CVE-2021-40444
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMicrosoft 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThis is a remote code execution vulnerability in Microsoft MSHTML (the HTML rendering engine used by Internet Explorer and embedded in Microsoft Office). Attackers craft malicious Office documents containing specially-designed ActiveX controls that, when opened by the victim, trigger the vulnerability and allow arbitrary code execution on the targeted system.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 10.0.10240.19060< 10.0.14393.4651< 10.0.17763.2183< 10.0.18363.1801< 10.0.19041.1237< 10.0.19042.1237< 10.0.19043.1237all versionsCVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- High
- Availability
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Windows 10 version build numberOpen Command Prompt or PowerShell and run 'winver' or 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to obtain the exact build number (e.g., 10.0.19043.xxxx)Affected if The build number is less than 10.0.10240.19060 (1507), less than 10.0.14393.4651 (1607), less than 10.0.17763.2183 (1809), less than 10.0.18363.1801 (1909), less than 10.0.19041.1237 (2004), less than 10.0.19042.1237 (20h2), or less than 10.0.19043.1237 (21h1)
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Check if running Windows 7Run 'winver' or check System Properties to confirm the Windows editionAffected if The system is running any version of Windows 7 (all versions are affected)
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Verify September 2021 security update is installedOpen Control Panel > Programs and Features > View installed updates, or run 'wmic qfe list' in Command Prompt, and look for KB5005565 or subsequent MSHTML-related security updates released September 2021 or laterAffected if The September 2021 cumulative security update for MSHTML (or subsequent relevant patch) is not installed
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Confirm MSHTML engine versionNavigate to C:\Windows\System32\ and locate mshtml.dll, then right-click > Properties > Details to view the File VersionAffected if The mshtml.dll file version is older than the version included in the September 2021 patches (exact version varies by Windows build)
The system is affected if it runs Windows 7 (any version) or Windows 10 with a build number below the thresholds listed, and the September 2021 Microsoft security updates for MSHTML are not installed.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped10.0.10240.1906010.0.14393.465110.0.17763.2183
Install the September 2021 Microsoft security updates for MSHTML/Windows and ensure Microsoft Defender antivirus definitions are updated to detection build 1.349.22.0 or newer. Users should avoid opening untrusted Office documents from unknown sources.
September 2021 Security Updates (KB5005565 and related patches for each Windows version)
- Identify current Windows version by running 'winver' or checking System Properties
- Determine the installed build number (e.g., 10.0.19041.1237)
- For Windows 10 1507: Upgrade to build 10.0.10240.19060 or later
- For Windows 10 1607: Upgrade to build 10.0.14393.4651 or later
- For Windows 10 1809: Upgrade to build 10.0.17763.2183 or later
- For Windows 10 1909: Upgrade to build 10.0.18363.1801 or later
- For Windows 10 2004: Upgrade to build 10.0.19041.1237 or later
- For Windows 10 20h2: Upgrade to build 10.0.19042.1237 or later
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,120.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-40444 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-40444 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data