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Release Date:



OS Build 10240.18842

NEW 8/5/21

IMPORTANT As of 8/5/2021, this KB is no longer available from Windows Update, the Microsoft Update Catalog, or other release channels. We recommend that you update your devices to the latest security quality update.


NEW 2/12/21
Important There is a Known Issue that halts the installation progress of the February 9, 2021 security update. To address this issue, we have released a new servicing stack update (SSU), KB5001079. You must install this new SSU before installing the February 9, 2021 security update.

For information about Windows update terminology, see the article about the types of Windows updates and the monthly quality update types. To view other notes and messages, see the Windows 10, version 1507 update history home page.


  • Corrects historical daylight savings time (DST) information for the Palestinian Authority.

  • Updates for verifying usernames and passwords.

  • Updates an issue with German translations of Central European Time.

Improvements and fixes

This security update includes quality improvements. Key changes include:

  • Enables administrators to disable standalone Internet Explorer using a Group Policy while continuing to use Microsoft Edge's IE Mode.

  • Updates Internet Explorer’s About dialog to use the standard modern dialog.

  • Corrects historical daylight savings time (DST) information for the Palestinian Authority.

  • Addresses an issue with German translations of Central European Time.

  • Addresses an issue that causes LSASS.exe to stop working because of a race condition that results in a double free error in Schannel. The exception code is c0000374, and the Event Log displays Schannel event 36888, fatal error code 20, and error state 960. This issue occurs after installing Windows updates from September 2020 and later.

  • Security updates to Windows App Platform and Frameworks, Windows Apps, Windows Authentication, Windows Fundamentals, and Windows Core Networking.

If you installed earlier updates, only the new fixes contained in this package will be downloaded and installed on your device.

For more information about the resolved security vulnerabilities, please refer to the new Security Update Guide website.

Windows Update Improvements

Microsoft has released an update directly to the Windows Update client to improve reliability. Any device running Windows 10 configured to receive updates automatically from Windows Update, including Enterprise and Pro editions, will be offered the latest Windows 10 feature update based on device compatibility and Windows Update for Business deferral policy. This doesn't apply to long-term servicing editions.

Known issues in this update



After installing the SSU KB4601390, installation of the Cumulative Update from Windows Update might not progress past 24%.

This issue was resolved in KB5001079.

To mitigate this issue on devices that have already installed KB4601390 and are not making progress installing KB4601331, restart your device and then follow only steps 1, 2 and 4a from Reset Windows Update components manually. Then restart your device again. KB5001079 should now install from Windows Update when you select "check for updates" or you can wait for it to install automatically. You should then be able to install the latest Cumulative Update from Windows Update.

Note KB4601390 has been removed and will no longer be offered to devices.

How to get this update

Before installing this update


You must install the new servicing stack update (SSU) KB5001079 before installing this cumulative update (LCU). SSUs improve the reliability of the update process to mitigate potential issues while installing the LCU and applying Microsoft security fixes. For general information about SSUs, see Servicing stack updates and Servicing Stack Updates (SSU): Frequently Asked Questions.

If you are using Windows Update, the latest SSU (KB5001079) will be offered to you automatically. To get the standalone package for the latest SSU, search for it in the Microsoft Update Catalog.

Install this update

Release Channel


Next Step

Windows Update and Microsoft Update


None. This update will be downloaded and installed automatically from Windows Update.

Microsoft Update Catalog


To get the standalone package for this update, go to the Microsoft Update Catalog website.

Windows Server Update Services (WSUS)


This update will automatically sync with WSUS if you configure Products and Classifications as follows:

Product: Windows 10

Classification: Security Updates

File information

For a list of the files that are provided in this update, download the file information for cumulative update 4601331

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