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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 17 Nov 2021. Known ransomware use
Windows 10 1507Operating system · Microsoft

CVE-2021-36955

HIGH · 7.8 CVSS v3.1 Published 2021-09-15
Fix available
A fix is available. Upgrade to 10.0.10240.19060 / 10.0.14393.4651 or later.
See remediation →
96/100
Remediation priority · Urgent
In the wild Ransomware Zero-click Patch available

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
Windows Common Log File System Driver Elevation of Privilege Vulnerability

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

Windows Common Log File System (CLFS) driver contains an elevation of privilege vulnerability that allows a local authenticated attacker to execute arbitrary code in kernel mode. The vulnerability stems from improper handling of objects in the CLFS driver, enabling privilege escalation from a low-privilege user to SYSTEM.

MitigationApply the Microsoft security update KB5005112 (August 2021 Patch Tuesday) or subsequent relevant security patches that address CVE-2021-36955. Organizations should prioritize patching affected Windows servers and workstations given the low attack complexity and high impact.

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 1507Operating system
Affected:< 10.0.10240.19060
Windows 10 1607Operating system
Affected:< 10.0.14393.4651
Windows 10 1809Operating system
Affected:< 10.0.17763.2183
Windows 10 1909Operating system
Affected:< 10.0.18363.1801
Windows 10 2004Operating system
Affected:< 10.0.19041.1237
Windows 10 20h2Operating system
Affected:< 10.0.19042.1237
Windows 10 21h1Operating system
Affected:< 10.0.19043.1237
Windows 7Operating system
Affected:all versions

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. Identify Windows version and build number
    Open Command Prompt or PowerShell and run 'winver' or 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to get the exact Windows version and build number.
    Affected if The build number is lower than the fixed versions (19060 for 1507, 4651 for 1607, 2183 for 1809, 1801 for 1909, 1237 for 2004/20h2/21h1), or if running any version of Windows 7.
  2. Confirm CLFS driver is present
    Check that the Common Log File System driver exists by running 'dir C:\Windows\System32\drivers\clfs.sys' in Command Prompt.
    Affected if The clfs.sys file exists on the system, which is required for the vulnerability to be exploitable.
  3. Check running kernel version of clfs.sys
    Run 'driverquery /v | findstr clfs' or check file properties of C:\Windows\System32\drivers\clfs.sys (right-click, Properties, Details tab) to see the file version.
    Affected if The clfs.sys version corresponds to an unpatched Windows build that matches the affected version ranges.
  4. Verify local user access context
    Determine if standard user accounts exist on the system using 'net user' or check user memberships with 'net localgroup Users'. The vulnerability requires a locally authenticated user.
    Affected if The system has locally created standard user accounts (non-domain, non-admin) that could potentially exploit this vulnerability.

The system is affected if it runs Windows 7 (any version) or Windows 10 build numbers below 19060 (1507), 4651 (1607), 2183 (1809), 1801 (1909), or 1237 (2004/20h2/21h1), and the CLFS driver is present with standard local user accounts on the machine.

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 10.0.10240.19060 / 10.0.14393.4651 / 10.0.17763.2183 or later
Fixed in 10.0.10240.1906010.0.14393.465110.0.17763.2183
Vendor patch portal.msrc.microsoft.com →
Interim mitigation

Apply the Microsoft security update KB5005112 (August 2021 Patch Tuesday) or subsequent relevant security patches that address CVE-2021-36955. Organizations should prioritize patching affected Windows servers and workstations given the low attack complexity and high impact.

Recommended fix High confidence

Windows 10 1507: build 10.0.10240.19060+ | Windows 10 1607: build 10.0.14393.4651+ | Windows 10 1809: build 10.0.17763.2183+ | Windows 10 1909: build 10.0.18363.1801+ | Windows 10 2004: build 10.0.19041.1237+ | Windows 10 20h2: build 10.0.19042.1237+ | Windows 10 21h1: build 10.0.19043.1237+ | Windo

  1. Identify the current Windows version by running 'winver' or 'systeminfo' in Command Prompt
  2. Determine which Windows 10 version branch (1507, 1607, 1809, 1909, 2004, 20h2, or 21h1) is installed
  3. For Windows 10: Navigate to Windows Update > Check for Updates and install all cumulative updates until the build number matches or exceeds the fixed version for your branch
  4. For Windows 10 21h1: Install KB5005101 or later cumulative update to reach build 10.0.19043.1288 or above
  5. For Windows 10 20h2: Install KB5005102 or later cumulative update to reach build 10.0.19042.1288 or above
  6. For Windows 10 2004: Install KB5005043 or later cumulative update
  7. For Windows 10 1909: Install KB5005040 or later cumulative update
  8. For Windows 10 1809: Install KB5005039 or later cumulative update
Caveat Windows 7 is end-of-life; ESU requires paid enrollment and only provides critical security updates for limited time

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

Fix this in Windows 10 1507 Exploited in the wild — priority engagement
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,984.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-36955 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

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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-2021-36955 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