Windows MSHTML zero-day defenses bypassed as new info emerges


New details have emerged about the recent Windows CVE-2021-40444 zero-day vulnerability, how it is being exploited in attacks, and the threat actor’s ultimate goal of taking over corporate networks.

This Internet Explorer MSHTML remote code execution vulnerability, tracked as CVE-2021-40444, was disclosed by Microsoft on Tuesday but with few details as it has not been patched yet.

The only information shared by Microsoft was that the vulnerability uses malicious ActiveX controls to exploit Office 365 and Office 2019 on Windows 10 to download and install malware on an affected computer.

Since then, researchers have found the malicious Word documents used in the attacks and have learned new information about how the vulnerability is exploited.

Why the CVE-2021-40444 zero-day is so critical

Since the release of this vulnerability, security researchers have taken to Twitter to warn how dangerous it is even though Microsoft Office’s ‘Protected View’ feature will block the exploit.

When Office opens a document it checks if it is tagged with a “Mark of the Web” (MoTW), which means it originated from the Internet.

If this tag exists, Microsoft will open the document in read-only mode, effectively blocking the exploit unless a user clicks on the ‘Enable Editing’ buttons.

Word document opened in Protected View
Word document opened in Protected View

As the “Protected View” feature mitigates the exploit, we reached out to Will Dormann, a vulnerability analyst for CERT/CC, to learn why security researchers are so concerned about this vulnerability.

Dormann told BleepingComputer that even if the user is initially protected via Office’s ‘Protected View’ feature, history has shown that many users ignore this warning and click on the ‘Enable Editing’ button anyway.

Dormann also warns that there are numerous ways for a document not to receive the MoTW flag, effectively negating this defense.

“If the document is in a container that is processed by something that is not MotW-aware, then the fact that the container was downloaded from the Internet will be moot. For example, if 7Zip opens an archive that came from the Internet, the extracted contents will have no indication that it came from the Internet. So no MotW, no Protected…