Microsoft Windows Security Updates April 2021 overview


Microsoft released security and non-security updates for supported versions of Microsoft Windows and other company products such as Office on the April 2021 Patch Tuesday.

Our guide provides administrators and home users with information on the released updates. It links to all support pages, lists known issues and direct patch downloads, and lists all security and non-security update releases of the day.

Click here to open the March 2021 Patch Day overview in case you missed it or want to revisit it.

The Microsoft Windows Security Updates: April 2021

Click on the following link to download an Excel spreadsheet with the released security updates: Security-updates-windows-april-2021

Executive Summary

  • Security updates are available for all supported versions of Windows (client and server).
  • Microsoft released security updates for other company products including Azure, Microsoft Edge, Exchange Server, Microsoft Office, Visual Studio and Visual Studio Code, and Windows Media Player.
  • If you manage Exchange Servers, check out this blog post on the April 2021 security updates.
  • Several updates have known issues, including those for Windows 7 and 8.1, various Windows 10 versions, Windows Server versions, and Microsoft Exchange Server.
  • Windows 10 version 1909 for Home customers runs out of support after the May 2021 Patch Day.

Operating System Distribution

  • Windows 7  (extended support only): 50 vulnerabilities: 14 critical and 36 important
    • CVE-2021-27095 — Windows Media Video Decoder Remote Code Execution Vulnerability
    • CVE-2021-28315 — Windows Media Video Decoder Remote Code Execution Vulnerability
    • CVE-2021-28329 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28330 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28331 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28332 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28333 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
      CVE-2021-28334  — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28335 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28336 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28337 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28338 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28339 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28343 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
  • Windows 8.1: 5 5vulnerabilities: 14 critical and 41 important
    • same as Windows 7
  •  Windows 10 version 1903 and 1909: 77 vulnerabilities: 14 critical and 62 important and 1 moderate
    • same as Windows 7
  • Windows 10 version 2004 and 20H2: 79 vulnerabilities, 14 critical and 64 important and 1 moderate
    • same as Windows 7

Windows Server products

  • Windows Server 2008 R2 (extended support only): 47 vulnerabilities: 14 critical and 33 important
    • CVE-2021-27095 — Windows Media Video Decoder Remote Code Execution Vulnerability
    • CVE-2021-28315 — Windows Media Video Decoder Remote Code Execution Vulnerability
    • CVE-2021-28329 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28330 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28331 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28332 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28333 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
      CVE-2021-28334  — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28335 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28336 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28337 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28338 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28339 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
    • CVE-2021-28343 — Remote Procedure Call Runtime Remote Code Execution Vulnerability
  • Windows Server 2012 R2: 55  vulnerabilities: 14 critical and 41 important
    • same as Windows Sever 2008 R2
  • Windows Server 2016: 63 vulnerabilities: 14 critical and 49 important.
    • same as Windows Sever 2008 R2
  • Windows Server 2019: 77 vulnerabilities: 14 critical and 62 important and 1 moderate
    • same as Windows Sever 2008 R2

Windows Security Updates

Windows 7 SP1 and Windows Server 2008 R2

Updates and improvements:

  • Time Zone change for Volgograd, Russia and The Republic of South Sudan.
  • Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
  • Addressed an issue that prevents users from specifying up to 255 columns when using the Jet Text installable indexed sequential access method (IISAM) format. (Monthly Rollup only)
  • Fixed a Windows Backup Event ID reporting issue. (Monthly Rollup only)
  • Fixed security issues.

Windows 8.1 and Windows Server 2012 R2

Updates and improvements:

  • Time Zone change for Volgograd, Russia and The Republic of South Sudan.
  • Fixed security issues.
  • Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
  • Addressed an issue that prevents users from specifying up to 255 columns when using the Jet Text installable indexed sequential access method (IISAM) format. (Monthly Rollup only)

Windows 10 version 1909

Updates and improvements:

  • Fixed a “potential elevation of privilege vulnerability in the way Azure Active Directory web sign-in allows arbitrary browsing”. See Policy CSP Authentication for more information.
  • Fixed an issue in “which a principal in a trusted MIT realm fails to obtain a Kerberos service ticket from Active Directory domain controllers (DC)”.
  • Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
  • Security updates.

Windows 10 version 2004 and 20H2

Updates and improvements:

  • same as Windows 10 version 1909

Other security updates

2021-04 Cumulative Update for Windows 10 Version 1507 (KB5001340)

2021-04 Cumulative Update for Windows 10 Version 1803 (KB5001339)

2021-04 Cumulative Update for Windows 10 Version 1809 (KB5001342)

2021-04 Security Only Quality Update for Windows Server 2008 (KB5001332)

2021-04 Security Monthly Quality Rollup for Windows Server 2008 (KB5001389)

2021-04 Security Only Quality Update for Windows Embedded 8 Standard and Windows Server 2012 (KB5001383)

2021-04 Security Monthly Quality Rollup for Windows Embedded 8 Standard and Windows Server 2012 (KB5001387)

2021-04 Cumulative Update for Windows Server 2016 and Windows 10 Version 1607 (KB5001347)

Known Issues

Windows 7 and Windows Server 2008 R2

  • Updates may be reverted if the machine is not supported for ESU.
  • Some operations may fail on Cluster Shared Volumes. Workaround available.

Windows 8.1 and Windows Server 2012 R2

  • Some operations may fail on Cluster Shared Volumes. Workaround available.

Windows 10 version 1909

  • System and user certificates may be lost when updating to a newer version of Windows. Workaround available, Microsoft is working on a solution.

Windows 10 version 2004 and 20H2

  • System and user certificates may be lost when updating to a newer version of Windows. Workaround available, Microsoft is working on a solution.
  • Issue with the Microsoft Japanese Input Method Editor and Kanji / Furigana characters.
  • Some devices, created using custom offline media or ISO images, may have Edge Legacy removed but not replaced by the new Edge. Microsoft has a workaround for this.

Security advisories and updates

ADV 990001 — Latest Servicing Stack Updates

Non-security related updates

2021-04 Update for Windows 8.1, Windows Server 2012 R2, Windows Embedded 8 Standard, Windows Server 2012, Windows 7, Windows Server 2008 R2, and Windows Server 2008 (KB4601275)

Windows Malicious Software Removal Tool – v5.88 (KB890830)

Microsoft Office Updates

You find Office update information here.

How to download and install the April 2021 security updates

microsoft windows security updates april 2021

Security updates are installed automatically on most Windows devices thanks to the built-in automatic updating functionality. Administrators may run manual checks for updates to download and install updates early, or download updates directly from the Microsoft Update Catalog website or by using first-party or third-party update management tools and services.

Note: we recommend to create backups before the installation of updates, as updates may have issues.

Windows administrators may run the following steps to check for updates manually on Windows devices:

  1. Select Start, type Windows Update and load the Windows Update item that is displayed.
  2. Select check for updates to run a manual check for updates.

Direct update downloads

Below are resource pages with direct download links, if you prefer to download the updates to install them manually.

Windows 7 and Server 2008 R2

  • KB5001335 — 2021-04 Security Monthly Quality Rollup for Windows 7
  • KB5001392 — 2021-04 Security Only Quality Update for Windows 7

Windows 8.1 and Windows Server 2012 R2

  • KB5001382 — 2021-04 Security Monthly Quality Rollup for Windows 8.1
  • KB5001393 — 2021-04 Security Only Quality Update for Windows 8.1

Windows 10 (version 1909)

  • KB5001337  — 2021-04 Cumulative Update for Windows 10 Version 1909

Windows 10 (version 2004)

  • KB5001330  — 2021-04 Cumulative Update for Windows 10 Version 2004

Windows 10 (version 20H2)

  • KB5001330 — 2021-04 Cumulative Update for Windows 10 Version  20H2

Additional resources

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