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CVE-2021-40444Microsoft MSHTML Remote Code Execution Vulnerability

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.

Exploited in the wild

CISA lists this vulnerability in its Known Exploited Vulnerabilities catalog as Microsoft MSHTML Remote Code Execution Vulnerability.

Added
3 Nov 2021
Fix due (US federal)
17 Nov 2021
Used by ransomware
Known

Apply updates per vendor instructions.

ENISA's EU Vulnerability Database lists it as exploited, from CISA's KEV, since 3 Nov 2021.

Public exploit

An exploit or proof of concept is public: anyone can try it against an unpatched system. The public collections list a Metasploit module for it. CISA's SSVC rates its exploitation: it is being exploited.

Show the exploits1

Metasploit modules1 module

Modules, templates and exploits as listed by the Metasploit Framework (BSD-3-Clause), ProjectDiscovery's Nuclei templates (MIT) and the Exploit Database (GPLv2), read once a day. Where the data comes from

Timeline

  1. ID reserved

    By microsoft

  2. · 13 days later

    Published

  3. · 49 days later

    Added to CISA KEV

    Known exploited, also by ransomware

  4. · 5 years later

    Record updated

Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.

Scores

CVSS scores of CVE-2021-40444, by who scored it
Scored byCVSSScoreVector
microsoftCNA (assigner)3.1High8.8CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C
NVDNVD · Analyzed3.1High8.8CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
EPSS (exploitation in 30 days)
97.8% · top 0.1% of all CVEs (13 Nov 2026)
CISA SSVC
Exploitation: active · Automatable: no · Technical impact: total
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 8.8.

Affected products

References

References6

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .