5124012

This cumulative update for Windows 11, version 26H1 (KB5124012) includes the latest security fixes and improvements, along with non-security updates from last month's optional preview release. Visit the Windows release health dashboard for the latest status on this release.

This update includes new features and quality improvements that were part of the following update:

August 27, 2026-KB5120996 (OS Build 28000.2804) Preview

The following summary outlines key quality improvements addressed by this update. The bold text within the brackets indicates the item or area of the change.

[Security updates] This update provides security improvements. For more information the security vulnerabilities resolved by this update, see the Security Update Guide

[Security updates] This update provides security improvements. For more information the security vulnerabilities resolved by this update, see the Security Update Guide

[Secure Boot] This update includes additional high confidence device targeting data, increasing coverage of devices eligible to automatically receive new Secure Boot certificates . Certificate deployment via Windows updates continues across supported PCs and non-managed business devices in the coming months.

[Secure Boot] This update includes additional high confidence device targeting data, increasing coverage of devices eligible to automatically receive new Secure Boot certificates . Certificate deployment via Windows updates continues across supported PCs and non-managed business devices in the coming months.

[Teams and Outlook on Arm64 PCs] Fixed: This update addresses an issue that could cause Microsoft Teams and Microsoft Outlook to unexpectedly close on Arm64-based PCs.

[Teams and Outlook on Arm64 PCs] Fixed: This update addresses an issue that could cause Microsoft Teams and Microsoft Outlook to unexpectedly close on Arm64-based PCs.

[Date and Time] This update adjusts Morocco Standard Time to reflect Morocco's transition to permanent UTC+00:00 effective September 20, 2026. This change ensures that the correct local time is displayed after the transition.

[Date and Time] This update adjusts Morocco Standard Time to reflect Morocco's transition to permanent UTC+00:00 effective September 20, 2026. This change ensures that the correct local time is displayed after the transition.

[Remote Desktop Audio Redirection] This update addresses an issue affecting Remote Desktop audio redirection that could prevent audio from a remote session from playing on the local device in certain configurations.

[Remote Desktop Audio Redirection] This update addresses an issue affecting Remote Desktop audio redirection that could prevent audio from a remote session from playing on the local device in certain configurations.

[OMA-DM Client Logging] This update improves diagnostic logging for the OMA-DM client by adding certificate chain information for server connections, helping administrators troubleshoot device management connectivity issues.

[OMA-DM Client Logging] This update improves diagnostic logging for the OMA-DM client by adding certificate chain information for server connections, helping administrators troubleshoot device management connectivity issues.

If you've already installed updates, your device will download and install only the new updates included in this package.

This release updates the following AI components to version 1.2608.951.0: Image , Content Extraction, Semantic Analysis, and Settings Model.

Known issues in this update

Host folder shares might be unavailable in Hyper-V-based Linux VMs

After installing the September 2026 security update (KB5124012), applications that use HCS-managed virtual machines might experience issues when sharing host folder with Linux VMs using Plan9. Affected virtual machines start normally, but folders shared from the Windows host using Plan9 do not appear or cannot be accessed in the guest environment.

Applications or sandbox environments that depend on these shared folders might display an error indicating that no Plan9 drive shares were mounted. Claude Cowork and the Windows Subsystem for Linux (WSL) are two of the applications affected by this issue. Standard Hyper-V virtual machines that do not use the Plan9 feature are not affected by this issue.

Microsoft is working on a resolution and will update this documentation when more information is available.

How to get this update

Before you install this update

Microsoft combines the latest servicing stack update (SSU) for your operating system with the latest cumulative update (LCU). For general information SSUs, see Servicing stack updates .

If you deploy dynamic updates such as this update to an existing Windows image, ensure the boot.stl file is included as part of the installation media. Failure to include the file might prevent devices from successfully starting from the installation media and can result in error code 0xc0430001 .

The boot.stl file is used during Secure Boot validation and must match the Windows version and architecture of the image you are updating.

To ensure the boot.stl file is included as part of the installation media, do one of the following:

Use the Update WinPE script to update an existing Windows image. (Recommended)

Manually copy the boot.stl file from the device Windows\Boot\EFI folder to the corresponding folder on your installation media before deploying the update.

For information how to apply Dynamic Update packages to existing Windows images, see Update Windows installation media with Dynamic Update .

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

Microsoft Update Catalog

Windows Server Update Services

Download this updates .msu files

Download all MSU files for update KB5124012 from the Microsoft Update Catalog and place them in the same folder (for example, C:/Packages). Ensure there are no other files in the folder. This update consists of the following update .msu file(s):

windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu

Method 1: Install all MSU files together

Use Deployment Image Servicing and Management (DISM.exe) to install the target update. DISM will use the folder specified in PackagePath to discover and install one or more prerequisite MSU files as needed.

Updating a Windows PC

To apply this update to a running Windows PC, run the following command from an elevated Command Prompt:

DISM /Online /Add-Package /PackagePath:c:\packages\windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu

Or, run the following command from an elevated Windows PowerShell prompt:

Add-WindowsPackage -Online -PackagePath "c:\\packages\\windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu"

Or use Windows Update Standalone Installer to install the target update.

Updating Windows Installation media

To apply this update to Windows Installation media, see Update Windows installation media with Dynamic Update .

Note: When downloading other Dynamic Update packages, ensure they match the same month as this KB. If the SafeOS Dynamic Update or Setup Dynamic Update is not available for the same month as this KB, use the most recently published version of each.

To add this update to a mounted image, run the following command from an elevated Command Prompt:

DISM /Image:mountdir /Add-Package /PackagePath:windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu

Or, run the following command from an elevated Windows PowerShell prompt:

Add-WindowsPackage -Path "c:\\offline" -PackagePath "windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu" -PreventPending

Method 2: Install each MSU file individually, in order

Download and install each MSU file individually (if there are more than one .msu file) either using DISM or Windows Update Standalone Installer in the following order:

windows11.0-kb5124012-arm64_96333a7811e165eafd98ac975832f8ca297de78c.msu

Classification : Security Updates

For a list of the files provided in this update, download the file information for cumulative update KB5124012​​​​​​​​​ .

For a list of the files provided in the servicing stack update, download the file information for the SSU (KB5125104) - version 28000.2950 .

Windows monthly updates explained

Description of the standard terminology used for Microsoft software updates

Windows release health