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Why Did My Windows File Explorer Preview Stop Working After the October 2025 Update?
The October 2025 security updates for Windows have caused an issue for some users where the file preview in File Explorer stops working. This problem mostly affects documents located on network shares. Instead of seeing a preview, users see a warning message.
Understand the Problem
When you select a file, the preview pane may show a warning that says, “The file you are attempting to preview could harm your computer.” This prevents the preview from loading.
- Affected Files: The issue is common with PDF and Microsoft Office files like Word and Excel documents.
- Unaffected Files: Graphic and image files usually continue to show previews correctly.
- Affected Systems: Reports confirm this problem occurs on Windows 10, Windows 11 (specifically with update KB5066835), and Windows Server 2019.
The root cause is related to a security feature called the Mark-of-the-Web (MoW). Windows applies this mark to files that come from sources it doesn’t fully trust, such as internet downloads or network drives. The latest update made the preview handler more cautious about files with this mark, causing it to block the preview and show a warning instead.
Fix the File Preview
Uninstalling the security update will resolve the issue, but this is not recommended as it leaves your system unprotected. The best solution is to tell Windows to trust the location where your files are stored. You can do this by adding your network drive to the “Local Intranet” zone.
- Open the Start Menu, type “Internet Options,” and press Enter.
- Navigate to the Security tab in the window that appears.
- Select the Local intranet icon, then click the Sites button.
- In the new window, click the Advanced button.
- In the “Add this website to the zone” field, enter the path to your network share (for example, \\your-server-name).
- Click Add, then Close, and finally OK on the remaining windows.
After you complete these steps, File Explorer will trust the files from that network location, and the previews for your documents should work again without showing a warning.