CVEs / CVE-2021-40444

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

• Known exploited in the CISA KEV catalog since 3 Nov 2021
• medium 6.8 CVSS 2.0 (NVD)
• EPSS 97.8% chance of exploitation in the next 30 days, higher than 99% of CVEs (30 Oct 2026)
• Last changed at its sources 10 Aug 2026

Summary

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

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.

Affected and fixed versions

Exploitation

Severity

References

• CVE-2021-40444 on NVD
• CVE-2021-40444 in the CISA KEV catalog
• 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