high 8.8

CVE-2021-40444 kev+ ransomware in the critical feed json

Microsoft MSHTML Remote Code Execution Vulnerability

published 2021-09-15 5 y ago modified 2026-08-10 2 mo ago also EUVD-2021-27621

cvss: 8.8; version 3.1, CNA
cisa kev: Exploited; added 2021-11-03, used in ransomware
epss: 97.8%; EPSS 97.8% chance of exploitation in 30 days, percentile 99.9%
public exploits: 1; Metasploit

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.

Patch / fix

cisa required action Apply updates per vendor instructions.

fixed in 25 distros 1 links 2

Microsoft Windows 10 Version 1507 10.0.10240.19060
Microsoft Windows 10 Version 1607 10.0.14393.4651
Microsoft Windows 10 Version 1809 10.0.17763.2183
Microsoft Windows 10 Version 1909 10.0.18363.1801
Microsoft Windows 10 Version 2004 10.0.19041.1237
Microsoft Windows 10 Version 20H2 10.0.19042.1237
Microsoft Windows 10 Version 21H1 10.0.19043.1237
Microsoft Windows 7 6.1.7601.25712
Microsoft Windows 7 Service Pack 1 6.1.7601.25712
Microsoft Windows 8.1 6.3.9600.20120
Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25712
Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25712
Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21218
Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21218
Microsoft Windows Server 2012 6.2.9200.23462
Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23462
Microsoft Windows Server 2012 R2 6.3.9600.20120
Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20120
Microsoft Windows Server 2016 10.0.14393.4651
Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4651
Microsoft Windows Server 2019 10.0.17763.2183
Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2183
Microsoft Windows Server 2022 10.0.20348.230
Microsoft Windows Server version 2004 10.0.19041.1237
Microsoft Windows Server version 20H2 10.0.19042.1237

python-oletools openSUSE Tumbleweed 0.60.2-2.4

opensuse advisories openSUSE-SU-2026:12022-1

Timeline

2021-09-08 CISA SSVC: exploitation active
2021-09-14 0-day patched (Project Zero)
2021-09-15 Published
2021-09-23 First public exploit (Metasploit exploit/windows/fileformat/word_mshtml_rce)
2021-11-03 Added to CISA KEV
2021-11-17 CISA patch deadline

Known exploited

Added: 5 y ago (2021-11-03)
Patch by: 2021-11-17 (4 y ago)
Ransomware: Known to be used in ransomware campaigns
Required action: Apply updates per vendor instructions.
Vendor, product: Microsoft, MSHTML
CISA name: Microsoft MSHTML Remote Code Execution Vulnerability

Zero-day

Detected in real attacks while still unknown to the vendor and the public, per Google Project Zero's 0day In-the-Wild tracker.

Patched: 2021-09-14
Details: project zero tracker

Ransomware

Groups reported to have exploited it in intrusions.

Conti ransomware gang

reported by microsoft.com

Ransomware Vulnerability Matrix by BushidoUK (github.com/BushidoUK/Ransomware-Vulnerability-Matrix), reformatted per CVE. CC BY 4.0

CISA SSVC

Exploitation: Active, exploited in the wild
Automatable: no
Technical impact: total
Assessed: 5 y ago (2021-09-08)

Exploit prediction

97.84% chance of exploitation in the next 30 days, higher than 99.9% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

Metasploit exploit/windows/fileformat/word_mshtml_rce Microsoft Office Word Malicious MSHTML RCE metasploit exploit, excellent rank, 2021-09-23
packetstormsecurity.com http://packetstormsecurity.com/files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html referenced PoC, tagged "exploit" by a source, unverified
packetstormsecurity.com http://packetstormsecurity.com/files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.8	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
CNA	3.1	8.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
CNA	3.1	7.8	CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
NVD	2.0	6.8	AV:N/AC:M/Au:N/C:P/I:P/A:P

Affected products

Microsoft Windows 10 Version 1507
Microsoft Windows 10 Version 1607
Microsoft Windows 10 Version 1809
Microsoft Windows 10 Version 1909
Microsoft Windows 10 Version 2004
Microsoft Windows 10 Version 20H2
Microsoft Windows 10 Version 21H1
Microsoft Windows 7
Microsoft Windows 7 Service Pack 1
Microsoft Windows 8.1
Microsoft Windows Server 2008 R2 Service Pack 1
Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation)
Microsoft Windows Server 2008 Service Pack 2
Microsoft Windows Server 2008 Service Pack 2 (Server Core installation)
Microsoft Windows Server 2012
Microsoft Windows Server 2012 (Server Core installation)
Microsoft Windows Server 2012 R2
Microsoft Windows Server 2012 R2 (Server Core installation)
Microsoft Windows Server 2016
Microsoft Windows Server 2016 (Server Core installation)
Microsoft Windows Server 2019
Microsoft Windows Server 2019 (Server Core installation)
Microsoft Windows Server 2022
Microsoft Windows Server version 2004
Microsoft Windows Server version 20H2

Classification

Weakness

CWE-22 Improper Limitation of a Pathname to a Restricted Directory

References

msrc.microsoft.com /update-guide/vulnerability/CVE-2021-40444
vendor-advisory, patch
portal.msrc.microsoft.com /en-US/security-guidance/advisory/CVE-2021-40444
mitigation, patch, vendor-advisory
packetstormsecurity.com /files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html
exploit, third-party-advisory, vdb-entry
packetstormsecurity.com /files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html
exploit, third-party-advisory, vdb-entry
packetstormsecurity.com /files/164210/Microsoft-Windows-MSHTML-Overview.html
third-party-advisory, vdb-entry
cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40444
government-resource, us-government-resource

Sources

CVE List CVE-2021-40444
NVD CVE-2021-40444

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary