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CVE Record

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

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.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A flaw in the X.Org X server and Xwayland can be triggered by a local user who is already logged in to the graphical session. By connecting twice and misusing a synchronization feature, they can crash the display server or, if it runs as root, gain higher privileges on the machine. Red Hat has shipped patches across supported Enterprise Linux versions.

Executive priority

Treat as a standard high-severity patch cycle item for Linux desktops, VDI, and jump hosts. It is not remotely exploitable and is not on CISA KEV, so it does not require emergency change control, but should be scheduled in the next patch window to close a credible local privilege-escalation path.

Technical view

CVE-2026-50257 is a CWE-416 use-after-free in miSyncDestroyFence() within X.Org X server and Xwayland. A client sets up multiple fence triggers and awaits a fence; a second client connection destroys that fence, leaving a dangling function pointer that is later called. CVSS 3.1 is 7.8 (AV:L/AC:L/PR:L/UI:N/C:H/I:H/A:H). Impact ranges from server crash to local privilege escalation when the X server runs as root.

Likely exposure

Exposure is limited to hosts that run X.Org X server, Xwayland, or TigerVNC-Xorg, primarily Linux workstations, jump hosts, and RHEL-based systems with graphical sessions. Red Hat lists RHEL 7 ELS, 8, 9, and 10 streams as affected. Servers without a graphical stack or that run rootless Xwayland face reduced impact.

Exploitation context

Not listed in CISA KEV and no public exploitation is cited in the source bundle. The attack vector is local with low privileges and no user interaction, so any authenticated user with X access could attempt it. Highest impact occurs where the X server still runs as root, which enables local privilege escalation rather than remote compromise.

Researcher notes

Root cause is a dangling function pointer left by miSyncDestroyFence() when one connection destroys a fence still awaited by another. CVSS vector confirms local, low-privilege, no-UI, high CIA impact. Red Hat has issued a wide fan-out of RHSAs (26562 through 36798) spanning RHEL 7 ELS, 8, 8.4/8.6 AUS/EUS, 8.8 TUS/E4S, 9, 9.2/9.4 E4S, 9.6 EUS, and 10, plus TigerVNC 1.15.0 rebuilds. No KEV entry and no public PoC referenced in the bundle at this time.

Mitigation direction

  • Apply the Red Hat RHSA errata for xorg-x11-server, xorg-x11-server-Xwayland, and tigervnc on affected RHEL 7 ELS, 8, 9, and 10 systems.
  • Prioritize multi-user hosts, jump boxes, and VDI where local users share the same graphical stack.
  • Where feasible, run Xwayland or X rootless rather than as root to reduce privilege-escalation impact.
  • Restrict local and interactive logon on sensitive hosts to trusted administrators until patches land.
  • Track downstream distributions and follow vendor guidance for any package not covered by Red Hat errata.

Validation and detection

  • Inventory hosts with xorg-x11-server, xorg-x11-server-Xwayland, or tigervnc installed and compare versions to the fixed NVRs in the RHSA advisories.
  • Query package managers (rpm -q, dnf updateinfo) for CVE-2026-50257 and confirm the errata IDs are applied.
  • Verify Xwayland or Xorg processes are running with least privilege and restart graphical sessions after patching.
  • Review authentication and session logs on multi-user Linux hosts for unexpected local logins during the exposure window.
  • Confirm configuration management or vulnerability scanners flag the CVE as remediated post-update.
Prepared
Confidence
high
Sources
12

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-416: Exact CWE lookup

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description · low confidence lookup

Privilege behavior lookup

The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2026-50257 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
7Timeline events
2ADP providers
25Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9redhat
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9redhat-SADP

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2026-50257Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. ADP timelineredhat-SADP

    Reported to Red Hat.

  2. Source timelineredhat

    Reported to Red Hat.

  3. ADP timelineredhat-SADP

    Made public.

  4. Source timelineredhat

    Made public.

  5. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  6. CVE publishedCVE Program

    The CVE record was published.

  7. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPxorg-x11-server: xorg-x11-server-Xwayland: xorg-x11-server: use-after-free in miSyncDestroyFence()
other:Red Hat severity ratingcvssV3_1
  • 2026-05-15T03:14:46.000Z: Reported to Red Hat.
  • 2026-06-02T00:00:00.000Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
Red HatRed Hat Enterprise Linux 10xorg-x11-server-Xwayland, 0:24.1.9-4.el10_2.2affected
Red HatRed Hat Enterprise Linux 10.0 Extended Update Supportxorg-x11-server-Xwayland, 0:24.1.5-6.el10_0.1affected
Red HatRed Hat Enterprise Linux 7 Extended Lifecycle Supportxorg-x11-server, 0:1.20.4-35.el7_9affected
Red HatRed Hat Enterprise Linux 8xorg-x11-server-Xwayland, 0:21.1.3-20.el8_10.2affected
Red HatRed Hat Enterprise Linux 8xorg-x11-server, 0:1.20.11-28.el8_10.2affected
Red HatRed Hat Enterprise Linux 8tigervnc, 0:1.15.0-10.el8_10affected
Red HatRed Hat Enterprise Linux 8.4 Advanced Mission Critical Update Supportxorg-x11-server, 0:1.20.10-5.el8_4affected
Red HatRed Hat Enterprise Linux 8.4 Extended Update Support Long-Life Add-Onxorg-x11-server, 0:1.20.10-5.el8_4affected
Red HatRed Hat Enterprise Linux 8.6 Advanced Mission Critical Update Supportxorg-x11-server, 0:1.20.11-8.el8_6affected
Red HatRed Hat Enterprise Linux 8.6 Extended Update Support Long-Life Add-Onxorg-x11-server, 0:1.20.11-8.el8_6affected
Red HatRed Hat Enterprise Linux 8.8 Telecommunications Update Servicexorg-x11-server, 0:1.20.11-19.el8_8affected
Red HatRed Hat Enterprise Linux 8.8 Update Services for SAP Solutionsxorg-x11-server, 0:1.20.11-19.el8_8affected
Red HatRed Hat Enterprise Linux 9xorg-x11-server-Xwayland, 0:24.1.9-4.el9_8.2affected
Red HatRed Hat Enterprise Linux 9xorg-x11-server, 0:1.20.11-34.el9_8.2affected
Red HatRed Hat Enterprise Linux 9tigervnc, 0:1.15.0-7.el9_8.2affected
Red HatRed Hat Enterprise Linux 9.2 Update Services for SAP Solutionsxorg-x11-server, 0:1.20.11-21.el9_2affected
Red HatRed Hat Enterprise Linux 9.2 Update Services for SAP Solutionsxorg-x11-server-Xwayland, 0:21.1.3-10.el9_2.1affected
Red HatRed Hat Enterprise Linux 9.4 Update Services for SAP Solutionsxorg-x11-server, 0:1.20.11-29.el9_4affected
Red HatRed Hat Enterprise Linux 9.4 Update Services for SAP Solutionsxorg-x11-server-Xwayland, 0:22.1.9-8.el9_4.1affected
Red HatRed Hat Enterprise Linux 9.6 Extended Update Supportxorg-x11-server, 0:1.20.11-34.el9_6affected
Red HatRed Hat Enterprise Linux 9.6 Extended Update Supportxorg-x11-server-Xwayland, 0:23.2.7-6.el9_6.1affected
Red HatRed Hat Enterprise Linux 6tigervncaffected
Red HatRed Hat Enterprise Linux 6xorg-x11-serverunknown
Red HatRed Hat Enterprise Linux 7tigervncaffected
Weakness

CWE details

CWE-416

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-416 · source CWE mapping

Use After Free

Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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