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.
The record classifies CVE-2021-40444 under weakness class CWE-22. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs a user to take an action first, and has high impact on integrity. 19 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 1846 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 8.8 — HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L - EPSS (FIRST)
- 94.3% probability of exploitation in the next 30 days, higher than 99.9% of all scored CVEs
- CISA KEV
- Listed since 2021-11-03, federal remediation deadline 2021-11-17 — used in known ransomware campaigns
- Threadlinqs priority
- 10/10 — CISA lists it as used in ransomware, which Threadlinqs scores at the maximum
- Published
- 2021-09-15, last modified 2025-10-30
Is CVE-2021-40444 being exploited?
CISA added CVE-2021-40444 to the Known Exploited Vulnerabilities catalog on 2021-11-03, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2021-11-17 under BOD 22-01. CISA flags the vulnerability as one used in known ransomware campaigns. It currently carries a trending score of 45 in the Threadlinqs vulnerability feed.
Affected products and versions
- Microsoft: Windows 10 1507, Windows 10 1607, Windows 10 1809, Windows 10 1909, Windows 10 2004, Windows 10 20h2, Windows 10 21h1, Windows 7, Windows 8.1, Windows Rt 8.1, Windows Server 2004, Windows Server 2008, Windows Server 2008 r2, Windows Server 2012, Windows Server 2016, Windows Server 2019, Windows Server 2022, Windows Server 20h2
Showing 18 of 19 recorded product entries.
How to fix CVE-2021-40444
The record marks a vendor fix as available for CVE-2021-40444. Patch reference: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444. Vendor advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40444. Because CVE-2021-40444 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2021-11-17. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2021-40444
No threat campaign in the Threadlinqs corpus currently references CVE-2021-40444, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.
Sources
Seeded from nvd and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.