Known exploited vulnerability · CISA KEV

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

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

CVE-2021-40444

Exploited in the wild. Fix this first.

CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03: attackers are using it right now. It is known to be used in ransomware campaigns.

CISA KEV: Listed; Added 2021-11-03 · federal fix-by 2021-11-17
EPSS (30 days): 98%; Higher than 99.9% of CVEs · 2026-11-01
CVSS v3.1: 8.8 High; CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L

What CISA says

Microsoft MSHTML Remote Code Execution Vulnerability

Apply updates per vendor instructions.

NVD description · published 2021-09-15

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 ac

Full record on NVD

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How to check if you are exposed to CVE-2021-40444

Find every Microsoft MSHTML system you or your clients run, including ones exposed to the internet. Those come first: CISA only lists flaws attackers are already using.
Compare each version with the fixed versions in the vendor advisory. Where no fix exists yet, apply the vendor’s mitigation or take the system off the internet.
Run a credentialed vulnerability scan to catch the systems you missed, then check that the next scan no longer reports CVE-2021-40444.

US federal agencies had to fix this by 2021-11-17. Everyone else should treat that deadline as a guide: if it has passed, this is overdue.

Affected versions

Versions the National Vulnerability Database lists as vulnerable to CVE-2021-40444. Check the vendor advisory for the exact fixed builds before you sign anything off.

Product	Vulnerable versions
Microsoft Windows 10 1507	before 10.0.10240.19060
Microsoft Windows 10 1607	before 10.0.14393.4651
Microsoft Windows 10 1809	before 10.0.17763.2183
Microsoft Windows 10 1909	before 10.0.18363.1801
Microsoft Windows 10 2004	before 10.0.19041.1237
Microsoft Windows 10 20h2	before 10.0.19042.1237

NVD lists 11 more affected products; see the full record on NVD.

Timeline

2021-09-15 · Published in the National Vulnerability Database
2021-11-03 · Added to CISA's Known Exploited Vulnerabilities catalog, 49 days after it was published
2021-11-17 · Fix-by date for US federal agencies, passed 1,810 days ago

Weakness type

CWE-22

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Sources: CISA Known Exploited Vulnerabilities catalog (version 2026.10.30); EPSS data from FIRST.org; CVSS and descriptions from the NIST National Vulnerability Database. Checked 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.