Microsoft is scanning the inside of password-protected zip files for malware

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Microsoft cloud services are scanning for malware by peeking inside users’ zip files, even when they’re protected by a password, several users reported on Mastodon on Monday.

Compressing file contents into archived zip files has long been a tactic threat actors use to conceal malware spreading through email or downloads. Eventually, some threat actors adapted by protecting their malicious zip files with a password the end user must type when converting the file back to its original form. Microsoft is one-upping this move by attempting to bypass password protection in zip files and, when successful, scanning them for malicious code.

While analysis of password-protected files in Microsoft cloud environments is well-known to some people, it came as a surprise to Andrew Brandt. The security researcher has long archived malware inside password-protected zip files before exchanging them with other researchers through SharePoint. On Monday, he took to Mastodon to report that the Microsoft collaboration tool had recently flagged a zip file, which had been protected with the password “infected.”