[tomek@vz]

Microsoft says that the File Explorer (formerly Windows Explorer) now automatically blocks previews for files downloaded from the Internet to block credential theft attacks via malicious documents, according to a report from BleepingComputer. This attack vector is particularly concerning because it requires no user interaction beyond selecting a file to preview and removes the need to trick a target into actually opening or executing it on their system.

For most users, no action is required since the protection is enabled automatically with the October 2025 security update, and existing workflows remain unaffected unless you regularly preview downloaded files.

"This change is designed to enhance security by preventing a vulnerability that could leak NTLM hashes when users preview potentially unsafe files," Microsoft says in a support document published Wednesday.

It is important to note that this may not take effect immediately and could require signing out and signing back in.