September 10, 2021

Security researchers have shed more light on how the Internet Explorer MSHTML remote code execution vulnerability tracked as CVE-2021-40444 is being exploited in attacks.

In analyzing the malicious Word document used to better understand how the exploit works, Beaumont discovered one of the known malicious Word attachments used in the attacks, called “A Letter before court 4.docx’ and claimed to be a letter from a lawyer.

As soon as the file is downloaded from the Internet, it is marked with the ‘Mark of the Web’ and opened in Protected View.

Once a user clicks on the ‘Enable Editing’ button, the exploit opens a URL with the ‘mhtml’ protocol to a ‘side.html’ file hosted on a remote page and loaded as a Word template and registered as ‘mhtml’.

Next, URLs in Internet Explorer will launch the browser to load HTML and its disguised JavaScript code that will exploit the CVE-2021-40444 vulnerability by creating a malicious ActiveX control.

While the ultimate payload tries to install a Cobalt Strike beacon that would allow the threat actor to gain remote access to the device, it can be used to install more malware, steal files or deploy ransomware once the attackers have obtained remote access to the victims’ computers.

Users are advised not to open attachments unless they come from a trusted source.

For more information, read the original story in Bleeping Computer.