Security researchers have discovered a new Microsoft Office zero-day vulnerability that is being used in attacks to execute malicious PowerShell commands via Microsoft Diagnostic Tool simply by opening a Word document.

The vulnerability, which has yet to receive a tracking number and is referred to by the infosec community as ‘Follina,’ is leveraged using malicious Word documents that execute PowerShell commands via the MSDT. This new Follina zero-day opens the door to a new critical attack vector leveraging Microsoft Office programs as it works without elevated privileges, bypasses Windows Defender detection, and does not need macro code to be enabled to execute binaries or scripts.

The HTML code then uses Microsoft’s MS-MSDT URI protocol scheme to load additional code and execute PowerShell code.

Multiple security researchers have analyzed the malicious document shared by nao sec and successfully reproduced the exploit with multiple versions of Microsoft Office.

For organizations relying on Microsoft Defender’s Attack Surface Reduction rules, Huntress advises activating the “Block all Office applications from creating child processes” in Block mode, which would prevent Follina exploits.

Another mitigation, from Didier Stevens, would be to remove the file type association for ms-msdt so that Microsoft Office won’t be able to invoke the tool when opening a malicious Folina document.