CVE-2021-4440

Kernel data disclosure via omitted MDS buffer clearing

Published Jun 25, 2024 · Updated May 11, 2026

Information disclosure in Linux kernel 5.10.215 through 5.10.217 allows local users to read kernel data through microarchitectural sampling. The 64-bit syscall return path runtime-patches USERGS_SYSRET64 to swapgs; sysretq without CLEAR_CPU_BUFFERS, leaving MDS-stale CPU buffers uncleared. Exposure requires an affected x86 kernel built with CONFIG_PARAVIRT_XXL; Xen need not run, and leaked data can include kernel pointers that defeat KASLR.

CVSS severity8.8
High
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Linux kernel 5.10.215 through 5.10.217 allows local users to read kernel data through microarchitectural sampling. The 64-bit syscall return path runtime-patches USERGS_SYSRET64 to swapgs; sysretq without CLEAR_CPU_BUFFERS, leaving MDS-stale CPU buffers uncleared. Exposure requires an affected x86 kernel built with CONFIG_PARAVIRT_XXL; Xen need not run, and leaked data can include kernel pointers that defeat KASLR.

The record

CVE
CVE-2021-4440
Published
Jun 25, 2024
Updated
May 11, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-400
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 25, 2024CVE publishedPublication date reported by the CVE source.
  2. May 11, 2026Record 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=5.10.215 <5.10.218
  2. Affected versionversion=cea750c99d8f6391080c420f811a46b21bad7cf4 <1424ab4bb386df9cc590c73afa55f13e9b00dea2

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=5.10.215 <5.10.218; version=cea750c99d8f6391080c420f811a46b21bad7cf4 <1424ab4bb386df9cc590c73afa55f13e9b00dea2
The Linux Kernel Organization · Linux Kernelversion=5.10.215 <5.10.218; version=cea750c99d8f <1424ab4bb386

Published CVSS scores

8.8UbuntuCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

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-400Uncontrolled Resource Consumption

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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