Host folder shares might be unavailable in Hyper-V-based Linux VMs

What changed

KB5129194 resolves missing or inaccessible Plan9 host-folder shares in HCS-managed Linux VMs after the September security update.

Why it matters

Missing Plan9 shares can break HCS-managed Linux VM, WSL and sandbox workflows even when the virtual machine starts normally.

Known limitations

• Claude Cowork and Windows Subsystem for Linux are listed as affected examples
• Standard Hyper-V virtual machines that do not use Plan9 are not affected
• Microsoft is working on a resolution

Now check

• Deploy KB5129194 or a later cumulative update
• Re-enable any temporary Group Policy mitigation before installing the OOB update and restart
• Retest Plan9 host-folder mounts in WSL and other HCS-managed Linux environments