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ExploitedIn CISA KEVFix available

CVE-2021-40444

Exploitation confirmed. First attested on 2021-11-03 by CISA-ADP, CISA KEV, GCVE · kevintel.

  • Published2021-09-15
  • WeaknessCWE-22

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2021-09-1512:15CVE publishedNVD
  2. 2021-11-03Added to the CISA KEV catalogCISA KEV
  3. 2021-11-03Listed as exploited by GCVEGCVE
  4. 2021-11-03Exploitation first reportedCISA KEV
  5. 2025-12-1617:57SSVC exploitation set to activeCISA-ADP

Observed activity

isMalicious corpus · updated 2026-11-03 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-40444?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-40444?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Microsoft windows server 2008
BranchAffectedFixed in
—all versionsno fix published
—= r2no fix published
Microsoft windows 10 1507
BranchAffectedFixed in
10.0< 10.0.10240.1906010.0.10240.19060
Microsoft windows 10 1607
BranchAffectedFixed in
10.0< 10.0.14393.465110.0.14393.4651

14 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Products declared by the CNA26 products

As microsoft, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • Microsoft Windows 10 Version 1507

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 10.0.10240.0, < 10.0.10240.19060fixed in 10.0.10240.19060custom
  • Microsoft Windows 10 Version 1607

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 10.0.14393.0, < 10.0.14393.4651fixed in 10.0.14393.4651custom
  • Microsoft Windows 10 Version 1809

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 10.0.17763.0, < 10.0.17763.2183fixed in 10.0.17763.2183custom
  • Microsoft Windows 10 Version 1809

    platforms: ARM64-based Systems

    Affected
    • ≥ 10.0.0, < 10.0.17763.2183fixed in 10.0.17763.2183custom
  • Microsoft Windows 10 Version 1909

    platforms: 32-bit Systems, ARM64-based Systems, x64-based Systems

    Affected
    • ≥ 10.0.0, < 10.0.18363.1801fixed in 10.0.18363.1801custom
  • Microsoft Windows 10 Version 2004

    platforms: 32-bit Systems, ARM64-based Systems, x64-based Systems

    Affected
    • ≥ 10.0.0, < 10.0.19041.1237fixed in 10.0.19041.1237custom

20 more products in the CVE record.

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-08-10

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • python-oletoolsopenSUSE:Tumbleweedall versions · fixed in 0.60.2-2.4

Full record on NVD

Fix

  1. CISA’s required action: Apply updates per vendor instructions.due 2021-11-17

Advisories and patches

Detection

read 2026-11-03 22:42 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
20 repositories, unverifiedozergoker/CVE-2021-40444DarkSprings/CVE-2021-40444rfcxv/CVE-2021-40444-POCbambooqj/CVE-2021-40444_EXP_JSImmersive-Labs-Sec/cve-2021-40444-analysis
PoC-in-GitHub
Metasploit modules
1 moduleexploit/windows/fileformat/word_mshtml_rcerank: excellentModule metadata: Metasploit Framework, © Rapid7, Inc., BSD-3-Clause.
Metasploit Framework

CISA Known Exploited Vulnerability

Date Added
2021-11-03
Patch Due Date
2021-11-17
Ransomware Use
Known
Required Action
Apply updates per vendor instructions.

Sources

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published
2021-09-15
Last Modified
2026-08-10
NVD status
Analyzed
GitHub Advisory
GHSA-2h32-fhf6-xhh5

Frequently asked questions

What is CVE-2021-40444?

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.

Is CVE-2021-40444 actively exploited?

Yes. CVE-2021-40444 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, meaning it has been confirmed as actively exploited in the wild. CISA requires federal agencies to patch by 2021-11-17.

What is the CVSS score for CVE-2021-40444?

CVE-2021-40444 has a CVSS v3 base score of 7.8 (HIGH severity), with vector string CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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