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CVE Record

CVE-2026-50256: Xorg-x11-server: xorg-x11-server-xwayland: xorg-x11-server: stack buffer overflow in font alias resolution due to libxfont2 name length mismatch

A stack-based buffer overflow flaw was found in the X.Org X server and Xwayland. A mismatch between the X server and the libXfont2 library's maximum font name length can cause a stack buffer overflow during font alias resolution. The server allocates a 256 byte stack buffer but libXfont2's alias target name length is 1024 bytes. A font alias name between 257 and 1023 bytes causes the X server to copy that name into the undersized stack buffer without further checks. 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 lets a local user crash the graphical display service or potentially take it over. It happens when the server processes an unusually long font alias name and writes it into a memory area that is too small. On systems where the X server runs with high privileges, this could allow the attacker to gain root-level control of the machine.

Executive priority

Treat as a near-term patching priority for Linux desktops, VDI, and remote-graphics servers. It is not remotely exploitable and is not on CISA KEV, but the local privilege-escalation path on root-privileged X servers makes it a meaningful risk on shared or multi-user Linux systems. Fold into the next standard patch cycle and expedite for exposed multi-user hosts.

Technical view

A size mismatch between the X server and libXfont2 creates a stack-based buffer overflow (CWE-121) during font alias resolution. The X server allocates a 256-byte stack buffer while libXfont2 permits alias target names up to 1024 bytes. An alias name between 257 and 1023 bytes is copied into the smaller buffer without bounds checking. CVSS 3.1 is 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H), reflecting local, low-complexity, low-privilege exploitation with full impact.

Likely exposure

Red Hat lists RHEL 7 ELS, 8 (including AUS/EUS/E4S/TUS variants), 9, and 10 as affected via xorg-x11-server, xorg-x11-server-Xwayland, and tigervnc packages. Any workstation, thin client, VDI host, or remote-desktop server running X.Org or Xwayland is potentially exposed, especially multi-user systems where local accounts exist.

Exploitation context

Attack vector is local and requires authenticated low-privilege access; no user interaction is needed. Not listed in CISA KEV and no public exploitation has been cited in the source bundle. Impact escalates to privilege escalation when the X server runs as root, which is common on traditional X11 desktop and VNC deployments.

Researcher notes

Root cause is a constant mismatch: X server uses a 256-byte on-stack buffer for font alias resolution while libXfont2 exposes names up to 1024 bytes. Trigger surface is font alias handling, so review any path that lets a local user influence font configuration or font paths. Multiple RHSA errata cover RHEL 7 ELS through 10 including AUS/EUS/E4S/TUS streams; verify the correct advisory for your specific channel. No KEV listing and no public PoC cited in bundle.

Mitigation direction

  • Apply the Red Hat RHSA errata for xorg-x11-server, xorg-x11-server-Xwayland, and tigervnc matching your RHEL release channel.
  • Prioritize patching multi-user systems, jump hosts, VDI, and VNC/RDP gateways where local accounts exist.
  • Restrict local shell access on graphical hosts until packages are updated.
  • Where feasible, prefer rootless Xwayland or Wayland sessions over root-privileged X servers.
  • Monitor vendor guidance for downstream distributions not yet patched and track RHSA advisories for additional streams.

Validation and detection

  • Inventory hosts with xorg-x11-server, xorg-x11-server-Xwayland, libXfont2, or tigervnc installed.
  • Compare installed package versions against the fixed NEVRAs listed in the linked RHSA advisories for your RHEL stream.
  • Confirm no residual older X server binaries remain after update and restart affected sessions.
  • Review whether the X server on each host runs as root and record hosts where privilege escalation risk is highest.
  • Check auditd or journal logs for unexpected X server crashes that could indicate probing.
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-121: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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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-2026-50256 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
27Source 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-2026-50256Attack 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: stack buffer overflow in font alias resolution due to libXfont2 name length mismatch
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 Advanced Mission Critical Update Supportxorg-x11-server-Xwayland, 0:21.1.3-2.el8_6.7affected
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 8.6 Extended Update Support Long-Life Add-Onxorg-x11-server-Xwayland, 0:21.1.3-2.el8_6.7affected
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 Telecommunications Update Servicexorg-x11-server-Xwayland, 0:21.1.3-13.el8_8.1affected
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 8.8 Update Services for SAP Solutionsxorg-x11-server-Xwayland, 0:21.1.3-13.el8_8.1affected
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-121

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

CWE-121 · source CWE mapping

Stack-based Buffer Overflow

Stack-based Buffer Overflow represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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