Microsoft Windows MSHTML Remote Code Execution Vulnerability (CVE-2021-40444) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTSystems running Microsoft Windows OS utilizing the MSHTML (Internet Explorer) rendering engine and Microsoft Office suites
Successful exploitation allows remote attackers to execute arbitrary code (RCE) with the privileges of the active user by tricking them into opening a specially crafted Office document
Apply the official Microsoft security updates immediately, or implement registry workarounds to disable ActiveX controls as a temporary mitigation
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Apply the official cumulative security updates provided by Microsoft. You can force Windows Update installation using PowerShell:
# Install PSWindowsUpdate module and deploy latest security patches
Install-Module PSWindowsUpdate -Force -Verbose
Get-WindowsUpdate -AcceptAll -Install -AutoReboot2. Temporary Workaround & Hardening Configuration
If patching cannot be deployed immediately, disable the execution of all ActiveX controls in Internet Explorer via the registry to mitigate the attack vector. Run the following commands in an elevated Command Prompt:
# Disable ActiveX controls across all security zones in Internet Explorer
reg add 'HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\0' /v '1201' /t REG_DWORD /d 3 /f
reg add 'HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\1' /v '1201' /t REG_DWORD /d 3 /f
reg add 'HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\2' /v '1201' /t REG_DWORD /d 3 /f
reg add 'HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3' /v '1201' /t REG_DWORD /d 3 /f3. Log Audit & IoC Verification Commands
Search for indicators of compromise (IoCs), such as suspicious rundll32.exe executions loading .cpl files, using the following PowerShell script:
# Query Sysmon logs for suspicious rundll32 execution involving CPL files
Get-WinEvent -FilterHashtable @{
LogName='Microsoft-Windows-Sysmon/Operational';
Id=1
} -ErrorAction SilentlyContinue | Where-Object {
$_.Message -like '*rundll32.exe*' -and $_.Message -like '*.cpl*'
} | Select-Object TimeCreated, Message | Format-ListBlock Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability is a remote code execution (RCE) flaw in the MSHTML (Internet Explorer) rendering engine used by Microsoft Windows and Office applications. The root cause lies in a Path Traversal vulnerability (CWE-22) during the extraction of cabinet (.cab) files initiated by MSHTML. When processing a specially crafted document, the engine fails to properly sanitize file paths, allowing an attacker to escape the designated temporary directory and write malicious payloads to arbitrary locations on the local filesystem.
2. Exploitation Mechanics & Attack Scenario
An attacker crafts a malicious Microsoft Office document (such as Word or Excel) that embeds an ActiveX control pointing to an external malicious URL. When a user opens the document, MSHTML retrieves the remote HTML content, which triggers the download of a malicious cabinet (.cab) file. Due to the path traversal flaw, MSHTML extracts a malicious Control Panel (.cpl) file into a predictable directory. The exploit then leverages shell execution mechanisms (e.g., via rundll32.exe) to execute the dropped .cpl file, granting the attacker arbitrary code execution under the context of the current user.
3. Infrastructure & Business Impact
This vulnerability has been actively exploited in the wild (listed in CISA KEV) and carries an extremely high EPSS score of 0.9784, indicating imminent threat potential. If exploited, it can lead to initial access within an enterprise network, lateral movement, and full system compromise. Organizations where users operate with administrative privileges face severe risks, including ransomware deployment and intellectual property theft.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2021-40444)
FAQQ1. How critical is CVE-2021-40444 and is it actively exploited in the wild?
CVE-2021-40444 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 97.8%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-40444?
Primary impacted vendors include microsoft across windows, mshtml, office. Scope summary: All supported Windows operating systems (Windows 7/8.1/10/11, Windows Server 2008/2012/2016/2019/2022) and Microsoft Office applications utilizing MSHTML
Q3. What temporary mitigations or workarounds exist for CVE-2021-40444 before patching?
[Mitigation] Apply the official Microsoft security updates immediately, or implement registry workarounds to disable ActiveX controls as a temporary mitigation Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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