CVE-2021-4009

Root code execution via unchecked XFixes request length

Published Dec 17, 2021 · Updated Aug 3, 2024

Out-of-bounds write in X.Org Server before 1.20.14 and 21.1.0 through 21.1.1 allows local users to execute code as root. The SProcXFixesCreatePointerBarrier handler fails to validate a byte-swapped XFixes request length before accessing its fields, causing a write past the allocated request buffer. Exploitation requires low-privileged code execution and authorization to connect as an X client; success grants root-context code execution.

CVSS severity7.8
High
EPSS probability0.57%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in X.Org Server before 1.20.14 and 21.1.0 through 21.1.1 allows local users to execute code as root. The SProcXFixesCreatePointerBarrier handler fails to validate a byte-swapped XFixes request length before accessing its fields, causing a write past the allocated request buffer. Exploitation requires low-privileged code execution and authorization to connect as an X client; success grants root-context code execution.

The record

CVE
CVE-2021-4009
Published
Dec 17, 2021
Updated
Aug 3, 2024
Vendor
X.Org
Product
X.Org Server
Classifications
CWE-119, T1068
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Dec 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=xorg-x11-server 21.1.2, xorg-x11-server 1.20.14

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

X.Org · X.Org Serverversion=xorg-x11-server 21.1.2, xorg-x11-server 1.20.14

Published CVSS scores

7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.8Zero Day InitiativeCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.2NIST NVDAV:L/AC:L/Au:N/C:C/I:C/A:C

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-119Improper Restriction of Operations within the Bounds of a Memory Buffer
T1068Exploitation for Privilege Escalation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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