Windows 11 KB5129195 Emergency Update Rolls Out
Windows 11 KB5129195 is now rolling out, addressing multiple critical issues identified in our testing after the September 2026 update. MS told me that today’s emergency update downloads automatically on PCs and includes additional security protections against an elevation-of-privilege vulnerability. However, MS has not listed a fix for the AMD GPU errors we have been tracking.
KB5129195 is a mandatory cumulative out-of-band update, meaning 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.
A single reboot is usually enough to complete the installation. However, if you have a pending Secure Boot certificate update or a related firmware update, your PC may restart multiple times.
To verify that the emergency update is installed, open Settings > System > About and check the OS build. You should see 26200.9457 on Windows 11 25H2 or 26100.9457 on Windows 11 24H2.
On Windows 11 25H2, today’s emergency update appears as “2026-09 Security Update (KB5129195) (26200.9457)”. Downloading and installing it takes approximately 10 minutes, although the duration may vary by PC.
Windows 11 KB5129195 Fixes Critical Bugs, but Some Issues Remain
On September 8, 2026, MS rolled out Windows 11 KB5124008 as part of the Patch Tuesday cycle. It is one of the largest updates of 2026 because it restores Windows 10 features, such as the movable taskbar, while also addressing hundreds of security issues.
Unfortunately, the September update caused serious problems on some PCs, and Windows Latest documented several bugs before MS acknowledged them.
Elevation-of-Privilege Vulnerability
First, today’s security fix makes Windows 11 KB5129195 even more important. I noticed that MS updated its CVE-2026-62721 listing on the MS Security Response Center portal and noted that the initial update did not completely resolve the security issue.
The 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 Services Fix
After the September 2026 update, Windows Latest found that Remote Desktop Services (RDS) had stopped working on some PCs. Our tests confirmed that removing the patch restored the ability to connect.
On affected systems, RDS became unstable and could prevent users from establishing a connection. In most cases, users told us they could not sign in or move past the “Please wait for the Remote Desktop Configuration” screen.
Even when users managed to connect, they could still encounter an unresponsive File Explorer or MS Management Console.
After we flagged the issue through our testing, MS confirmed that it was aware of the reports and promised a fix. Windows 11 KB5129195 resolves the RDS issue, eliminating the need to use workarounds or remove security updates.
Multichannel Audio Fix and Remaining Sound Problems
Windows 11 KB5129195 also fixes a bug that caused multichannel audio features, including 8-channel and 3D audio modes, to stop working after the September 2026 update. MS says the problem affected some USB devices that continued to work in standard stereo mode.
Unfortunately, the update does not resolve all the audio problems we identified. In our tests, some PCs running the September 2026 update could no longer play any sound, and increasing the volume to 100% had no effect.
When audio problems occur on a Windows PC, users would typically run the audio troubleshooter through Settings. With this specific bug, however, that is not possible because the Windows Settings page for sound and audio crashes immediately, before users can determine what to adjust.
When we opened Device Manager and checked the audio devices, they appeared normally. However, opening their properties displayed an error that made the problem clear: “This device cannot start (Code 10).” This means Windows can detect the device but cannot start it.
MS is not fixing this issue in today’s update, so users may still have no audio after installing Windows 11 KB5129195. Because this confirmed problem affects only USB devices, affected users can consider switching to Bluetooth headphones while MS develops a patch.
We have also received reports of microphones not working on affected devices. MS has not listed a separate microphone fix in today’s emergency update.
Claude Cowork and Plan9 Folder-Sharing Fix
Immediately after installing the September 2026 update, KB5124008, I found a bug that broke Claude Cowork. In my tests, Claude Cowork’s workspace could no longer access local storage, even when I had explicitly granted access. This disrupted most Cowork actions that interacted with Windows storage.
The issue was widespread, and Anthropic was the first to confirm that the Windows update was responsible for breaking the functionality, well before MS issued a clarification. While waiting for MS to address the problem, Anthropic began displaying an error message in Claude Cowork that attributed the issue to the Windows update.
MS later confirmed that it was aware of an issue affecting applications that share Windows host folders with HCS-managed Linux virtual machines through Plan9.
“Applications that use HCS-managed virtual machines might experience issues when sharing a 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,” MS explained in a support document.
I can confirm that Windows 11 KB5129195 fixes this Claude Cowork bug. My custom WSL-based applications also work again and no longer encounter storage-access problems.
What About Explorer.exe Crashes, File History, and AMD GPU Issues?
We are still tracking reports of separate Explorer.exe crashes, File History problems, and AMD GPU issues following Windows 11 KB5124008, the September 2026 Patch Tuesday update. Unfortunately, today’s emergency update does not list fixes for these problems.
Explorer.exe Crashes
Windows Latest has received reports of Explorer.exe failing to start or crashing almost immediately after sign-in, leaving some PCs on a black screen without a functioning desktop or taskbar.
So far, these reports primarily involve enterprise environments using Citrix UPM, FSLogix, Horizon, or ProfileUnity ProfileDisks. Both Windows 11 24H2 and 25H2 appear to be affected. If you encounter the problem, restarting Explorer.exe through Task Manager should restore functionality.
File History Problems
File History is one of Windows’ lesser-known features. Although it has not received any significant updates in years, many people still use it because it stores backups of different file versions on an external drive, allowing users to restore earlier versions when necessary.
Unfortunately, the Windows 11 September 2026 update made the feature unusable for some users because File History could no longer detect external backup drives, even when they were connected.
MS has not yet confirmed the reports. However, many users told me that File History had been broken for months before the issue received greater attention after the September 2026 update, which appears to have affected more users.
File History remains unusable for affected users, and I can confirm that Windows 11 KB5129195 does not improve the legacy backup tool.
AMD Radeon GPU Issues
As soon as the September 2026 update was released, I began receiving reports of serious GPU driver problems on AMD Radeon PCs. GPU issues are always difficult to investigate because they may be connected to a Windows update, GPU driver update, or firmware update. In this case, however, there are reasons to believe the Windows update is responsible.
Affected users who install the September update may encounter black screens or system freezes even when they are not playing games. I have also seen reports on Reddit in which users confirmed that the GPU driver can time out. In the worst cases, the display may disappear entirely.
The reports involve PCs using the following GPUs:
- Radeon RX 6600
- Radeon RX 7700 XT
- Radeon RX 7800 XT
- Radeon RX 7900 GRE
- Radeon RX 7900 XTX
- Radeon RX 9070 XT
The problem is therefore not limited to a single GPU model. Switching to older GPU drivers does not appear to make a difference, making the Windows update the stronger suspect.
Are Major Security Fixes Behind the Increase in Windows Update Issues?
The larger question is what happened to MS’s promise to make Windows more stable in 2026. Why has Windows suddenly become so buggy? Although I do not have a statement from MS, my theory is that the sharp increase in problems is related to the growing number of major security fixes.
Based on our analysis, the September 2026 update fixes 611 unique security flaws affecting Windows 11 24H2 and 25H2. When the entire Windows product family, including Windows Server, is considered, the number increases to 723 fixes. It rises to 974 fixes when all MS products covered by the release are included.
I am not suggesting that security fixes are a bad thing, but the sudden change raises questions. Why is MS releasing approximately 10 times as many security fixes as it did last September?
For comparison, MS averaged approximately 100 security fixes per month from January through September 2025. That is no longer the case, partly because of AI.
This year, MS is using its in-house AI models to identify more security vulnerabilities. This is beneficial because patching these flaws makes Windows more secure, and MS has little choice as malicious actors accelerate their own vulnerability research using AI.
It is a cat-and-mouse game. If MS relies on manual vulnerability discovery, it may never keep pace with malicious actors using less expensive AI models.
In a Reddit post, MS employee Jason asked users experiencing Explorer.exe crashes to share memory dumps to help identify the root cause.
“The September Windows updates were extensive and need more data to understand and root cause,” Jason wrote on Reddit.
It is entirely possible that changes introduced through some of those 611 security fixes caused multiple features to break, but MS—or any company—is unlikely to publicly acknowledge that possibility.
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