dbcve.org
Windows 10 1607Operating system · Microsoft

CVE-2026-20820

HIGH · 7.8 CVSS v3.1 Published 2026-01-13
Fix available
A fix is available. Upgrade to 10.0.14393.8783 / 10.0.17763.8276 or later.
See remediation →
80/100
Remediation priority · High
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Heap-based buffer overflow in Windows Common Log File System Driver allows an authorized attacker to elevate privileges locally.

In the news

Third-party coverage
Trending covered by 2 outlets this week · latest 7mo ago

Surfaced from public web coverage — external links open in a new tab.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

Heap-based buffer overflow in the Windows Common Log File System Driver (CLFS.sys) allows a local authenticated attacker to execute code with elevated kernel privileges, potentially achieving SYSTEM-level access.

MitigationApply the Microsoft security update for this vulnerability once released; verify patch deployment via vulnerability scanning or system inventory; disable CLFS.sys if not required in the environment.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Windows 10 1607Operating system
Affected:< 10.0.14393.8783
Windows 10 1809Operating system
Affected:< 10.0.17763.8276
Windows 10 21h2Operating system
Affected:< 10.0.19044.6809
Windows 10 22h2Operating system
Affected:< 10.0.19045.6809
Windows 11 23h2Operating system
Affected:< 10.0.22631.6491
Windows 11 24h2Operating system
Affected:< 10.0.26100.7623
Windows 11 25h2Operating system
Affected:< 10.0.26200.7623
Windows Server 2008Operating system
Affected:all versions= r2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed Windows version
    Run `winver` or `systeminfo | findstr /B /C:"OS"` to display the Windows version and build number
    Affected if The displayed version and build number fall within the affected ranges (Windows 10 1607 < 10.0.14393.8783, 1809 < 10.0.17763.8276, 21h2 < 10.0.19044.6809, 22h2 < 10.0.19045.6809; Windows 11 23h2 < 10.0.22631.6491, 24h2 < 10.0.26100.7623, 25h2 < 10.0.26200.7623; or Windows Server 2008 R2 any version)
  2. Verify CLFS.sys driver version
    Open File Explorer, navigate to `C:\Windows\System32\drivers\`, right-click clfs.sys, select Properties, and view the File Version on the Details tab; alternatively run `powershell -command "(Get-Item 'C:\Windows\System32\drivers\clfs.sys').VersionInfo.FileVersion"`
    Affected if The CLFS.sys file version is lower than the version corresponding to the patched build for your Windows version
  3. Confirm kernel-level access exposure
    Verify the system has CLFS.sys loaded by running `driverquery /v | findstr -i clfs` or checking the presence of the driver in the registry under `HKLM\SYSTEM\CurrentControlSet\Services\CLFS`
    Affected if The CLFS.sys driver is present and loaded on the system, exposing the vulnerable code path

You are affected if your Windows version or CLFS.sys driver version is below the patched build numbers listed in the affected version ranges, and the CLFS driver is enabled on your system.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 10.0.14393.8783 / 10.0.17763.8276 / 10.0.19044.6809 or later
Fixed in 10.0.14393.878310.0.17763.827610.0.19044.6809
Interim mitigation

Apply the Microsoft security update for this vulnerability once released; verify patch deployment via vulnerability scanning or system inventory; disable CLFS.sys if not required in the environment.

Recommended fix High confidence

Install the latest Windows cumulative security update that includes the fix for CVE-2026-20820, targeting the minimum fixed build for your Windows version (e.g., 10.0.19045.6809 for Win10 22h2)

  1. Identify current Windows build version by running 'winver' or 'systeminfo | findstr /B /C:"OS Build"'
  2. Download and install the appropriate Windows Update from Microsoft Update Catalog or Windows Update for the specific version
  3. For Windows 10 1607: Install update build 10.0.14393.8783 or later
  4. For Windows 10 1809: Install update build 10.0.17763.8276 or later
  5. For Windows 10 21h2: Install update build 10.0.19044.6809 or later
  6. For Windows 10 22h2: Install update build 10.0.19045.6809 or later
  7. For Windows 11 23h2: Install update build 10.0.22631.6491 or later
  8. For Windows 11 24h2: Install update build 10.0.26100.7623 or later
Caveat Standard Windows update risks apply - ensure backups and compatibility testing for critical systems; Windows Server 2008 R2 is end-of-life with no patch available

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Windows 10 1607 Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing8.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,720
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,352.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-20820 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data