Security Experts:

Connect with us

Hi, what are you looking for?


Malware & Threats

Microsoft Office Patch Bypassed for Malware Distribution in Apparent ‘Dry Run’

Cybercriminals have found a way to bypass the patch for a recent Microsoft Office vulnerability and leveraged it to briefly distribute Formbook malware, Sophos reports.

Cybercriminals have found a way to bypass the patch for a recent Microsoft Office vulnerability and leveraged it to briefly distribute Formbook malware, Sophos reports.

Tracked as CVE-2021-40444 (CVSS score of 8.8), and affecting the MSHTML file format, the security defect can be exploited to achieve remote code execution on vulnerable systems. An attacker looking to exploit the bug needs to trick the indented victim into opening a maliciously crafted document.

Publicly disclosed on September 7, after attacks exploiting it were identified, the security error was addressed with the September 2021 Patch Tuesday updates. Proof-of-concept code targeting the bug was also published and exploitation activity intensified.

[ READ: Microsoft Confirms ‘NotLegit’ Azure Flaw Exposed Source Code Repositories ]

The patch that Microsoft provided was meant to prevent the execution of code to download a Microsoft Cabinet (CAB) archive containing a malicious executable. However, it appears that attackers found a way to bypass the patch by incorporating a Word document in a specially crafted RAR archive.

Sophos says the attackers distributed the archives as part of a spam email campaign that lasted for roughly 36 hours – on October 24 and 25 – before completely disappearing, which would suggest the attack was only a “dry run” experiment.

A PowerShell script was used to prepend the malicious Word document inside the archive and, as soon as the victim opened the archive to access the document, the script was executed, leading to an infection with Formbook malware.

“The attachments represent an escalation of the attacker’s abuse of the -40444 bug and demonstrate that even a patch can’t always mitigate the actions of a motivated and sufficiently skilled attacker,” Sophos says.

According to the cybersecurity company, the attack was possible because the provided patch was narrowly focused and because of the manner in which WinRAR treats files containing the correct magic bytes, regardless of where in the file these bytes are located.

Related: Microsoft Urges Customers to Patch Recent Active Directory Vulnerabilities

Related: PoC Exploit Published for Latest Microsoft Exchange Zero-Day

Related: Severe OMIGOD Vulnerabilities Expose Thousands of Azure Users

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

Click to comment

Expert Insights

Related Content

Mobile & Wireless

Technical details published for an Arm Mali GPU flaw leading to arbitrary kernel code execution and root on Pixel 6.

Mobile & Wireless

Apple rolled out iOS 16.3 and macOS Ventura 13.2 to cover serious security vulnerabilities.

Cloud Security

VMware vRealize Log Insight vulnerability allows an unauthenticated attacker to take full control of a target system.

Mobile & Wireless

Apple’s iOS 12.5.7 update patches CVE-2022-42856, an actively exploited vulnerability, in old iPhones and iPads.

Malware & Threats

Microsoft plans to improve the protection of Office users by blocking XLL add-ins from the internet.


Security researchers have observed an uptick in attacks targeting CVE-2021-35394, an RCE vulnerability in Realtek Jungle SDK.

Mobile & Wireless

Two vulnerabilities in Samsung’s Galaxy Store that could be exploited to install applications or execute JavaScript code by launching a web page.


Several vulnerabilities have been patched in OpenText’s enterprise content management (ECM) product.