Your Windows PC Is Getting a Crucial Security Upgrade: Why File Previews Are Changing
The File Explorer preview pane is one of Windows’ most convenient features. It lets you peek inside a document, image, or PDF without actually opening the file. But what if that simple convenience could be exploited by attackers? A new security enhancement rolling out for Windows 11 aims to close this dangerous loophole for good.
Microsoft is fundamentally changing how File Explorer handles previews for files downloaded from the internet, a move designed to protect users from sophisticated “zero-click” attacks.
The Hidden Danger in File Previews
Security researchers have long known that the process of generating a file preview isn’t always harmless. To show you the contents of a file, Windows has to execute a small part of that file’s code. Cybercriminals have learned to abuse this by embedding malicious code within seemingly innocent files like text documents or PDFs.
Here’s how the attack works:
- An attacker tricks you into downloading a malicious file.
- You open File Explorer and simply click on the file once to select it.
- The preview pane automatically attempts to generate a preview.
- In doing so, it executes the hidden malicious code, potentially compromising your system without you ever opening the file itself.
This is a zero-click exploit because it requires no interaction beyond selecting the file. It’s a stealthy and effective way to bypass traditional security measures, as many users wouldn’t suspect that merely previewing a file could be dangerous.
How Microsoft Is Fixing the Flaw
To combat this threat, Windows is now leveraging a long-standing security feature called the Mark of the Web (MotW). This is a special, hidden attribute that Windows automatically applies to any file downloaded from the internet. You might have seen its effects before when Windows warns you before opening a downloaded program.
The new update makes File Explorer “MotW-aware.” This means that when you select a downloaded file:
- File Explorer will check for the Mark of the Web.
- If the MotW is present, File Explorer will not generate a file preview.
- This prevents the malicious code from ever being executed, effectively neutralizing the threat.
This change represents a crucial shift in philosophy: security is now being prioritized over convenience for files that originate from untrusted sources.
What This Means for You
For the average user, this change will be a minor adjustment to their workflow but a major boost to their digital safety.
When you download a file from the internet, you will no longer see an automatic preview in File Explorer. The preview pane will simply appear blank. Previews for files you create yourself or that are already on your local system will continue to work as normal.
If you have downloaded a file from a source you trust and want to enable the preview, you can easily remove the Mark of the Web.
How to Manually Trust a Downloaded File:
- Right-click on the downloaded file.
- Select Properties.
- In the General tab, at the bottom, you will see a security message: “This file came from another computer and might be blocked to help protect this computer.”
- Check the “Unblock” box and click Apply or OK.
Once you have unblocked the file, you are telling Windows that you trust it, and File Explorer will be able to generate previews for it once again.
Key Security Tips and Takeaways
This update is a powerful reminder that vigilance is key to staying safe online.
- A Smart Default: Disabling previews for downloaded files is a smart, secure-by-default setting that protects users who may not be aware of this threat vector.
- Trust but Verify: Only unblock files from sources you know and trust completely. If you have any doubt about a file’s origin, do not unblock it.
- Stay Updated: This feature is rolling out to Windows 11 users, beginning with those in the Windows Insider program. Ensuring your system is always up-to-date is the best way to receive the latest security protections.
By making this small but significant change, Microsoft is hardening the Windows operating system against an entire class of stealthy attacks, making your computer safer one click—or lack thereof—at a time.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-preview-pane-for-downloads-to-block-ntlm-theft-attacks/
