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.
Improvements
This update includes new features and quality improvements that were part of the following update:
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 about 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.
[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.
[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.
If you've already installed previous updates, your device will download and install only the new updates included in this package.
Component updates
AI components
This release updates the following AI components to version 1.2608.951.0: Image Search, Content Extraction, Semantic Analysis, and Settings Model.
To learn more, see Release information for AI components.
Servicing stack update
Includes KB5125104 (Build 28000.2950), which improves the reliability of the Windows update installation process. To learn more about SSUs, see [Simplifying on-premises deployment of servicing stack updates](https://learn.microsoft.com/windows/deployment/update/servicing-stack-updates#simplifying-on-premises-deployment-of-servicing-stack-updates).Known issues in this update
USB audio devices might fail to start or produce no sound
Symptoms
After installing the September 8, 2026, Windows security update (KB5124012), some USB Audio Class 1.0 devices might fail to start or produce audio. Affected devices might experience one or more of the following symptoms:
- The device displays an error in Device Manager: "This device cannot start (Code 10).”
- No audio output.
- Volume controls are unresponsive or remain at zero.
- Sound settings are unresponsive or unavailable.
- Some devices might function in standard stereo configurations but fail when using multichannel audio features, including 8-channel or 3D audio modes. Some customers have reported that they're able to restore audio in these cases by switching to 2-channel mode.
This issue is limited to USB Audio Class 1.0 devices.
Next steps
Microsoft is working on a resolution and will update this documentation when more information is available.
Host folder shares might be unavailable in Hyper-V-based Linux VMs
Symptoms
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.
Next steps
Microsoft is working on a resolution and will update this documentation when more information is available.
Remote Desktop Services might stop responding after September 2026 security update
Symptoms
After installing the September 2026 Windows security update, some organizations might experience issues with Remote Desktop Services (RDS).
In some environments, RDS might become unstable, resulting in RDP connections failing after several minutes, sign-in issues, or servers hanging at "Please wait for the Remote Desktop Configuration". Related tools, including Microsoft Management Console (MMC), RDS Licensing Diagnoser, and File Explorer might also become unresponsive. Additionally, the Windows Update page might stop responding and continuously display a loading indicator.
Microsoft Support: IT administrators who need an immediate workaround should contact Microsoft Support for Business for assistance.
Resolution
Microsoft is working to address this issue in a future Windows update and will provide additional details when 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 about SSUs, see Servicing stack updates.
Deployment
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.
Note
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 about how to apply Dynamic Update packages to existing Windows images, see Update Windows installation media with Dynamic Update.
Install this update
To install this update, use one of the following Windows and Microsoft release channels.
| Available | Next Step |
|---|---|
|
This update downloads and installs automatically from Windows Update and Microsoft Update. |
File information
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.
Change log
| Change date | Change description |
|---|---|
| September 11, 2026 | Added a known issue, "Host folder shares might be unavailable in Hyper-V-based Linux VMs". |
| September 12, 2026 | Added a known issue, "Remote Desktop Services might stop responding after September 2026 security update". |
| September 12, 2026 | Added a known issue, "USB audio devices might fail to start or produce no sound". |
Related topics
Windows monthly updates explained
Description of the standard terminology used for Microsoft software updates