IRONSMITHINTEL
HIGHCVSS7.5
|
Actively Exploited
|CISA KEV|CVE-2021-33742|Auth: none — unauthenticated|Reboot: required|Manual only

KB5003635: Windows Server Security Update (June 2021)

Microsoft Windows MSHTML Platform contains an unspecified vulnerability that allows for remote code execution.

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

A remote 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 is a Out-of-bounds Write (CWE-787) vulnerability in Microsoft Windows. Windows MSHTML Platform Remote Code Execution Vulnerability Exploitation requires remote network access, higher attack complexity, no authentication required, and user interaction required.

📧

Phishing link

→

🖼

Malicious file

🔓

Server compromised

Am I affected?Quick check

Probably yes if any of these apply:

●Windows Administrators
●Systems Engineers
●IT Security
●Running windows 10 1507: v < 10.0.10240.18967; windows 10 1607: v < 10.0.14393.4467; windows 10 1809: v < 10.0.17763.1999; 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 7: -; windows 8.1: -; windows rt 8.1: -; windows server 2008: r2, sp2; windows server 2012: -, r2; windows server 2016: v < 10.0.14393.4467; windows server 2019: v < 10.0.17763.1999
Fixed inKB5003635, KB5003636, KB5003637, KB5003638, KB5003646, KB5003661, KB5003667, KB5003671, KB5003687, KB5003697 (applies to 30 product versions) — build 1.0.0.0, 10.0.10240.18967+
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
    KB5003636 — https://support.microsoft.com/help/5003636
    1
    KB5003637 — https://support.microsoft.com/help/5003637
    1
    KB5003638 — https://support.microsoft.com/help/5003638
    1
    KB5003646 — https://support.microsoft.com/help/5003646
    1
    KB5003661 — https://support.microsoft.com/help/5003661
    1
    KB5003667 — https://support.microsoft.com/help/5003667
    1
    KB5003671 — https://support.microsoft.com/help/5003671
    1
    KB5003687 — https://support.microsoft.com/help/5003687
    1
    KB5003697 — https://support.microsoft.com/help/5003697

Supersedes: KB5003165, KB5003169, KB5003171, KB5003172, KB5003173, KB5003197, KB5003208, KB5003209, KB5003210, KB5003233

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 7 for 32-bit Systems Service Pack 1
    1
    Windows 7 for x64-based Systems Service Pack 1
    1
    (…10 more product versions)

Fixed Build Numbers

    1
    1.0.0.0
    1
    10.0.10240.18967
    1
    10.0.14393.4467
    1
    10.0.17763.1999
    1
    10.0.18363.1621
    1
    10.0.19041.1052
    1
    10.0.19042.1052
    1
    10.0.19043.1052
    1
    6.0.6003.21137
    1
    6.1.7601.25632
    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 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=KB5003636
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003637
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003638
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003646
    1
    (…5 more)

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5003635','KB5003636','KB5003637','KB5003638','KB5003646','KB5003661','KB5003667','KB5003671','KB5003687','KB5003697') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33742
    1
    KB article: https://support.microsoft.com/help/5003635
    1
    KB article: https://support.microsoft.com/help/5003637
    1
    KB article: https://support.microsoft.com/help/5003646
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-33742
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33742

Discovery Credit

Clément Lecigne of Google’s Threat Analysis Group

PowerShell automationComing soon

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

References

↗ CVE-2021-33742↗ MSRC Advisory↗ NVD↗ KBKB5003635