Windows 11 KB5129195 is out after Microsoft confirms major issues with the September 2026 update, but it won’t fix AMD GPU errors

Windows 11 KB5129195 is rolling out now, and it addresses multiple critical issues we found in our tests, including a bug that broke audio on some PCs. Microsoft told me today’s emergency update downloads automatically on all PCs, and it also includes additional security protections against an “elevation of privilege” vulnerability.

KB5129195 is a mandatory, cumulative out-of-band update, which means it includes fixes from previous releases, including the September 2026 Patch Tuesday update. It advances Windows 11 25H2 to Build 26200.9457 and version 24H2 to Build 26100.9457.

Today’s emergency update shows up as “2026-09 Security Update (KB5129195) (26200.9457)” on Windows 11 25H2 and takes approximately 10 minutes to finish downloading and installing, although this can vary by PC.

A single reboot is usually enough to finish applying it, but if you have a pending Secure Boot certificate update or a related firmware update, your PC may reboot multiple times.

Windows 11 KB5129195 fixes critical bugs, but some issues remain

On September 8, 2026, Microsoft rolled out Windows 11 KB5124008 as part of the Patch Tuesday cycle, and it’s one of the biggest updates of 2026. That’s because it restores Windows 10 features, such as the movable taskbar, and also addresses hundreds of security issues.

Unfortunately, the September update caused havoc on some PCs, and Windows Latest documented several bugs before Microsoft acknowledged them.

Today’s emergency update addresses some of those problems, along with a security issue that needed another fix.

Additional security fix for CVE-2026-62721

First, let’s talk about the security fix rolling out today, which makes Windows 11 KB5129195 (emergency update) even more important. I noticed that Microsoft updated its CVE-2026-62721 listing on MSRC (Microsoft’s security portal) and noted that the initial update didn’t completely resolve the security issue.

This vulnerability affects the Windows User-Mode Power Service (UMPS). It could allow an attacker with access to the machine to gain SYSTEM-level privileges by manipulating the service’s power-management requests.

Remote Desktop issues fixed

After the September 2026 Update, Windows Latest found that Remote Desktop Services (RDS) had stopped working on some PCs, and our tests confirmed that removing the patch allowed connections to work again. In affected environments, RDS became unstable, connections failed frequently, and some users couldn’t sign in or get past the “Please wait for the Remote Desktop Configuration” screen.

And if you managed to get through, you could still run into an unresponsive File Explorer or Microsoft Management Console.

Microsoft says today’s KB5129195 emergency update fixes this RDS issue, so you no longer need to resort to workarounds or remove security updates to address it.

Windows audio issues partially fixed

Windows 11 KB5129195 also fixes a bug where multichannel audio features, such as 8-channel or 3D audio modes, stopped working after the September 2026 Update. Microsoft says this affected some USB Audio Class 1.0 devices that continued to work normally in standard stereo mode.

But there’s another audio issue that remains unresolved.

In our tests, we found that some PCs with the September 2026 Update could no longer play any sound, and nothing happened when we increased the volume to 100%.

We also noticed that the Windows Settings page for troubleshooting sound would crash when we tried to repair the problem, while Device Manager reported: “This device cannot start (Code 10).”

Microsoft is not fixing that issue today, so you might still end up with no audio after installing Windows 11 KB5129195.

Claude Cowork and other affected apps now work again on Windows

As soon as I installed the September 2026 Update (KB5124008), I found a bug that broke Claude Cowork. In my tests, Claude Cowork’s workspace could no longer access my local storage, even when I had explicitly given it access, and that broke most Cowork actions that interacted with Windows storage.

Anthropic confirmed that the Windows update was responsible for breaking this functionality. While waiting for Microsoft to do something about it, Anthropic started showing an error message in Claude Cowork, pinning the blame on Microsoft:

With the Windows 11 KB5129195 emergency update, Microsoft has patched the bug affecting Claude Cowork and other apps that share Windows host folders with HCS-managed Linux virtual machines using Plan9.

“Folders shared from the Windows host using Plan9 did not appear or could not be accessed in the guest environment,” Microsoft noted.

What about Explorer.exe crashes, File History, and AMD GPU issues?

Windows 11 KB5124008 (September 2026 Patch Tuesday) has at least three unconfirmed issues that we’re tracking. Unfortunately, today’s emergency update doesn’t list fixes for the separate Explorer.exe crashes, File History problems, or AMD GPU issues we’ve reported.

We’ve seen reports of Explorer.exe failing to start or crashing almost immediately after you sign in, leaving some PCs stuck on a black screen without a working desktop or taskbar. So far, these reports mostly involve enterprise environments using Citrix UPM, FSLogix, Horizon, or ProfileUnity ProfileDisks, and both Windows 11 24H2 and 25H2 appear to be affected.

On top of that, multiple users told Windows Latest that File History could no longer detect their external backup drives, even when the drives were connected.

Some AMD Radeon users have also reported driver timeouts, black screens, missing displays, and complete system freezes. In some cases, the GPU driver appeared to break again immediately after being reinstalled.

Microsoft hasn’t confirmed the File History or AMD GPU problems yet. Some users told us that removing the September update restores File History or resolves their GPU issues, but doing so also removes the security fixes included in that update.

Are major security fixes behind the spike in Windows update issues?

Based on our analysis, the September 2026 Update fixes 611 unique security flaws affecting Windows 11 24H2 and 25H2. This is only for consumer editions (Home, Pro). But when you take the entire Windows product family into account, which includes Windows Server, the number of fixes increases to 723. The number rises to 974 when you include all Microsoft products that can run on Windows.

There’s no denying that the September 2026 Update has been one of the biggest releases, and I made a table that better explains it:

I am not trying to make a point that bug fixes are a bad thing, but what suddenly happened? Why is Windows getting 10 times more fixes than last year? For comparison, Windows was getting no more than 100 security fixes a month last year, but that is no longer the case today, and it’s because of… AI.

This year, Microsoft is using its in-house AI models to find more security bugs. It’s a good thing because patching these flaws makes Windows more secure, and Microsoft doesn’t have much choice as bad actors accelerate their own hunting using AI.

It’s a cat-and-mouse game because if Microsoft resorts to manually finding bugs, it may never be able to match the bad actors using cheaper AI models.

As one Microsoft employee noted in a Reddit post, the September Windows updates were an “extensive” release, and that complicates the debugging process. He said Microsoft needed more data to pinpoint the root cause and asked affected users experiencing Explorer.exe crashes to share memory dumps.

Does that statement explain why Windows is having so many issues out of nowhere? Maybe. It’s entirely possible that changes made for some of those 611 security fixes contributed to the broken features on Windows 11 25H2/24H2, but you can’t expect Microsoft to come out loudly and admit that.