIMARY 7.8 HIGH
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
SUSE HIGH
Red Hat 7.8 HIGH

Primary rating from Vendor (redhat).

CVSS VectorVendor: redhat

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

DescriptionCVE.org

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.

Zero Day Initiative Advisories

ZDI-26-391 X.Org Server miSyncDestroyFence Use-After-Free Privilege Escalation Vulnerability Jun 2026

Executive ViewAI

Why It Matters

A privilege escalation vulnerability in X.Org X server and Xwayland (CVE-2026-50257, CVSS 7.8) allows authenticated local users to crash the display server or escalate to root privileges on legacy and embedded Linux systems. This affects organizations operating development workstations, embedded IoT devices, and legacy deployments where X.Org retains elevated privileges, creating direct risk to system integrity.

Business Impact: High
Compensating Controls

No vendor-released patch identified at time of analysis. An upstream fix has been committed by X.Org maintainers but is not yet available in production releases. Interim mitigations: (1) Restrict local user access on systems where X.Org runs with root privileges; (2) Deploy AppArmor or SELinux mandatory access control policies to limit X server capabilities and fence object access; (3) Migrate to Wayland deployments where feasible, as this vulnerability does not affect Wayland; (4) Disable X.Org network listening and restrict to local socket connections only; (5) Implement system monitoring for display server crashes and unexpected privilege escalation attempts.

Threat Intelligence Overview

EPSS Score
0.0%
Top 98% exploitable
Data Quality
7 sources
Confidence: High
EUVD NVD CVE.org EPSS Vulnrichment MSRC Vendor VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.0%
Patch available No
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Scheduled
Exploitation
None
System Exposure· derived
Small
Automatable
No
Human Impact
High
Technical Impact
Total

Priority Score

39
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +39
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Privilege Escalation HIGH

Privilege escalation occurs when an attacker leverages flaws in access control mechanisms to gain permissions beyond what they were originally granted.

Use After Free HIGH

Use-after-free vulnerabilities occur when a program continues to access memory through a pointer after that memory has been deallocated.

Memory Corruption HIGH

Memory corruption occurs when a program writes data beyond the boundaries of allocated memory regions or accesses memory in unintended ways, violating the integrity of the process's address space.

Affected Products (CPE)CVE.org

cpe:2.3:a:red_hat:red_hat_enterprise_linux_10:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_6:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_7:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_8:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_8:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_9:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_9:*:*:*:*:*:*:*:*

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Same weakness CWE-416 – Use After Free

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected
SUSE Linux Enterprise Module for Basesystem 15 SP7 Affected
SUSE Linux Enterprise Module for Development Tools 15 SP7 Affected

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-50257 ADVISORY https://vuldb.com/vuln/368892 REF https://access.redhat.com/errata/RHSA-2026:26562 REF https://access.redhat.com/errata/RHSA-2026:26566 REF https://access.redhat.com/errata/RHSA-2026:26590 REF https://access.redhat.com/errata/RHSA-2026:26610 REF https://access.redhat.com/errata/RHSA-2026:26709 REF https://access.redhat.com/errata/RHSA-2026:28923 REF https://access.redhat.com/errata/RHSA-2026:29844 REF https://access.redhat.com/errata/RHSA-2026:36083 REF https://access.redhat.com/errata/RHSA-2026:36085 REF https://access.redhat.com/errata/RHSA-2026:36086 REF https://access.redhat.com/errata/RHSA-2026:36087 REF https://access.redhat.com/errata/RHSA-2026:36632 REF https://access.redhat.com/errata/RHSA-2026:36633 REF https://access.redhat.com/errata/RHSA-2026:36634 REF https://access.redhat.com/errata/RHSA-2026:36768 REF https://access.redhat.com/errata/RHSA-2026:36791 REF https://access.redhat.com/errata/RHSA-2026:36792 REF https://access.redhat.com/errata/RHSA-2026:36798 REF https://access.redhat.com/errata/RHSA-2026:38502 REF https://access.redhat.com/errata/RHSA-2026:38810 REF https://access.redhat.com/errata/RHSA-2026:46377 REF https://access.redhat.com/errata/RHSA-2026:46382 REF https://access.redhat.com/errata/RHSA-2026:46385 REF https://access.redhat.com/errata/RHSA-2026:46392 REF https://access.redhat.com/errata/RHSA-2026:46456 REF https://access.redhat.com/errata/RHSA-2026:46460 REF https://access.redhat.com/errata/RHSA-2026:46473 REF https://access.redhat.com/errata/RHSA-2026:49519 REF https://access.redhat.com/security/cve/CVE-2026-50257 REF https://bugzilla.redhat.com/show_bug.cgi?id=2485382 REF https://gitlab.freedesktop.org/xorg/xserver/-/commit/f5abfb61994471023d8c6470428c8e30c411cc0b REF https://lists.x.org/archives/xorg-announce/2026-June/003702.html REF https://redhat.atlassian.net/browse/PSIRTSUPT-16950 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-50257.json

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