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

HarborFront

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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 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.”

"While I totally understand doing this for anyone other than a malware analyst, this kind of nosy, get-inside-your-business way of handling this is going to become a big problem for people like me who need to send their colleagues malware samples,” Brandt wrote. “The available space to do this just keeps shrinking and it will impact the ability of malware researchers to do their jobs.”

Fellow researcher Kevin Beaumont joined the discussion to say that Microsoft has multiple methods for scanning the contents of password-protected zip files and uses them not just on files stored in SharePoint but all its 365 cloud services. One way is to extract any possible passwords from the bodies of email or the name of the file itself. Another is by testing the file to see if it’s protected with one of the passwords contained in a list.

“If you mail yourself something and type something like 'ZIP password is Soph0s', ZIP up EICAR and ZIP password it with Soph0s, it'll find (the) password, extract and find (and feed MS detection),” he wrote.

Brandt said that last year Microsoft’s OneDrive started backing up malicious files he had stored in one of his Windows folders after creating an exception (i.e., allow listing) in his endpoint security tools. He later discovered that once the files made their way to OneDrive, they were wiped off of his laptop hard drive and detected as malware in his OneDrive account.

“I lost the whole bunch,” he said.

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SeriousHoax

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Mar 16, 2019
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Yeah, this is a known thing and extremely annoying for anyone wanting to test malware. Often, I had to turn off MD to stop it from detecting password protected malicious zips. It happens for zips with "infected" password. But this is new to me that it even extracts passwords from email bodies.
 

Anthony Qian

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Apr 17, 2021
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I noticed this behavior of MD before. A password-protected (password : infected) zip archive containing clean files will also be detected by Microsoft’s “machine learning” model.
 
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