NTLM theft attacks have pushed Microsoft to disable the File Explorer preview pane for internet downloads. The company introduced this change to stop attackers from stealing user credentials through malicious files. The update enhances protection for Windows 11 and Windows Server users by preventing automatic previews of files marked as unsafe.
Microsoft’s Security Update Explained
In October 2025, Microsoft released a security update that blocks the File Explorer preview pane for files carrying the Mark of the Web (MotW). Files downloaded from the internet receive this tag automatically, warning Windows that they may be untrusted.
Previously, when a user selected a file in File Explorer, the preview pane generated a thumbnail or content preview. In certain cases, this triggered external connections that exposed NTLM credentials. Cybercriminals could exploit this behavior by embedding malicious HTML or external image references in a document.
With the new update, these previews are no longer shown automatically for internet files, closing a potential entry point for NTLM theft attacks.
How NTLM Theft Attacks Work
NTLM theft attacks rely on tricking a system into sending authentication hashes to a remote server. These hashes can then be cracked or replayed to gain unauthorized access to networks. By simply previewing a compromised file, users could unknowingly send sensitive credentials to attackers.
This attack vector became increasingly common, especially through phishing campaigns and shared corporate drives. Disabling previews for MotW-tagged files prevents Windows from making external connections that could reveal credentials.
How Users Can Manage Trusted Files
While this new protection is automatic, Microsoft allows users to restore previews for trusted files. They can do so by unblocking the file through its Properties menu or by adding the file’s location to the Trusted Sites or Local Intranet zone in Internet Options.
This flexibility helps balance convenience and security, giving users control without re-exposing themselves to NTLM theft attacks.
Strengthening Enterprise Defenses
For organizations, this change reduces the risk of credential leaks from accidental file previews. System administrators should ensure the latest Windows 11 and Windows Server updates are applied across all endpoints. They should also educate employees about safe file-handling practices and the importance of not bypassing built-in protections.
By addressing a subtle but significant weakness in Windows file handling, Microsoft adds another layer to its defense against NTLM theft attacks.
Conclusion
Microsoft’s decision to disable the File Explorer preview pane for internet downloads directly targets NTLM theft attacks. This proactive step eliminates a hidden vulnerability that exposed credentials through simple file previews. The update marks a strong move toward more resilient Windows security and underscores the need for continued awareness of credential protection.
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