Advisory September 14, 2026: Windows Update Breaks Claude Cowork Sandbox

Status: Active — Last updated September 14, 2026

We are aware of a compatibility issue affecting Windows users of Claude Cowork who have installed a recent Windows update (KB5124008). Because this issue is caused by a Windows 11 cumulative update, users will experience it both inside and outside of Blue Border and there is nothing Venn can do to resolve the issue.

What users are experiencing: Cowork tasks that read, write, search, or edit files, or run code in the sandbox terminal are failing on Windows devices that have installed a recent Windows update (KB5124008). Users may see "failure to set VHDX path" or "failure to start Claude's workspaces" errors. This failure is happening both inside and outside of Blue Border.

What users can do in the meantime: If your users have installed this update and they are blocked by this issue, uninstalling it will restore Cowork's sandbox functionality. Note that this update also contains patches for two actively exploited security vulnerabilities, so removing it carries a real security trade-off — weigh this against your organization's own risk tolerance, and plan to reapply the update once a permanent fix is available. macOS is not affected.

Identified: Approximately September 8, 2026

Microsoft has acknowledged the issue and says they are working on a resolution, but no timeline has been provided. Our team is tracking Microsoft's documentation and will update this article as soon as a permanent fix is available.