CVE-2021-40444: Microsoft MSHTML Remote Code Execution Vulnerability

High severity, CVSS 8.8. Actively exploited: in CISA KEV since 2021-11-03. EPSS: 97.5% chance of exploitation in the next 30 days.

Microsoft is investigating reports of a remote code execution vulnerability in MSHTML that affects Microsoft Windows. Microsoft is aware of targeted attacks that attempt to exploit this vulnerability by using specially-crafted Microsoft Office documents. An attacker could craft a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine. The attacker would then have to convince the user to open the malicious document. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights. 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 utilize automatic updates do not need to take additional action. 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”. Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. UPDATE September 14, 2021: Microsoft has released security updates to address this vulnerability. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. Please see the FAQ for important information about which updates are applicable to your system.

Affected products

  • Microsoft Windows 10 1507: before 10.0.10240.19060 (fixed in 10.0.10240.19060)
  • Microsoft Windows 10 1607: before 10.0.14393.4651 (fixed in 10.0.14393.4651)
  • Microsoft Windows 10 1809: before 10.0.17763.2183 (fixed in 10.0.17763.2183)
  • Microsoft Windows 10 1909: before 10.0.18363.1801 (fixed in 10.0.18363.1801)
  • Microsoft Windows 10 2004: before 10.0.19041.1237 (fixed in 10.0.19041.1237)
  • Microsoft Windows 10 20h2: before 10.0.19042.1237 (fixed in 10.0.19042.1237)
  • Microsoft Windows 10 21h1: before 10.0.19043.1237 (fixed in 10.0.19043.1237)
  • Microsoft Windows 7: affected versions not specified
  • Microsoft Windows 8.1: affected versions not specified
  • Microsoft Windows Rt 8.1: affected versions not specified
  • Microsoft Windows Server 2004: before 10.0.19041.1237 (fixed in 10.0.19041.1237)
  • Microsoft Windows Server 2008: affected versions not specified; version r2 only
  • Microsoft Windows Server 2012: affected versions not specified
  • Microsoft Windows Server 2016: before 10.0.14393.4651 (fixed in 10.0.14393.4651)
  • Microsoft Windows Server 2019: before 10.0.17763.2183 (fixed in 10.0.17763.2183)
  • Microsoft Windows Server 2022: before 10.0.20348.230 (fixed in 10.0.20348.230)
  • Microsoft Windows Server 20h2: before 10.0.19042.1237 (fixed in 10.0.19042.1237)

Published 2021-09-15. Last modified 2026-08-10.