Windows update KB5124008 breaks Anthropic Cowork's sandbox shell; uninstall is the only fix
standard_deviant_Q · reddit · 2026-09-10
- Windows cumulative security update KB5124008 (build 26200.9445, installed Sep 9) breaks Cowork's sandbox on Windows: every shell command fails with sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared
- File read/write via the connected folder still works; only the folder-sharing step into the Linux VM breaks. A 6am scheduled task fails the same way.
- Restart, re-adding the folder, wsl --update / wsl --shutdown did NOT help (WSL2 was healthy — Cowork's share mechanism isn't drvfs)
- Fix: wusa /uninstall /kb:5124008 in admin PowerShell, then reboot — shell came back immediately. Author paused Windows Update and set a daily check.
- Caveat: it's a security cumulative, so the workaround means staying unpatched until Anthropic ships a fix. Author asks if anyone found a way to keep the KB and still get the mount working.
Related event: Windows Update Breaks Claude Desktop's Local File Access(3 posts)→
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