What's new in KB5124008 (Windows 11 build 26100.9445)
- Windows 11 version 24H2 (OS Build 26100.9445)
- Windows 11 version 25H2 (OS Build 26200.9445)
Maya Reyes3 min readQuick answer
KB5124008 is a mandatory security update released on September 8, 2026, for Windows 11 version 24H2 and version 25H2. It updates system build numbers to 26100.9445 and 26200.9445 while delivering core security maintenance, stability fixes, and cumulative improvements. Users can install it through Windows Update or download standalone packages from the Microsoft Update Catalog.
Microsoft published update KB5124008 on September 8, 2026, as a monthly Patch Tuesday release for Windows 11. This cumulative update is designed specifically for systems running Windows 11 version 24H2 and Windows 11 version 25H2. When installed, it increments the operating system build numbers to OS Build 26100.9445 for version 24H2 and OS Build 26200.9445 for version 25H2.
As part of Microsoft's regular update cadence, KB5124008 focuses on underlying operating system security, reliability updates, and general maintenance fixes. Because version 24H2 and version 25H2 share a common servicing baseline, both releases receive the same underlying platform updates simultaneously. The package incorporates all changes from previous updates, including non-security improvements introduced in preview rollups, ensuring that devices remain protected against security vulnerabilities while keeping background components functioning predictably.
Highlights
- Updates the operating system build to OS Build 26100.9445 for devices running Windows 11 version 24H2.
- Advances systems running Windows 11 version 25H2 to OS Build 26200.9445.
- Delivers essential September 8, 2026 Patch Tuesday security fixes and maintenance patches across Windows 11 core sub-systems.
- Incorporates cumulative non-security fixes released in the August 27, 2026 preview update KB5120998 (OS Builds 26200.9278 and 26100.9278).
- Includes background servicing stack rollups and quality improvements established in earlier updates such as KB5121003 (OS Builds 26200.9168 and 26100.9168).
Known issues
Microsoft reports no known issues for this update.
Should you install it?
KB5124008 is a mandatory Patch Tuesday security release, meaning it includes important security patches intended to keep Windows 11 secure against potential exploits. All users running supported hardware on Windows 11 version 24H2 or version 25H2 should install this update to maintain device security and operational health.
Because Microsoft reports no known issues with this release, standard consumer devices, laptops, and home desktop PCs can safely apply the update as soon as it becomes available. If you operate in an enterprise environment or run critical specialized software, you may choose to briefly test the build before broad deployment, but general users should proceed with installation without unnecessary delays.
How to get it
You can install KB5124008 directly through the standard Windows Update interface or obtain standalone installers for offline installation.
- Open the Start menu and select Settings, or press the Windows key + I keyboard shortcut.
- In the left navigation bar, select Windows Update.
- Click the Check for updates button to search for newly published updates.
- Once KB5124008 appears in your update list, click Download & install.
- Allow Windows Update to complete the download, then click Restart now to finish applying build 26100.9445 or build 26200.9445 to your machine.
If you prefer to install the update manually or if Windows Update fails to download the package, you can retrieve the installer directly from Microsoft's servers:
- Open your web browser and navigate to the official Microsoft Update Catalog website.
- Type KB5124008 into the search box located at the top right of the page and press Enter.
- Locate the package matching your specific Windows 11 version (24H2 or 25H2) and processor architecture (x64 or ARM64).
- Click Download next to the entry, click the file link inside the popup window, and save the installer to your device.
- Run the downloaded installer file, follow the prompts on your screen, and restart your computer when prompted.
To verify that the installation completed successfully, go to Settings > System > About and check under Windows specifications. If your OS build displays as 26100.9445 or 26200.9445, the installation was successful. If you encounter errors during the process, run the Windows Update troubleshooter in Settings to clear the update cache and reset system update components.
Sources
Frequently asked questions
How do I check if KB5124008 is installed on my PC?
Go to Settings > System > About and check your OS build number. If it shows 26100.9445 or 26200.9445, KB5124008 is installed on your device.
Is KB5124008 a required update for Windows 11?
Yes, KB5124008 is a mandatory Patch Tuesday security update released on September 8, 2026, and will install automatically via Windows Update.
Which versions of Windows 11 receive build 26100.9445?
Windows 11 version 24H2 updates to OS Build 26100.9445 under KB5124008, while Windows 11 version 25H2 updates to OS Build 26200.9445.
Can I manually install KB5124008 if Windows Update gets stuck?
Yes, you can search for KB5124008 on the Microsoft Update Catalog web portal, download the standalone installer, and run it manually.
Checked against Microsoft's release notes on ; re-checked when a new build ships.
Covers Windows 11 update releases and the problems that follow them. Tracks Microsoft's release-health notes daily and flags known issues the same day they appear.
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