Updated

Updated · PCWorld · Sep 15

Microsoft Ships 2 Emergency Windows Patches After 700-Plus September Flaws Triggered New Bugs

Updated

Updated · PCWorld · Sep 15

Microsoft Ships 2 Emergency Windows Patches After 700-Plus September Flaws Triggered New Bugs

Q: What hidden flaw in the latest Windows patch forced an emergency fix for virtual machines?

The hidden flaw was a **Hyper-V regression in Plan9 host-folder sharing** introduced by the September Windows 11 security updates. It affected **Host Compute Service-managed Linux virtual machines**, where folders shared from the Windows host using Plan9 would either **not appear inside the guest** or become **inaccessible**. Microsoft’s own release notes for the emergency patches say the bug hit applications that rely on **HCS-managed VMs** and **Plan9-based file sharing**. That made it especially disruptive for sandbox-style workloads and tools such as **WSL** and **Claude Cowork**, which depend on those shared mounts to access host files. Evidence from user reports and Microsoft’s known-issues documentation points to the same failure pattern: the VM itself would boot normally, but the **Plan9/virtiofs mount layer broke**, producing errors like missing shares or failed mounts. Standard Hyper-V virtual machines that do not use Plan9 were not broadly affected. Microsoft addressed the issue in the September 14 out-of-band updates for Windows 11, including **KB5129195** for versions 24H2 and 25H2 and **KB5129194** for version 26H1. In short, the emergency fix was forced by a patch-introduced bug that **broke Plan9-based host folder mounts in Hyper-V Linux VMs**.

3 articles · Updated · PCWorld · Sep 15

KB5129195 for Windows 11 and KB5129236 for Windows 10 were released as out-of-band updates to fix Patch Tuesday fallout and close a newly discovered elevation-of-privilege flaw.
The patches target USB audio failures, Remote Desktop Services instability and Hyper-V host-folder sharing problems with Linux virtual machines; Microsoft says Windows 11 devices will receive the update automatically.
Windows 11 users still face unresolved issues with AMD Radeon GPUs, plus remaining File Explorer, File History and some audio problems, even after the emergency release.
The fixes follow Microsoft's September Patch Tuesday, which addressed more than 700 vulnerabilities but also introduced bugs severe enough to require a restart-heavy emergency rollout about a week later.