Software issue · Evidence-backed record

Claude Cowork / WSL host folders missing after KB5124008

TraceCheck source review · Verification method · Request a correction

Direct answer

Microsoft confirmed a Plan9 host-folder sharing failure in HCS-managed Linux VMs after the September 2026 Windows security update. Claude Cowork and WSL are named examples. For Windows 11 24H2/25H2, KB5129195 and subsequent updates resolve the issue; ordinary Hyper-V VMs without Plan9 are outside this documented scope.

Resolved Microsoft [1] · Microsoft [2]

Check affected environments Find a remedy for your version

Status: Resolved FACT
Affected: Windows 11 (4 versions); Windows 10 (2 versions) · Version table
First observed: Unknown
Last verified: Oct 11, 2026, 2:36 PM UTC
Confidence: Vendor confirmed
Severity: Unknown

TraceCheck interpretation

Does this explain your situation?

Use these checks to interpret the source. This is a reading guide, not a device test or a vendor diagnosis.

Match the sharing path
A missing folder alone is insufficient to match this record. Check whether the affected application uses an HCS-managed Linux VM and Plan9 host shares. A VM that starts normally can still have this specific sharing failure.
Source: Microsoft
Choose the package for the host version
KB5129195 is scoped here to Windows 11 24H2/25H2. Microsoft lists KB5129194 for 26H1. Windows 10 and 23H2 also appear in the affected list, but their fixing packages are not mapped in this index. Use the vendor instructions for that host version.
Source: Microsoft
Check previous administrator mitigation
If an administrator deployed the temporary Group Policy mitigation, Microsoft requires re-enabling that policy, installing the update and restarting. Record whether that mitigation was used before deciding that installing a package alone finishes the procedure.
Source: Microsoft

Symptoms

The Linux guest starts, but Plan9 host shares are missing or inaccessible. Source: Microsoft
Applications depending on those shares may report that no Plan9 drives were mounted. Source: Microsoft

Affected environments

Evidence-backed affected targets. An omitted dimension has no recorded conclusion.
Entity	Version	Originating update	Virtualization path	Evidence
Windows 11	26H1	Unknown	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft
Windows 11	25H2	KB5124008	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft
Windows 11	24H2	KB5124008	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft
Windows 11	23H2	Unknown	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft
Windows 10	22H2	Unknown	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft
Windows 10	21H2	Unknown	HCS-managed Linux VM; Plan9 host folder sharing	Source: Microsoft

What caused it?

Root cause has not been confirmed.

Fixes and workarounds

A version match identifies recorded guidance, not a diagnosis or a guarantee. Check the symptom, device and application requirements in each source before proceeding.

Install the applicable Windows update for Plan9 sharing

FACT Official fix · vendor fix

Microsoft documents KB5129195 or subsequent updates for Windows 11 24H2/25H2. Use the current applicable cumulative update. Previous Group Policy mitigation also requires the vendor re-enable/install/restart procedure. Other host versions need their own packages.

Documented symptom resolution

Applies to: Windows 11 24H2, Windows 11 25H2.

Risk: medium (TraceCheck editorial assessment, not a vendor success rate)

Verification: Oct 11, 2026, 2:36 PM UTC

Fixing update: KB5129195 · Microsoft [1] · Microsoft [2]

Read Microsoft's instructions ↗Read Microsoft's instructions ↗

Current status

Resolved This is the recorded state as of the last verification, not a live device diagnosis. Microsoft [1] · Microsoft [2]

Vendor acknowledgement / confirmation: 2026-09-11 09:22:00 UTC.

Resolution recorded: 2026-09-14 17:00:00 UTC.

Updates linked to documented corrections: KB5129195

For the recorded vendor guidance, see Plan9 host shares — Windows 11 24H2 release health Plan9 fix — KB5129195.

Evidence

Plan9 host shares — Windows 11 24H2 release health
Microsoft · Vendor documentation
Supports: Affected platforms, named applications, vendor acknowledgement/resolution dates and prior Group Policy mitigation procedure.
Retrieved Oct 11, 2026, 2:36 PM UTC
Plan9 fix — KB5129195
Microsoft · Vendor documentation
Supports: The HCS-managed Plan9 sharing correction and Windows 11 24H2/25H2 update scope.
Retrieved Oct 11, 2026, 2:36 PM UTC
Plan9 sharing — Windows 11 26H1 resolved issues
Microsoft · Vendor documentation
Supports: The 26H1 entry names different originating and fixing KBs; do not reuse the 24H2/25H2 package.
Retrieved Oct 11, 2026, 2:36 PM UTC

Change history

2026-09-14 17:00:00 UTC
Confirmed → Resolved
Microsoft records resolution with the September 14 updates and later.
Source: Microsoft
2026-09-11 09:22:00 UTC
Confirmed
Microsoft opened the Plan9 sharing record. This is an acknowledgement timestamp, not the first device occurrence.
Source: Microsoft

Mitigated Tracked software issue

USB Audio Class 1.0 Code 10 / no sound after KB5124008

Microsoft tracks a USB Audio Class 1.0 failure affecting startup, sound and controls. The current vendor state is mitigated. On Windows 11 24H2/25H2, KB5129195 fixes the 8-channel/3D audio subset; it does not establish that Code 10 or every no-sound symptom is resolved.

15 documented versions across Windows 11, Windows 10, Windows Server · Affected environments

Recorded update fix: Partial — remaining symptoms unresolved · Windows 11 24H2, Windows 11 25H2 · Check instructions

FACT Vendor confirmed · Source checked Oct 11, 2026, 2:43 PM UTC

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