Microsoft is taking a more aggressive stance against a sneaky credential-stealing technique by disabling a convenient Windows feature that many users take for granted. According to security reports, the latest Windows updates now automatically block File Explorer previews for files downloaded from the internet, closing what analysts describe as a significant hash leakage vulnerability.
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The Hash Leak Problem
Security researchers have identified what might seem like an obscure but potentially dangerous attack vector. When users preview certain downloaded files containing HTML tags in File Explorer, the system could allegedly leak NTLM authentication hashes to remote servers. These cryptographic hash values represent user credentials and could be captured by attackers to compromise accounts.
Sources indicate the vulnerability specifically involves files containing tags like <link> and <src> that reference external paths. Previewing such files triggers requests that transmit the user’s NTLM hash without their knowledge. Microsoft’s documentation reportedly explains this could expose “sensitive credentials” through what security professionals call a hash leakage attack.
How Windows Is Responding
The solution, while effective, removes a feature many Windows users rely on daily. Now when you download files from the internet, File Explorer will display a security warning instead of a preview: “The file you are attempting to preview could harm your computer.”
This protection leverages what’s known as “Mark of the Web” metadata—essentially a digital tag that identifies files originating from online sources. Windows Defender pays extra attention to these files, and now the preview pane joins the security party too.
What’s particularly interesting is how Microsoft has implemented this change. Rather than asking users to opt-in to protection, they’ve made security the default behavior. It’s part of a broader industry shift toward what security experts call “secure by design” principles, where safety features are built in rather than bolted on.
The User Experience Trade-off
For power users who frequently download and preview files, this creates additional steps. To restore preview functionality for trusted downloads, you’ll need to right-click each file, select Properties, and check the Unblock option. According to reports, this manual process must be repeated for every file and might require a logout and login to take effect.
Security analysts suggest this friction is intentional—making users consciously decide to trust each file rather than automatically previewing potentially dangerous content. It’s a classic security versus convenience tradeoff that Microsoft appears to have decided in favor of protection.
The change reflects growing concerns about NTLM hash vulnerabilities that have persisted despite newer authentication protocols. While modern systems support more secure options, NTLM remains widely used in corporate environments, making hash protection particularly important for business users.
Industry Implications
This move represents another step in Microsoft’s ongoing effort to harden Windows against increasingly sophisticated attacks. What’s notable is the company targeting what might seem like a minor feature—the preview pane—that turns out to have significant security implications.
Security professionals have largely praised the approach, noting that blocking previews prevents attacks before they can execute. Meanwhile, enterprise IT teams will need to consider how this change affects workflows that rely heavily on quick file previews from downloaded content.
As organizations balance productivity against security, Microsoft’s decision to disable previews by default shows how even seemingly innocent features can become attack vectors in today’s threat landscape. The update serves as a reminder that in cybersecurity, sometimes the most dangerous threats come through the doors we leave unlocked without realizing it.
