IRONSMITHINTEL
HIGHCVSS7.8
|
Actively Exploited
|CISA KEV|CVE-2021-40449|Auth: low — authenticated user|Reboot: required|Manual only

KB5006667: Windows Server Security Update (October 2021)

Unspecified vulnerability allows for an authenticated user to escalate privileges.

Published Oct 13, 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 2021-12-01 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Use After Free (CWE-416) vulnerability in Microsoft Windows. 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 < 10.0.10240.19086; windows 10 1607: v < 10.0.14393.4704; windows 10 1809: v < 10.0.17763.2237; windows 10 1909: v < 10.0.18363.1854; windows 10 2004: v < 10.0.19041.1288; windows 10 20h2: v < 10.0.19041.1288; windows 10 21h1: v < 10.0.19041.1288; windows 11 21h2: v < 10.0.22000.258; windows 7: -; windows 8.1: -; windows rt 8.1: -; windows server 2004: v < 10.0.19041.1288; windows server 2008: -, r2; windows server 2012: -, r2; windows server 2016: v < 10.0.14393.4704; windows server 2019: v < 10.0.17763.2237; windows server 2022: v < 10.0.20348.288; windows server 20h2: v < 10.0.19042.1288
Fixed inKB5006667, KB5006669, KB5006670, KB5006672, KB5006674, KB5006675, KB5006699, KB5006714, KB5006715, KB5006728, KB5006729, KB5006732, KB5006736, KB5006739, KB5006743 (applies to 43 product versions) — build 10.0.10240.19086, 10.0.14393.4704+
Real-world incidentsWhat we've seen

Used in known ransomware campaigns. Threat-research write-up: http://packetstormsecurity.com/files/164926/Win32k-NtGdiResetDC-Use-After-Free-Local-Privilege-Escalation.html

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 CatalogKB5006667

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Updates

    1
    KB5006667 — https://support.microsoft.com/help/5006667
    1
    KB5006669 — https://support.microsoft.com/help/5006669
    1
    KB5006670 — https://support.microsoft.com/help/5006670
    1
    KB5006672 — https://support.microsoft.com/help/5006672
    1
    KB5006674 — https://support.microsoft.com/help/5006674
    1
    KB5006675 — https://support.microsoft.com/help/5006675
    1
    KB5006699 — https://support.microsoft.com/help/5006699
    1
    KB5006714 — https://support.microsoft.com/help/5006714
    1
    KB5006715 — https://support.microsoft.com/help/5006715
    1
    KB5006728 — https://support.microsoft.com/help/5006728
    1
    KB5006729 — https://support.microsoft.com/help/5006729
    1
    KB5006732 — https://support.microsoft.com/help/5006732
    1
    KB5006736 — https://support.microsoft.com/help/5006736
    1
    KB5006739 — https://support.microsoft.com/help/5006739
    1
    KB5006743 — https://support.microsoft.com/help/5006743

Supersedes: KB5005565, KB5005566, KB5005568, KB5005569, KB5005573, KB5005575, KB5005606, KB5005613, KB5005623, KB5005633

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 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 11 version 21H2 for ARM64-based Systems
    1
    Windows 11 version 21H2 for x64-based Systems
    1
    (…23 more product versions)

Fixed Build Numbers

    1
    10.0.10240.19086
    1
    10.0.14393.4704
    1
    10.0.17763.2237
    1
    10.0.18363.1854
    1
    10.0.19041.1288
    1
    10.0.19042.1288
    1
    10.0.20348.288
    1
    10.0.22000.258
    1
    6.0.6003.21251
    1
    6.1.7601.25740
    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 KB5006667
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 KB5006667 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5006667
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5006669
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5006670
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5006672
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5006674
    1
    (…10 more)

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5006667','KB5006669','KB5006670','KB5006672','KB5006674','KB5006675','KB5006699','KB5006714','KB5006715','KB5006728','KB5006729','KB5006732','KB5006736','KB5006739','KB5006743') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40449
    1
    KB article: https://support.microsoft.com/help/5006670
    1
    KB article: https://support.microsoft.com/help/5006672
    1
    KB article: https://support.microsoft.com/help/5006714
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-40449
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40449

Discovery Credit

Boris Larin (oct0xor) with Kaspersky

PowerShell automationComing soon

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

References

↗ CVE-2021-40449↗ MSRC Advisory↗ NVD↗ KBKB5006667