Microsoft has rolled out a security update to Windows File Explorer starting October 14, 2025, automatically disabling the preview pane for downloaded files.
This move directly targets a vulnerability that could expose users’ NTLM hashes, sensitive credentials used for network authentication that attackers have long exploited to compromise accounts and gain unauthorized access to corporate networks.
The vulnerability at the heart of this update involves a deceptively simple attack vector.
When users preview files downloaded from the internet, malicious files embed HTML elements like <link> or <src> Tags that can trigger unauthorized network requests in the background.
These requests were often used by attackers to harvest NTLM hashes from unsuspecting users, potentially leading to lateral movement across networks or complete account takeovers.
By proactively disabling previews, Microsoft removes one pathway for this type of credential theft.
The new behavior relies on the “Mark of the Web” attribute that Windows applies to files from untrusted sources.
Once tagged with this marker, files will no longer display previews in File Explorer. Instead, users see a clear warning: “The file you are attempting to preview could harm your computer.
If you trust the file and the source from which you received it, you may open it to view its contents.”
For most users, the impact remains minimal. Local documents and files from trusted network shares continue to preview normally.
The protection activates automatically without requiring any configuration or user intervention. Microsoft’s approach prioritizes security while maintaining usability for legitimate workflows.
Microsoft recognizes that users sometimes need to preview downloaded files.
For trusted downloads, overriding the protection is deliberate but straightforward. Users can right-click the file in File Explorer, select Properties, and check the “Unblock” box.
These changes take effect after the next login.
For entire file shares in Internet Zones, administrators can add the share’s address to Local Intranet or Trusted Sites through Internet Options in Control Panel, though this approach should be reserved for verified networks since it lowers defenses for all files from that source.
Enterprise and Administrative Benefits
IT administrators and security-conscious users will appreciate the broad protection that covers both downloaded files and remote shares.
This reduces the attack surface in enterprise environments where NTLM weaknesses persist despite ongoing pushes toward modern authentication methods like Kerberos.
Rather than imposing a complete lockdown, Microsoft’s update encourages safer security habits through smart defaults.
The change demonstrates a measured approach to Windows security that protects users without unnecessarily disrupting legitimate workflows.
As cyber threats continue to evolve, such incremental improvements help keep Windows systems more resilient against credential theft attacks without overcomplicating daily operations.
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