Windows 11 Emergency Update KB5129195: What It Fixes and What to Do

Microsoft released an emergency, out-of-band update for Windows 11 on 14 September 2026 after the regular September security update broke Remote Desktop, Hyper-V folder sharing with Linux virtual machines, and some USB audio devices. On Windows 11 25H2 and 24H2 the fix is KB5129195. It installs through Windows Update and most PCs should take it.

Here is what went wrong, which update applies to your version, how to confirm it is installed, and the one problem that is still open.

What is the Windows 11 emergency update?

It is an out-of-band update, meaning Microsoft shipped it outside the normal monthly schedule because the problems were too disruptive to leave for a month. It is cumulative, so it contains the September security fixes plus the repairs. If you skipped the September update, this one brings you fully up to date on its own.

The September security update arrived on 8 September 2026. Within days, administrators were reporting Remote Desktop servers that stopped accepting sign-ins, and Microsoft confirmed the faults before releasing the fix six days later.

Which update do you need for your version of Windows?

The update number depends on the Windows version you run. Windows 11 and Windows 10 PCs receive it through Windows Update, while the Windows Server packages are published in the Microsoft Update Catalog for manual or managed deployment.

Not sure which version you have? Press Windows + R, type winver and press Enter. The version and build number appear in the window that opens.

What did the September update break?

Three things. Remote Desktop Services could stop responding, Hyper-V-based Linux virtual machines could lose access to folders shared from the Windows host, and some USB audio devices failed in multichannel modes. Everyday home use was mostly unaffected, which is why many people noticed nothing.

• Remote Desktop. New sessions could not be created and both remote and local sign-in could fail, with some servers becoming unresponsive. Related tools such as the management console, File Explorer and the Windows Update page could hang on affected machines.
• Hyper-V and Linux folder sharing. Applications that run Linux virtual machines through the Host Compute Service found that shared Windows folders either did not appear inside the guest or could not be opened. Developers using container and Linux tooling were the most likely to see this.
• USB audio. Some USB Audio Class 1.0 devices failed when switched to 8-channel or 3D audio modes, while plain stereo kept working.

The update also carries a security fix for an elevation-of-privilege flaw in a Windows power service, tracked as CVE-2026-62721. That is a reason to install it even if none of the three faults touched your PC.

How do you install the emergency update?

Open Settings, go to Windows Update and select Check for updates. The out-of-band update downloads and installs like a normal monthly update, then asks for a restart. On unmanaged home and small-office PCs it arrives automatically, so checking manually simply gets it sooner.

1. Press Windows + I and open Windows Update.
2. Select Check for updates and let the September 2026 out-of-band update download.
3. Restart when prompted, saving your work first.
4. Run winver and compare the build number with the table above.

If the download stalls or fails with an error code, our guide to fixing Windows Update install error 0x80070103 covers the usual causes. If your PC is still on an older feature release, see how to get the Windows 11 24H2 update first.

What is still not fixed?

One USB audio problem remains open. Some USB Audio Class 1.0 devices may fail to start after the September update, showing "This device cannot start (Code 10)" in Device Manager, producing no sound, or leaving the volume controls stuck. Microsoft has said it is working on a fix, with no date given at the time of writing.

If you rely on an affected audio interface for work, check Microsoft's release notes for KB5129195 before each new update until the fix is confirmed.

Should you uninstall the September update instead?

In most cases, no. Removing the September update does restore Remote Desktop, but it also removes that month's security fixes, which press reports say included vulnerabilities already being exploited. Installing the out-of-band update gives you the repairs and keeps the protection, so it is the better route.

A note from our support desk for customers running Remote Desktop Services on Windows Server: if sessions began failing after 8 September, the cause is this update and not your licences. Your RDS CALs and server activation are unaffected, so there is no need to reinstall licensing or re-enter keys. Apply the matching server package from the table and restart.

Frequently asked questions

Do you need KB5129195 if you never use Remote Desktop?

Yes, it is still worth installing. The update includes a security fix as well as the reliability repairs, and it is cumulative, so it also delivers the September security patches if you had not installed them. Home PCs will receive it automatically through Windows Update.

How do you check that the emergency update is installed?

Press Windows + R, type winver and press Enter. On Windows 11 25H2 the build should read 26200.9457 or later, and on 24H2 it should read 26100.9457 or later. You can also look under Settings, Windows Update, Update history for KB5129195.

What does out-of-band mean?

It means the update was released outside Microsoft's regular schedule, which delivers security updates on the second Tuesday of each month. Microsoft uses out-of-band releases for urgent security issues or for faults serious enough that waiting for the next monthly update would cause too much disruption.

Does the update affect Windows activation or your product key?

No. Cumulative and out-of-band updates do not change your licence or activation state. If Windows shows as not activated after an update, the cause lies elsewhere, and our guide to fixing an activation key that is not working walks through the checks.

The short version

Install the September 2026 out-of-band update, confirm the build number with winver, and keep an eye on Microsoft's release notes if you use a USB audio interface. If this episode has you thinking about moving an older Windows 10 PC onto a supported version, our comparison of Windows 11 Pro vs Windows 10 Home sets out the options, and a genuine Windows 11 Pro licence is available in our store when you are ready.