Microsoft MSHTML Remote Code Execution Vulnerability (CVE-2021-40444) Threat Alert

Microsoft MSHTML Remote Code Execution Vulnerability (CVE-2021-40444) Threat Alert

October 12, 2021 | Jie Ji

Overview

On September 8, Beijing time, NSFOCUS CERT found that Microsoft released a security bulletin to disclose a remote code execution vulnerability (CVE-2021-40444) in Microsoft MSHTML. Attackers could craft a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine, and convince the user to open a malicious document to trigger arbitrary code execution on the target system with privileges of this user. In this bulletin, Microsoft indicated that this vulnerability is already exploited in the wild and users should take preventive measures as soon as possible.

MSHTML (also known as Trident) is a proprietary browser engine for Internet Explorer developed by Microsoft. It is also used by Microsoft Office applications (Word, Excel, or PowerPoint) for rendering web-based contents. AcitveX controls are products of Microsoft’s Component Object Module (COM) architecture. They are also widely used in the Microsoft Office Suite and Internet Explorer in Windows systems. AcitveX controls can be used for interaction with MSHTML components.

Reference link: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444

Scope of Impact

Affected Versions

  • Windows Server, version 20H2 (Server Core Installation)
  • Windows Server, version 2004 (Server Core installation)
  • Windows Server 2022 (Server Core installation)
  • Windows Server 2022
  • Windows Server 2019  (Server Core installation)
  • Windows Server 2019
  • Windows Server 2016  (Server Core installation)
  • Windows Server 2016
  • Windows Server 2012 R2 (Server Core installation)
  • Windows Server 2012 R2
  • Windows Server 2012 (Server Core installation)
  • Windows Server 2012
  • Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation)
  • Windows Server 2008 for x64-based Systems Service Pack 2
  • Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation)
  • Windows Server 2008 for 32-bit Systems Service Pack 2
  • Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation)
  • Windows Server 2008 R2 for x64-based Systems Service Pack 1
  • Windows RT 8.1
  • Windows 8.1 for x64-based systems
  • Windows 8.1 for 32-bit systems
  • Windows 7 for x64-based Systems Service Pack 1
  • Windows 7 for 32-bit Systems Service Pack 1
  • Windows 10 for x64-based Systems
  • Windows 10 for 32-bit Systems
  • Windows 10 Version 21H1 for x64-based Systems
  • Windows 10 Version 21H1 for ARM64-based Systems
  • Windows 10 Version 21H1 for 32-bit Systems
  • Windows 10 Version 20H2 for x64-based Systems
  • Windows 10 Version 20H2 for ARM64-based Systems
  • Windows 10 Version 20H2 for 32-bit Systems
  • Windows 10 Version 2004 for x64-based Systems
  • Windows 10 Version 2004 for ARM64-based Systems
  • Windows 10 Version 2004 for 32-bit Systems
  • Windows 10 Version 1909 for x64-based Systems
  • Windows 10 Version 1909 for ARM64-based Systems
  • Windows 10 Version 1909 for 32-bit Systems
  • Windows 10 Version 1809 for x64-based Systems
  • Windows 10 Version 1809 for ARM64-based Systems
  • Windows 10 Version 1809 for 32-bit Systems
  • Windows 10 Version 1607 for x64-based Systems
  • Windows 10 Version 1607 for 32-bit Systems

Mitigation

Disabling the installation of all ActiveX controls in Internet Explorer in all zones mitigates attacks against this vulnerability. You can create a registry file to disable ActiveX controls (make a backup in advance)

1. Paste the following contents into a text file and save it as a .reg file.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\0]

“1001”=dword:00000003

“1004”=dword:00000003

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\1]

“1001”=dword:00000003

“1004”=dword:00000003

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\2]

“1001”=dword:00000003

“1004”=dword:00000003

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3]

“1001”=dword:00000003

“1004”=dword:00000003

2. Double-click the .reg file and apply it to your Policy hive.

3. Reboot the system to ensure the new configuration is applied.

Note: This sets the URLACTION_DOWNLOAD_SIGNED_ACTIVEX (0x1001) and URLACTION_DOWNLOAD_UNSIGNED_ACTIVEX (0x1004) to DISABLED (3) for all internet zones for 64-bit and 32-bit processes. New ActiveX controls will not be installed. Previously-installed ActiveX controls will continue to run.

Undo the workaround:

Delete the registry keys that were added in implementing this workaround.

Product Update for Protection

Currently, Microsoft Defender Antivirus and Microsoft Defender for Endpoint both provide detection and protections for the known vulnerability. Customers should keep antimalware products up to date. Customers who use automatic updates do not need to take additional actions. Enterprise customers who manage updates should select the detection build 1.349.22.0 or newer and deploy it across their environments. Microsoft Defender for Endpoint alerts will be displayed as: “Suspicious Cpl File Execution.”

About NSFOCUS

NSFOCUS, Inc., a global network and cyber security leader, protects enterprises and carriers from advanced cyber attacks. The company’s Intelligent Hybrid Security strategy utilizes both cloud and on-premises security platforms, built on a foundation of real-time global threat intelligence, to provide multi-layered, unified and dynamic protection against advanced cyber attacks.

NSFOCUS works with Fortune Global 500 companies, including four of the world’s five largest financial institutions, organizations in insurance, retail, healthcare, critical infrastructure industries as well as government agencies. NSFOCUS has technology and channel partners in more than 60 countries, is a member of both the Microsoft Active Protections Program (MAPP), and the Cloud Security Alliance (CSA).

A wholly owned subsidiary of NSFOCUS Technologies Group Co., Ltd., the company has operations in the Americas, Europe, the Middle East and Asia Pacific.