Infosec teams are being warned about a vulnerability in Windows’ file encryption capability which may mean temporarily abandoning the EFS encryption functionality

Windows has long offered the EFS file and folder capability in the Pro, Professional, Business, Ultimate, Enterprise and Education editions of the operating system. Organizations may find this built-in functionality useful for protecting certain data, although an enterprise-toughened encryption solution may be required for large corporations.

But in a blog this morning, SafeBreach Labs reported a proof-of-concept attack that would in effect turn EPS on itself, using the built-in encryption as a ransomware weapon to encrypt an entire disk.

This “EFS ransomware” type of attack was tested with Windows 10 64-bit versions 1803, 1809 and 1903, the blog says, but should also work on Windows 32-bit operating systems, and on earlier versions of Windows as far back as Vista.

Related: Ransomware now stealing data

To a threat actor, this type of attack has the advantage of encrypting files deep in the kernel at the NTFS driver level, and the modification wouldn’t be noticed by file-system filter drivers. It also doesn’t require administrator rights or human interaction.

On the other hand when files and/or folders are encrypted, a small yellow padlock icon is displayed at the top right corner of the file/folder main icon, which might tip off a user of something suspicious. And if a Data Recovery Agent is defined for the machine (this is not the default for standalone/workgroup machines, but it is the default for domain-joined machines), then recovery is trivial using the Data Recovery Agent.

What worried SafeBreach is that a test of several anti-ransomware solutions — including Windows 10 Controlled Folder Access — failed to catch the attack SafeBreach had crafted. Last fall it quietly contacted a number of security providers such as Avast, ESET Kaspersky and others. Many of them have since updated their software to detect such an attack or are about to issue updates.

SafeBreach said Microsoft told it last October that it considers Controlled Folder Access a defence-in-depth feature. “We assessed this submittal (by SafeBreach) to be a moderate class defence-in-depth issue, which does not meet the Microsoft Security Servicing Criteria for Windows. Microsoft may consider addressing this in a future product.”

Still, some organizations may want to stop using or turn off EFS until they are sure detection of this kind of attack is assured, or they may want to find another encryption solution.

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