What's new in KB5124008 (Windows 11 build 26200.9445)
- Windows 11 version 24H2 (OS Build 26100.9445)
- Windows 11 version 25H2 (OS Build 26200.9445)
Priya Natarajan3 min readQuick answer
KB5124008 is a mandatory security update released on September 8, 2026, for Windows 11 version 24H2 and version 25H2, advancing systems to OS builds 26100.9445 and 26200.9445. It combines core security fixes with cumulative quality improvements rolled over from the previous preview update, KB5120998. You can download it directly via Windows Update or manually through the Microsoft Update Catalog.
Microsoft released KB5124008 on September 8, 2026, as part of its monthly Patch Tuesday security deployment. This cumulative update applies to computers running Windows 11 version 24H2 and Windows 11 version 25H2, advancing system builds to OS build 26100.9445 and OS build 26200.9445 respectively. If your system is set to receive updates automatically, Windows Update will download and install this patch during your standard maintenance window.
Understanding what is included in mandatory monthly updates helps you keep your PC secure and performing at its best. Because KB5124008 carries forward system enhancements tested in recent preview updates like KB5120998 alongside essential security vulnerability patches, installing it promptly ensures your device remains fully supported and protected.
Highlights
- Windows 11 25H2 build update: Advances systems on Windows 11 version 25H2 to OS Build 26200.9445 to deliver the latest security patches.
- Windows 11 24H2 build update: Updates Windows 11 version 24H2 installations to OS Build 26100.9445 across all active editions.
- Security updates: Incorporates critical security enhancements and vulnerability patches released during the September 8, 2026 update cycle.
- Preview fix integration: Rollups fixes tested in the August 27, 2026 preview update KB5120998 (OS Builds 26200.9278 and 26100.9278) into the standard production channel.
- Servicing stack alignment: Delivers underlying servicing stack updates to ensure smooth installation and reliability for subsequent Windows 11 updates.
Known issues
Microsoft reports no known issues for this update.
Should you install it?
Yes, you should install KB5124008 as soon as it becomes available for your device. As a standard Patch Tuesday release, it contains essential security fixes designed to protect Windows 11 version 24H2 and version 25H2 systems against newly discovered vulnerabilities. Because this is a mandatory security update rather than an optional preview, Windows Update will automatically install it on your PC over time.
If you use your PC for business or critical daily tasks, taking a quick backup or creating a restore point before installing updates is always good practice. However, since Microsoft reports no known issues with KB5124008, home and workplace users can proceed with the installation confidently.
How to get it
You can acquire KB5124008 automatically through Windows Update or download it manually if your machine is offline or experiencing update errors.
Method 1: Using Windows Update
- Open the Settings app on your Windows 11 device.
- Select Windows Update from the left-hand navigation pane.
- Click the Check for updates button on the right panel.
- Wait for Windows Update to search for available patches for your operating system build.
- Click Download and install if the download process does not start automatically.
- Restart your computer when prompted to finalize the installation.
To verify your build number after updating, navigate to Settings > System > About. Scroll down to the Windows specifications section and confirm that your OS build displays 26100.9445 (for version 24H2) or 26200.9445 (for version 25H2).
Alternative: PowerShell update management
If you manage updates via command line, you can verify Windows Update services directly through PowerShell.
Caution: Running administrative command-line tools requires elevated privileges. Make sure you enter commands carefully.
- Right-click the Start menu icon and select Terminal (Admin) or PowerShell (Admin).
- Type
Get-Service wuauservand press Enter to check the status of the Windows Update service. - If the service is stopped, type
Start-Service wuauservand press Enter to start it.
Method 2: Download from the Microsoft Update Catalog
If Windows Update is failing or stuck, you can download the standalone installer manually:
- Open your web browser and navigate to the official Microsoft Update Catalog.
- Enter KB5124008 into the search bar and press Search.
- Locate the download matching your system architecture (x64 or ARM64) and Windows 11 version (24H2 or 25H2).
- Click Download next to the correct entry.
- Double-click the downloaded
.msupackage file to run the installer. - Follow the prompt instructions and restart your system when installation completes.
If you run into persistent installation errors, run the Windows Update troubleshooter by going to Settings > System > Troubleshoot > Other troubleshooters and clicking Run next to Windows Update.
Sources
Frequently asked questions
What is KB5124008?
KB5124008 is a mandatory cumulative security update released on September 8, 2026, for Windows 11 version 24H2 and version 25H2. It updates operating system builds to 26100.9445 and 26200.9445.
Do I need to install KB5124008?
Yes, KB5124008 is a standard Patch Tuesday update containing critical security patches and quality improvements. Windows Update will automatically install it if automatic updates are enabled.
How do I check if KB5124008 is installed on my computer?
You can check by opening Settings > System > About on your computer. Under Windows specifications, verify whether your OS build displays 26100.9445 or 26200.9445.
What should I do if KB5124008 fails to install?
If the update fails or gets stuck, run the Windows Update troubleshooter via Settings > System > Troubleshoot > Other troubleshooters, or manually download the standalone installer from the Microsoft Update Catalog.
Checked against Microsoft's release notes on ; re-checked when a new build ships.
Writes Thea's how-to guides for new and changed Windows 11 features: where a setting moved, how to turn it on or off, and what it actually does. Every guide is checked against Microsoft's release notes and updated when a new build changes the steps.
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