CVE-2021-40444: Windows 10 MSHTML RCE Vulnerability

CVE-2021-40444 Overview

CVE-2021-40444 is a remote code execution vulnerability in Microsoft MSHTML, the HTML rendering engine used by Microsoft Windows and Microsoft Office. This vulnerability allows attackers to craft malicious ActiveX controls embedded in Microsoft Office documents that can execute arbitrary code when the victim opens the document. The vulnerability has been actively exploited in targeted attacks using specially-crafted Office documents.

An attacker could leverage this vulnerability by creating a malicious ActiveX control and embedding it in a Microsoft Office document that uses the browser rendering engine. The attack requires user interaction—the victim must open the malicious document. Users with limited user rights on the system experience reduced impact compared to those operating with administrative privileges.

Critical Impact

This vulnerability enables remote code execution through malicious Office documents and has been actively exploited in the wild. It is listed in CISA's Known Exploited Vulnerabilities Catalog and requires immediate patching.

Affected Products

• Microsoft Windows 10 (versions 1507, 1607, 1809, 1909, 2004, 20H2, 21H1)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1 and Windows RT 8.1
• Microsoft Windows Server 2008 SP2, 2008 R2 SP1, 2012, 2012 R2
• Microsoft Windows Server 2016, 2019, 2022
• Microsoft Windows Server versions 2004 and 20H2

Discovery Timeline

• September 14, 2021 - Microsoft releases security updates to address the vulnerability
• September 15, 2021 - CVE-2021-40444 published to NVD
• October 30, 2025 - Last updated in NVD database

Technical Details for CVE-2021-40444

Vulnerability Analysis

This vulnerability resides in MSHTML (also known as Trident), the proprietary browser engine for Internet Explorer that is also used by Microsoft Office applications for rendering HTML content. The flaw enables attackers to execute arbitrary code by exploiting how Office documents process embedded ActiveX controls through the MSHTML component.

The attack chain involves crafting a malicious Office document (typically Word documents with .docx extension) that contains a specially-crafted ActiveX control. When the victim opens the document, the MSHTML engine processes the embedded content, leading to remote code execution. Microsoft Defender Antivirus and Microsoft Defender for Endpoint detect exploitation attempts, displaying alerts as "Suspicious Cpl File Execution."

Root Cause

The vulnerability is classified under CWE-22 (Path Traversal), indicating that improper neutralization of special elements in path names allows attackers to bypass security controls. The MSHTML engine fails to properly validate and sanitize input when processing ActiveX controls embedded in Office documents, allowing malicious payloads to escape the intended security boundaries and execute arbitrary code on the victim's system.

Attack Vector

The attack requires local access in the sense that the user must open a malicious file, though the file itself can be delivered remotely via email, compromised websites, or file-sharing services. The exploitation process follows these steps:

1. The attacker creates a malicious Office document containing a specially-crafted ActiveX control
2. The document is delivered to the victim through phishing emails, malicious websites, or other social engineering techniques
3. When the victim opens the document, Office invokes the MSHTML engine to render embedded HTML content
4. The malicious ActiveX control executes, allowing the attacker to run arbitrary code with the victim's privileges
5. If the user has administrative rights, the attacker gains full system control

The vulnerability does not require user authentication and exploits the trust relationship between Office applications and the MSHTML rendering engine.

Detection Methods for CVE-2021-40444

Indicators of Compromise

• Suspicious Office documents containing embedded ActiveX controls or OLE objects with unusual external references
• Process chains showing Office applications (WINWORD.EXE, EXCEL.EXE) spawning unexpected child processes
• Network connections from Office applications to unknown or suspicious external URLs
• Presence of .inf or .dll files in temporary directories following Office document access
• Microsoft Defender alerts for "Suspicious Cpl File Execution"

Detection Strategies

• Monitor for Office applications making unusual network connections or downloading executable content from external sources
• Implement behavioral detection rules for Office processes spawning command interpreters (cmd.exe, powershell.exe) or other executables
• Deploy Yara rules targeting malicious document structures with embedded ActiveX controls
• Enable Microsoft Defender for Endpoint with detection build 1.349.22.0 or newer for known exploit patterns

Monitoring Recommendations

• Configure endpoint detection and response (EDR) solutions to alert on suspicious Office document behavior
• Monitor Windows Event Logs for unusual MSHTML and ActiveX-related activities
• Implement network traffic analysis to detect command-and-control communications following document opening
• Review Microsoft Defender alerts and ensure antimalware definitions are current

How to Mitigate CVE-2021-40444

Immediate Actions Required

• Apply Microsoft security updates released on September 14, 2021, immediately for all affected Windows versions
• Ensure Microsoft Defender Antivirus definitions are updated to build 1.349.22.0 or newer
• Enable Protected View in Microsoft Office to prevent automatic execution of embedded content
• Disable ActiveX controls in Microsoft Office applications where not required for business operations
• Implement application control policies to restrict unauthorized code execution

Patch Information

Microsoft released security updates on September 14, 2021, to address this vulnerability. Organizations should consult the Microsoft Security Advisory CVE-2021-40444 for the specific update applicable to their Windows version. Automatic updates should apply the patches without additional action; enterprise environments managing updates manually should prioritize deployment immediately.

Workarounds

• Disable the installation of all ActiveX controls in Internet Explorer by modifying the registry to prevent MSHTML from instantiating ActiveX controls
• Configure Microsoft Office to open documents from the internet in Protected View by default
• Implement email gateway filtering to block Office documents with embedded OLE objects from external sources
• Use Microsoft Defender Application Guard to isolate potentially malicious documents
• Restrict user permissions to run with least privilege, reducing the impact of successful exploitation

# Registry modification to disable ActiveX controls in Internet Explorer
# Run these commands in an elevated command prompt to apply the workaround

reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\0" /v 1001 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\0" /v 1004 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\1" /v 1001 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\1" /v 1004 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\2" /v 1001 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\2" /v 1004 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3" /v 1001 /t REG_DWORD /d 3 /f
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3" /v 1004 /t REG_DWORD /d 3 /f