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

CISA Known Exploited Vulnerabilities

Name

Microsoft MSHTML Remote Code Execution Vulnerability

Vendor

Microsoft

Product

MSHTML

Added to the catalog

2021-11-03

Due date for US federal agencies

2021-11-17

Weaknesses

CWE-22

Catalog version

2026.10.30

Microsoft MSHTML contains a unspecified vulnerability that allows for remote code execution.

Required action (CISA)

Apply updates per vendor instructions.

Ransomware use: CISA marks this vulnerability as known to be used in ransomware campaigns. CISA does not say which group used it.

Links in CISA's notes

• https://nvd.nist.gov/vuln/detail/CVE-2021-40444

Read on 2026-11-02T23:39:54.475Z under licence cisa-kev-cc0@b0067899 from https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json

NVD analysis

Status

Analyzed

Published

2021-09-15T12:15:16.467

Last modified

2026-08-10T20:19:30.393

Weaknesses

CWE-22

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.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444 (Patch, Vendor Advisory)
• http://packetstormsecurity.com/files/164210/Microsoft-Windows-MSHTML-Overview.html (Third Party Advisory, VDB Entry)
• http://packetstormsecurity.com/files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html (Exploit, Third Party Advisory, VDB Entry)
• http://packetstormsecurity.com/files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html (Exploit, Third Party Advisory, VDB Entry)
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40444 (Mitigation, Patch, Vendor Advisory)
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40444 (US Government Resource)

This product uses the NVD API but is not endorsed or certified by the NVD.

Read on 2026-11-02T23:41:59.479Z under licence nvd-api-terms-of-use@ff2a5288 from https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2021-40444

CVE record

Title (CNA)

Microsoft MSHTML Remote Code Execution Vulnerability

Assigner

microsoft

State

PUBLISHED

Published

2021-09-15T11:24:26.000Z

Updated

2026-08-10T15:10:57.757Z

Problem types

Remote Code Execution

References

• Microsoft MSHTML Remote Code Execution Vulnerability

Read on 2026-11-03T01:08:53.819Z under licence cve-terms-of-use@81bfa2f3 from https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2021/40xxx/CVE-2021-40444.json

Detection rules (SigmaHQ)

SigmaHQ rules whose own tags name this CVE. Each rule page shows the rule as its author published it.

• Potential Exploitation Attempt From Office Application (status test, level high)
• Potential CVE-2021-40444 Exploitation Attempt (status test, level high)

Defensive techniques (MITRE D3FEND)

D3FEND maps these defensive techniques to the weakness (CWE) that NVD lists for this CVE, through the digital artifacts the weakness affects. D3FEND does not say they stop this vulnerability.

MITRE D3FEND™ knowledge graph. © 2025 The MITRE Corporation.
"The MITRE Corporation (MITRE) hereby grants you a non-exclusive, royalty-free license to use D3FEND for research, development, and commercial purposes. Any copy you make for such purposes is authorized provided that you reproduce MITRE's copyright designation and this license in any such copy." Terms: https://d3fend.mitre.org/tou/