Summary
Windows 11 KB5124008 breaks Plan9 host folder shares in Hyper-V-based Linux VMs.
WSL and Claude Cowork can’t read Plan9 shares; apps may show “no Plan9 drive shares mounted” errors.
Microsoft lists it as a known issue and is working on a fix, but no patch or home workaround yet.
Have you been having issues with either WSL or Claude Cowork on Windows 11 lately? IF so, you’re not alone. Microsoft has updated its documentation to report that something within the latest Windows 11 update has created problems with Plan9, and the company is working on a fix.
Windows 11 has a nasty Plan9 problem
It affects both WSL and Claude Cowork
As spotted by Windows Latest, Microsoft has updated its “Windows 11, version 25H2 known issues and notifications” documentation. Microsoft often revisits this page to list any problems it has identified for Windows 11, so it acts as a sort of bill of health for the operating system.
A new entry in the documentation is titled “Host folder shares might be unavailable in Hyper-V-based Linux VMs.” It says that, if you install the September 2026 security update KB5124008, any folders shared to Windows using Plan9 don’t appear and can’t be read.
This causes some nasty problems with some pretty popular apps:
Applications or sandbox environments that depend on these shared folders might display an error indicating that no Plan9 drive shares were mounted. Claude Cowork and the Windows Subsystem for Linux (WSL) are two of the applications affected by this issue. Standard Hyper-V virtual machines that do not use the Plan9 feature are not affected by this issue.
Unfortunately, Microsoft doesn’t have a solution just yet. There are no official patches out to fix the issue, and the company didn’t share any details on how people can fix this at home (if it can even be done). However, because it’s on the known issues page, affected users should be reassured that Microsoft is, at the very least, working on it right now. When the patch will appear is another story.
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