IRONSMITHINTEL
HIGHCVSS7.8
|
Actively Exploited
|CISA KEV|CVE-2021-36955|Auth: low — authenticated user|Reboot: required|Manual only

KB5005565: Windows Server Security Update (September 2021)

Microsoft Windows Common Log File System (CLFS) driver contains an unspecified vulnerability that allows for privilege escalation.

Published Sep 15, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A local attacker, with a low-privilege account, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. CISA has confirmed use of this vulnerability in known ransomware campaigns — treat as high priority for remediation. Federal agencies are required to remediate by 2021-11-17 under CISA BOD 22-01.

How the attack worksNo clicks needed

This vulnerability affects Microsoft Windows. Windows Common Log File System Driver Elevation of Privilege Vulnerability Exploitation requires local access, low attack complexity, a low-privilege authenticated account, and no user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

●Windows Administrators
●Systems Engineers
●IT Security
●Running windows 10 1507: v < 10.0.10240.19060; windows 10 1607: v < 10.0.14393.4651; windows 10 1809: v < 10.0.17763.2183; windows 10 1909: v < 10.0.18363.1801; windows 10 2004: v < 10.0.19041.1237; windows 10 20h2: v < 10.0.19042.1237; windows 10 21h1: v < 10.0.19043.1237; windows 7: -; windows 8.1: -; windows rt 8.1: -; windows server 2004: v < 10.0.19041.1237; windows server 2008: -, r2; windows server 2012: -; windows server 2016: v < 10.0.14393.4651; windows server 2019: v < 10.0.17763.2183; windows server 2022: v < 10.0.20348.230; windows server 20h2: v < 10.0.19042.1237
Fixed inKB5005565, KB5005566, KB5005568, KB5005569, KB5005573, KB5005575, KB5005606, KB5005607, KB5005613, KB5005615, KB5005618, KB5005623, KB5005627, KB5005633 (applies to 42 product versions) — build 10.0.10240.19060, 10.0.14393.4651+
Real-world incidentsWhat we've seen

CISA confirms this CVE has been used in known ransomware campaigns. Added to the KEV catalog on 2021-11-03; federal agencies required to remediate by 2021-11-17.

How to patch

Manual download

For air-gapped servers or out-of-band deployment. Microsoft Update Catalog returns every OS-version variant of this update.

↗ Microsoft Update CatalogKB5005565

Manual remediation steps

Apply the Microsoft Security Update

Microsoft has released an official security update that fixes this vulnerability.

Required KB Updates

    1
    KB5005565 — https://support.microsoft.com/help/5005565
    1
    KB5005566 — https://support.microsoft.com/help/5005566
    1
    KB5005568 — https://support.microsoft.com/help/5005568
    1
    KB5005569 — https://support.microsoft.com/help/5005569
    1
    KB5005573 — https://support.microsoft.com/help/5005573
    1
    KB5005575 — https://support.microsoft.com/help/5005575
    1
    KB5005606 — https://support.microsoft.com/help/5005606
    1
    KB5005607 — https://support.microsoft.com/help/5005607
    1
    KB5005613 — https://support.microsoft.com/help/5005613
    1
    KB5005615 — https://support.microsoft.com/help/5005615
    1
    KB5005618 — https://support.microsoft.com/help/5005618
    1
    KB5005623 — https://support.microsoft.com/help/5005623
    1
    KB5005627 — https://support.microsoft.com/help/5005627
    1
    KB5005633 — https://support.microsoft.com/help/5005633

Supersedes: KB5005030, KB5005031, KB5005033, KB5005040, KB5005043, KB5005076, KB5005088, KB5005090, KB5005094

Affected Products

    1
    Windows 10 Version 1607 for 32-bit Systems
    1
    Windows 10 Version 1607 for x64-based Systems
    1
    Windows 10 Version 1809 for 32-bit Systems
    1
    Windows 10 Version 1809 for ARM64-based Systems
    1
    Windows 10 Version 1809 for x64-based Systems
    1
    Windows 10 Version 1909 for 32-bit Systems
    1
    Windows 10 Version 1909 for ARM64-based Systems
    1
    Windows 10 Version 1909 for x64-based Systems
    1
    Windows 10 Version 2004 for 32-bit Systems
    1
    Windows 10 Version 2004 for ARM64-based Systems
    1
    Windows 10 Version 2004 for x64-based Systems
    1
    Windows 10 Version 20H2 for 32-bit Systems
    1
    Windows 10 Version 20H2 for ARM64-based Systems
    1
    Windows 10 Version 20H2 for x64-based Systems
    1
    Windows 10 Version 21H1 for 32-bit Systems
    1
    Windows 10 Version 21H1 for ARM64-based Systems
    1
    Windows 10 Version 21H1 for x64-based Systems
    1
    Windows 10 for 32-bit Systems
    1
    Windows 10 for x64-based Systems
    1
    Windows 7 for 32-bit Systems Service Pack 1
    1
    (…22 more product versions)

Fixed Build Numbers

    1
    10.0.10240.19060
    1
    10.0.14393.4651
    1
    10.0.17763.2183
    1
    10.0.18363.1801
    1
    10.0.19041.1237
    1
    10.0.19042.1237
    1
    10.0.19043.1237
    1
    10.0.20348.230
    1
    6.0.6003.21218
    1
    6.1.7601.25712
    1
    (…2 more builds)

Installation Methods

Windows Update (recommended)

1
Settings → Windows Update → Check for updates
2
The security update is offered if your system is in scope
3
Restart when prompted — a reboot IS required to complete the install

Microsoft Update Catalog (manual download)

1
Open https://catalog.update.microsoft.com
2
Search for KB5005565
3
Download the package matching your OS architecture and Windows build
4
Run the .msu installer with administrator privileges
5
Restart when prompted

WSUS / SCCM / Intune

Approve KB5005565 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5005565
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5005566
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5005568
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5005569
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5005573
    1
    (…9 more)

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5005565','KB5005566','KB5005568','KB5005569','KB5005573','KB5005575','KB5005606','KB5005607','KB5005613','KB5005615','KB5005618','KB5005623','KB5005627','KB5005633') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36955
    1
    KB article: https://support.microsoft.com/help/5005565
    1
    KB article: https://support.microsoft.com/help/5005568
    1
    KB article: https://support.microsoft.com/help/5005606
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-36955
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-36955
PowerShell automationComing soon

No tested PowerShell script for this entry yet. We’re prioritising automation based on user demand.

References

↗ CVE-2021-36955↗ MSRC Advisory↗ NVD↗ KBKB5005565