“The file you are attempting to preview could harm your computer”: fix

Why the preview is blocked

The full message reads: The file you are attempting to preview could harm your computer. If you trust the file and the source you received it from, open it to view its contents. Opening the file normally still works; only the preview pane is affected.

Browsers and many email apps tag downloaded files with a hidden marker, the Mark of the Web, which records that the file came from the internet zone. Microsoft’s support article says that Windows security updates released on or after October 14, 2025 stop File Explorer from previewing these files, and files on shares Windows treats as internet locations. The reason given is that previewing a file containing HTML that points to an outside address could leak the NTLM password hash of the signed-in user. Security sites widely link the change to CVE-2025-59214, though Microsoft’s article doesn’t name a CVE.

Microsoft’s article lists Windows 11 and Windows Server. Users also report the same block on Windows 10 22H2 after its October 2025 update.

Fix 1: Install the latest Windows 11 update

Microsoft relaxed the rule in the optional KB5124010 update of September 22, 2026 for Windows 11 versions 24H2 and later. Its notes say non-HTML files such as PDFs are now previewed automatically, and HTML files get a new Preview anyway button so you can acknowledge the warning. Press reports say the October 2026 Patch Tuesday update carries the same change to all users, rolling out gradually.

Open Settings > Windows Update, select Check for updates, install everything offered (including the optional update if you see it) and restart. Because the change rolls out in stages, it may take a little while to appear even on an up-to-date PC. Windows 10 PCs, including those on Extended Security Updates, don’t appear to get this change, so use the fixes below there.

Fix 2: Unblock a file you trust

1. Right-click the file in File Explorer and choose Properties.
2. On the General tab, find the Security line at the bottom and tick Unblock.
3. Select Apply, then OK.

Microsoft notes the change might not take effect immediately; if the pane still shows the warning, sign out and back in. If there’s no Unblock box, the file doesn’t carry the marker and something else is stopping the preview, such as a missing preview handler for that file type.

Only unblock files you expected and trust. Removing the marker also removes other protections: SmartScreen won’t check the file, Office won’t open it in Protected View, and macros in it are no longer blocked as internet files. If you’re unsure, scan it first; see how to scan for malware.

Fix 3: Unblock several files with PowerShell

Microsoft’s Unblock-File cmdlet does the same as the Unblock box. To unblock one file:

Unblock-File -Path "$env:USERPROFILE\Downloads\report.pdf"

To unblock every file in a folder you’ve checked (add -Recurse after the path for subfolders, and -WhatIf at the end to preview what would change):

Get-ChildItem "$env:USERPROFILE\Documents\Invoices" | Unblock-File

Microsoft warns not to run a bulk command like this until you’ve verified all the files are safe, so don’t point it at your whole Downloads folder.

Downloaded a ZIP? Windows’ built-in extraction copies the marker onto every file it extracts. Unblock the .zip file itself first (Fix 2), then extract it, and the extracted files won’t be blocked. 7-Zip doesn’t copy the marker by default, which is why files extracted with it may preview normally.

Fix 4: Files on a network share

If files on a NAS or another PC are blocked, Windows is treating that location as part of the internet zone. Microsoft’s fix is to add the share to the Local intranet or Trusted sites zone:

1. Press Win + R, type inetcpl.cpl and press Enter.
2. On the Security tab, select Local intranet, then Sites and Advanced.
3. Add the server, for example \\nas or file://192.168.1.20, select Add, then Close and OK.
4. Sign out and back in, then try the preview again.

Microsoft warns that this relaxes security for every file on that share, so only add servers you control. On a work PC, IT normally sets this through the Group Policy Site to Zone Assignment List. If the share won’t open at all, see “block unauthenticated guest access”.

Tweaks to avoid

Some guides suggest switching the block off for everything: editing the internet zone’s registry settings, enabling the policy Do not preserve zone information in file attachments, or copying files to a FAT32 drive and back to strip the marker. None of these are Microsoft fixes. The first two remove the protection for every downloaded file, which reopens the hash-leak risk the update was made to close and weakens SmartScreen and Office’s Protected View too. Stick with unblocking specific files you trust.

If File Explorer itself misbehaves while you’re trying this, see Explorer crashes on right-click. If downloads are also being flagged by SmartScreen, see SmartScreen can’t be reached.

Related guides

• SmartScreen can’t be reached right now: how to fix it
• Windows Explorer crashes on right-click: how to find the cause and fix it
• How to Scan for Malware on Windows 11 and 10 (Free, Built-In Tools)
• “Your organization’s security policies block unauthenticated guest access”: fix
• “Microsoft has blocked macros from running because the source of this file is untrusted”: fix

Official sources

• File Explorer automatically disables the preview feature for files downloaded from the internet (Microsoft Support)
• KB5124010, September 22, 2026 preview update (Microsoft Support)
• Unblock-File (Microsoft Learn)