Shared folders empty in Linux VMs after KB5124008 ("no Plan9 drive shares mounted")

Windows reports no error for this failure, so the build number plus the guest-side message is the only reliable way to tell it apart from an ordinary configuration mistake.

Applies to: Windows 11 23H2, 24H2, 25H2 and 26H1

1. Press Windows+R, type winver and press Enter. Note the OS build number shown in the second line.

   winver

2. You are on an affected build if it reads 26100.9445 or 26200.9445 (x64, from KB5124008) or 28000.2954 (ARM64 26H1, from KB5124012). If it reads 26100.9457 or 26200.9457 you already have the fix. Microsoft lists Windows 11 26H1, 25H2, 24H2 and 23H2 and Windows 10 22H2 and 21H2 as affected clients, and no server versions.
3. Check the guest-side message. This bug reports that no Plan9 drive shares were mounted, a mount failure of "invalid argument", or a zero count such as "mounted 0/9 Plan9 shares". A different error — a permission denial, or a path that does not exist — points somewhere else.
4. Confirm the timing. If sharing was already failing before 8 September 2026, the update is not your cause.

Confirmed by the vendor.

Source: Microsoft, Microsoft, Microsoft, GitHub, GitHub

Microsoft shipped a targeted out-of-band update on 14 September 2026 that restores Plan9 host folder sharing. It is cumulative, so it replaces KB5124008 rather than requiring you to remove it.

Applies to: Windows 11 24H2 and 25H2 (KB5129195), Windows 11 26H1 (KB5129194)

1. Select Start > Settings > Windows Update and select Check for updates. Microsoft states the out-of-band update downloads and installs automatically through Windows Update.
2. Install KB5129195 on Windows 11 24H2 or 25H2, or KB5129194 on 26H1, and restart. You can also download either as a standalone package from the Microsoft Update Catalog.
3. Confirm the build afterwards with winver. You want 26100.9457 (24H2) or 26200.9457 (25H2). Anything still reading .9445 means the update has not applied.

   winver

4. If you deployed Microsoft's temporary Group Policy mitigation, re-enable that Group Policy, install the update, and restart. Microsoft added this instruction on 14 September and says the issue is not resolved without it.
5. Start your VM again and confirm the shares mount. If they still do not, re-check the guest-side error — a different message points at a different problem.

Confirmed by the vendor.

Source: Microsoft, Microsoft, Microsoft, Microsoft

Microsoft names the Windows Subsystem for Linux as affected, but reporters found ordinary WSL2 file access still working. Knowing which of the two you have can save you from removing a security update you do not need to remove.

Applies to: WSL, Hyper-V-based sandbox VMs

1. If you use WSL, open a shell in your distribution and list a Windows drive through the usual mount point.

   ls /mnt/c

2. If that listing works, your everyday WSL drive access is fine. In the Microsoft Q&A thread one contributor reports WSL2 continuing to work with the update installed, and another reproduced the break on a machine with WSL not installed at all — so read Microsoft's "WSL is affected" as applying to Plan9 share attach specifically, not to all WSL file access.
3. If the application that hosts your own sandbox VM reports no Plan9 shares mounted, that is the confirmed regression and only the workaround below is known to address it.
4. If your VM does not use Plan9 host folder shares at all, stop here — Microsoft states those VMs are unaffected and your problem is something else.

Reported to work by 2 users; not confirmed by the vendor. 1 report it did not help.

Source: Microsoft, Microsoft Q&A

Now that a fix exists, this is a fallback for machines that cannot take the out-of-band update — a managed device held back by an update policy, for example. It restores the host components the September update replaced.

Applies to: Windows 11 24H2, 25H2 and 26H1

1. Try the out-of-band update first. It is cumulative, supersedes KB5124008 and keeps September's security fixes in place, which removing the update does not.
2. Select Start > Settings > Windows Update.
3. Under More options, select Update history.
4. Under Related settings, select Uninstall updates.
5. Select the September 2026 security update for your PC — KB5124008 on x64, KB5124012 on ARM64 26H1 — and select Uninstall. Restart when prompted. Microsoft states some updates cannot be uninstalled; if it is not listed, this route is closed to you.
6. Start your VM again and confirm the shares mount. Reporters who tested this before the fix shipped found sharing working normally on the next boot with no change to the application itself.
7. Install KB5129195 or KB5129194 as soon as you are able. That returns both the security fixes and working Plan9 sharing.

Reported to work by 3 users; not confirmed by the vendor.

Source: Microsoft, GitHub, GitHub, Microsoft Q&A