Microsoft has released an out-of-band emergency update for Windows 11, following significant stability issues introduced by its September Patch Tuesday rollout. The fix, KB5129195, is now mandatory for versions 24H2 and 25H2, moving them to builds 26100.9457 and 26200.9457 respectively. It installs automatically via Windows Update, and administrators can deploy it manually through Microsoft’s tools.
Patch Tuesday Fallout
The trouble traces back to KB5124008, the September 8 security update, which addressed nearly 960 CVEs—106 of them critical, including two already being exploited. While its intention was to close a broad range of vulnerabilities—among them flaws in the Windows Update Stack and Advanced Local Procedure Call—several regressions emerged in its wake.
Remote Desktop Services bore the brunt: observers reported that RDP sessions dropped after just a few minutes, sign-in failures occurred, and some servers froze on the “Please wait for the Remote Desktop Configuration” screen. Essential tools like Microsoft Management Console, File Explorer, RDS Licensing Diagnoser, and even Windows Update settings became unresponsive in certain cases. Some admins found themselves forced to roll back or restart machines manually to regain control.
Virtualization & Audio Problems Surface
Another serious bug broke virtualization workflows involving Linux guests: through Hyper-V, applications using the Plan9 protocol couldn’t share host folders with guests. Shared folders were either invisible or unmountable inside the guest OS. Windows Subsystem for Linux and Anthropic’s Claude Cowork were affected. The root origin was found in how the Plan9/9P filesystem-sharing layer handled attach requests, rejecting those that had functioned before. Standard Hyper-V VMs that don’t use Plan9 weren’t impacted.
Meanwhile, some USB Audio Class 1.0 devices stopped working, showing “Code 10” errors or no sound output. This emergency patch partially restores functionality for devices using 8-channel or 3D multichannel audio, though not all symptoms are addressed yet.
Security Fixes & What’s Still Broken
Beyond stability, the update addresses a security risk in Windows User-Mode Power Service (UMPS). Despite fixes in the original September update, Microsoft determined that the elevation-of-privilege vulnerability CVE-2026-62721 wasn’t fully closed, allowing a local attacker to gain SYSTEM-level access via power-management requests. The emergency release strengthens protection against this.
However, known issues persist: audio problems for certain USB devices remain unresolved, and graphics driver crashes on AMD Radeon hardware are still under investigation. Some users report that not all reported regressions have been fixed.
Recommended Actions for Admins
The KB5129195 patch takes about ten minutes to install and typically requires a single reboot. Machines with pending Secure Boot certificate updates may go through multiple restarts. It’s automatically pushed through Windows Update/Business, synced via WSUS, and deployable via Microsoft Update Catalog, Intune, or Configuration Manager.
Organizations that uninstalled the September update to restore Remote Desktop access are now advised to reapply protections using this out-of-band build. Microsoft suggests deploying first in test rings, prioritizing systems exposed to the most critical and actively exploited vulnerabilities, and monitoring the Windows release health dashboard for ongoing PCI (platform, driver, audio) issue updates.
Where things stand: KB5129195 fixes many but not all problems from September’s big Patch Tuesday. It enhances security and restores functions for affected workflows. Still, remaining bugs could disrupt audio and GPU performance until final patches arrive.
