CVE-2021-40444

HIGH
2021-09-15 [email protected]
8.8
CVSS 3.1
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CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
High
Availability
Low

Lifecycle Timeline

5
Analysis Generated
Mar 26, 2026 - 11:19 vuln.today
Added to CISA KEV
Oct 30, 2025 - 19:16 cisa
CISA KEV
PoC Detected
Oct 30, 2025 - 19:16 vuln.today
Public exploit code
Patch Released
Oct 30, 2025 - 19:16 nvd
Patch available
CVE Published
Sep 15, 2021 - 12:15 nvd
HIGH 8.8

Description

<p>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.</p> <p>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.</p> <p>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”.</p> <p>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.</p> <p>Please see the <strong>Mitigations</strong> and <strong>Workaround</strong> sections for important information about steps you can take to protect your system from this vulnerability.</p> <p><strong>UPDATE</strong> 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.</p>

Analysis

Windows MSHTML component contains a remote code execution vulnerability that allows attackers to craft malicious ActiveX controls in Office documents, exploited as a zero-day in targeted attacks before the September 2021 patch.

Technical Context

The CWE-22 path traversal in MSHTML allows crafted ActiveX controls in Office documents to download and execute cabinet (CAB) files from remote servers. The exploit uses a specially crafted .docx file that loads an MHTML URL containing a malicious ActiveX control, which downloads a CAB file and extracts a DLL for execution.

Affected Products

['Microsoft Windows (all supported versions with MSHTML)', 'Microsoft Office (document delivery vector)']

Remediation

Apply Microsoft security update. Disable ActiveX controls in Office via Group Policy. Block MHTML handling in Office documents. Disable the IE rendering engine's ability to process external content from Office.

Priority Score

218
Low Medium High Critical
KEV: +50
EPSS: +94.3
CVSS: +44
POC: +20

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CVE-2021-40444 vulnerability details – vuln.today

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