IRONSMITHINTEL
HIGHCVSS8.4
|
Actively Exploited
|CISA KEV|CVE-2021-33739|Auth: none — unauthenticated|Reboot: required|Manual only

KB5003635: Windows Server Security Update (June 2021)

Microsoft Desktop Window Manager (DWM) Core Library contains an unspecified vulnerability that allows for privilege escalation.

Published Jun 8, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A local attacker, without authentication, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. 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. Microsoft DWM Core Library Elevation of Privilege Vulnerability Exploitation requires local access, low attack complexity, no authentication required, 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 1909: v < 10.0.18363.1621; windows 10 2004: v < 10.0.19041.1052; windows 10 20h2: v < 10.0.19042.1052; windows 10 21h1: v < 10.0.19043.1052; windows server 2004: v < 10.0.19041.1052; windows server 20h2: v < 10.0.19042.1052
Fixed inKB5003635, KB5003637 (applies to 13 product versions) — build 10.0.18363.1621, 10.0.19041.1052+
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2021-11-03 based on evidence of active exploitation in the wild. 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 CatalogKB5003635

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Updates

    1
    KB5003635 — https://support.microsoft.com/help/5003635
    1
    KB5003637 — https://support.microsoft.com/help/5003637

Supersedes: KB5003169, KB5003173

Affected Products

    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 Server, version 2004 (Server Core installation)
    1
    Windows Server, version 20H2 (Server Core Installation)

Fixed Build Numbers

    1
    10.0.18363.1621
    1
    10.0.19041.1052
    1
    10.0.19042.1052
    1
    10.0.19043.1052

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 KB5003635
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 KB5003635 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003635
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003637

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5003635','KB5003637') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33739
    1
    KB article: https://support.microsoft.com/help/5003635
    1
    KB article: https://support.microsoft.com/help/5003637
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-33739
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33739

Discovery Credit

Tu XiaoYi with DBAPPSecurity Threat Intelligence Center, Dong Wu with DBAPPSecurity Threat Intelligence Center, Jinquan(@jq0904) with DBAPPSecurity Threat Intelligence Center

PowerShell automationComing soon

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

References

↗ CVE-2021-33739↗ MSRC Advisory↗ NVD↗ KBKB5003635