October 14, 2025—KB5066791 (OS Builds 19044.6456 and 19045.6456)

Windows Secure Boot certificate expiration

Important: Secure Boot certificates used by most Windows devices are set to expire starting in June 2026. This might affect the ability of certain personal and business devices to boot securely if not updated in time. To avoid disruption, we recommend reviewing the guidance and taking action to update certificates in advance. For details and preparation steps, see Windows Secure Boot certificate expiration and CA updates.

Windows updates do not install Microsoft Store application updates. If you are an enterprise user, see Microsoft Store apps - Configuration Manager. If you are a consumer user, see Get updates for apps and games in Microsoft Store.

End of support information

Support for Windows 10 has ended on October 14, 2025

After October 14, 2025, Microsoft will no longer provide free software updates from Windows Update, technical assistance, or security fixes for Windows 10. Your PC will still work, but we recommend moving to Windows 11.

Learn more

Summary

This article lists the security issues and quality improvements included in this security update.

Applies to: Windows 10 Enterprise LTSC 2021 and Windows 10 IoT Enterprise LTSC 2021

Important: Use EKB KB5003791 to update to Windows 10, version 21H2 on supported editions.

This security update includes fixes and quality improvements that are part of the following updates:

The following is a summary of the issues that this update addresses when you install this update. If there are new features, it lists them as well. The bold text within the brackets indicates the item or area of the change we are documenting.

[Input and Composition]

Fixed: An issue with the Chinese Input Method Editor (IME). Private Unicode characters were shown incorrectly and did not meet GB18030 standard.
Fixed: An issue that affects USER32 Edit controls. Surrogate pairs appear as empty boxes when text fields reach their length limit.

[Windows Remote Management (WinRM)]

Fixed: An issue that affects PowerShell Remoting and WinRM in which commands time out after 600 seconds.

[Licensing]

New! This update introduces a servicing stack update (SSU) which incorporates an updated certificate chain to improve Azure environment validation. For more information, see the servicing stack update section.

[Fax modem driver]

This update removes the ltmdm64.sys driver. Fax modem hardware dependent on this specific driver will no longer work in Windows.

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

For more information about security vulnerabilities, please refer to the new Security Update Guide website and the October 2025 Security Updates.

For information about Windows update terminology, see the article about the types of Windows updates and the monthly quality update types. For an overview of Windows 10, version 22H2, see its update history page.

Note Follow @WindowsUpdate to find out when new content is published to the Windows release health dashboard.

Known issues in this update

We are currently not aware of any issues with this update.

Windows 10 servicing stack update (KB5066790) - versions 19044.6449 and 19045.6449

Microsoft now combines the latest servicing stack update (SSU) for your operating system with the latest cumulative update (LCU). SSUs improve the reliability of the update process to mitigate potential issues while installing updates.

This update makes quality improvements to the servicing stack, which is the component that installs Windows updates. To learn more about SSUs, see Servicing stack updates and Servicing Stack Updates (SSU): Frequently Asked Questions.

This servicing stack update (SSU) includes enhanced logic to verify whether a device is hosted on Azure, leveraging an updated certificate chain for validation.

To ensure seamless installation of future updates on Azure-hosted devices, it is strongly recommended to install this latest SSU prior to applying any additional updates.

Additionally, please ensure that the device can access the required certificate update domains to successfully download and install certificate updates. For more information, refer to:
- Certificate downloads and revocation lists
- Azure Certificate Authority details

How to get this update

Before you install this update

Important You must have the latest servicing stack update (SSU) installed. Not installing the latest SSU before applying Windows updates might result in the Windows update not being offered until the latest SSU is installed.

Based on your installation scenario, choose one of the following:

For offline OS image servicing:

If your image does not have the July 25, 2023 (KB5028244) or later LCU, you must install the special standalone October 13, 2023 SSU (KB5031539) before installing this update.
For Windows Server Update Services (WSUS) deployment or when installing the standalone package from Microsoft Update Catalog:

If your devices do not have the May 11, 2021 (KB5003173) or later LCU, you must install the special standalone August 10, 2021 SSU (KB5005260) before installing this update.

Install this update

To install this update, use one of the following Windows and Microsoft release channels.

Available	Next Step
	This update will be downloaded and installed automatically from Windows Update and Microsoft Update.

If you want to remove the LCU

To remove the LCU after installing the combined SSU and LCU package, use the DISM/Remove-Package command line option with the LCU package name as the argument. You can find the package name by using this command: DISM /online /get-packages.

Running Windows Update Standalone Installer (wusa.exe) with the /uninstall switch on the combined package will not work because the combined package contains the SSU. You cannot remove the SSU from the system after installation.

File information

A list of the files that are included in this update are provided in a CSV (Comma delimited) (*.csv) file. The file can be opened in a text editor such as Notepad or in Microsoft Excel.

Download Icon Download the file information for this cumulative update KB5066791 now.