# What is CVE-2021-40444? > CVE-2021-40444 is a high-severity flaw (CVSS 8.8) in Windows 10 1507 from Microsoft, published on 15 September 2021. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at 98%. If you run an affected version, patch it first. Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-40444 ## At a glance - **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. - **Precursor Intelligence risk score:** 79 out of 100 (High); action: Emergency patch. Kept up to date as the evidence changes. - **Severity:** High (CVSS 8.8 out of 10) - **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited - **Chance of use in the next 30 days:** 98% (latest exploit-likelihood forecast) - **Published:** 15 September 2021 - **Record last updated:** 4 April 2025 - **Main product:** Windows 10 1507 from Microsoft - **Affected products:** 17 ## What is CVE-2021-40444? In plain terms It can be attacked from anywhere on the internet without logging in, but a person has to take an action first. If it works, the attacker can change data. ### The official 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. ### What kind of flaw is it? - CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') ### How an attack on it works - Where an attacker can be: Anywhere on the internet - How hard it is to pull off: Easy: no special conditions - Login needed first: No login needed - Does someone have to click: Yes, a person has to take an action - What an attacker gets: Can change data Severity vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L` ## Is CVE-2021-40444 being exploited? Yes Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 98%. Exploit likelihood over the last 7 days: 97%, 97%, 97%, 98%, 98%, 98%, 98%. ### Timeline - 15 September 2021: Published - 4 April 2025: Record last updated ### How Precursor Intelligence ranks it CVE-2021-40444 scores **79 out of 100 (High)**, with the action "Emergency patch". Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less: - Evidence it is being exploited: biggest factor - How severe it is: strong factor - How widely exposed it is: smaller factor - Threat activity around it: smaller factor 85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it. ## Who is affected by CVE-2021-40444? The published record lists 17 products. Versions are as published: "before 10.0.10240.19060" means every version up to, but not including, 10.0.10240.19060. - [Windows 10 1507](https://www.precursorintelligence.com/product/microsoft/windows_10_1507) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.10240.19060 - [Windows 10 1607](https://www.precursorintelligence.com/product/microsoft/windows_10_1607) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.14393.4651 - [Windows 10 1809](https://www.precursorintelligence.com/product/microsoft/windows_10_1809) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.17763.2183 - [Windows 10 1909](https://www.precursorintelligence.com/product/microsoft/windows_10_1909) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.18363.1801 - [Windows 10 2004](https://www.precursorintelligence.com/product/microsoft/windows_10_2004) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19041.1237 - [Windows 10 20H2](https://www.precursorintelligence.com/product/microsoft/windows_10_20h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19042.1237 - [Windows 10 21H1](https://www.precursorintelligence.com/product/microsoft/windows_10_21h1) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19043.1237 - [Windows 7](https://www.precursorintelligence.com/product/microsoft/windows_7) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 8.1](https://www.precursorintelligence.com/product/microsoft/windows_8.1) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows Rt 8.1](https://www.precursorintelligence.com/product/microsoft/windows_rt_8.1) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows Server 2004](https://www.precursorintelligence.com/product/microsoft/windows_server_2004) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19041.1237 - [Windows Server 2008](https://www.precursorintelligence.com/product/microsoft/windows_server_2008) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): r2-sp1 - [Windows Server 2012](https://www.precursorintelligence.com/product/microsoft/windows_server_2012) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows Server 2016](https://www.precursorintelligence.com/product/microsoft/windows_server_2016) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.14393.4651 - [Windows Server 2019](https://www.precursorintelligence.com/product/microsoft/windows_server_2019) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.17763.2183 - [Windows Server 2022](https://www.precursorintelligence.com/product/microsoft/windows_server_2022) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.20348.230 - [Windows Server 20H2](https://www.precursorintelligence.com/product/microsoft/windows_server_20h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19042.1237 ## How do I fix CVE-2021-40444? 1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list. 2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisory first in the list. 3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in. 4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance. ### References - [portal.msrc.microsoft.com](https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40444) (Mitigation, Patch, Vendor advisory) - [packetstormsecurity.com](http://packetstormsecurity.com/files/164210/Microsoft-Windows-MSHTML-Overview.html) (Third-party advisory, Vulnerability database) - [packetstormsecurity.com](http://packetstormsecurity.com/files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html) (Exploit, Third-party advisory, Vulnerability database) - [packetstormsecurity.com](http://packetstormsecurity.com/files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html) (Exploit, Third-party advisory, Vulnerability database) ## Questions ### What is CVE-2021-40444? CVE-2021-40444 is a high-severity flaw in Windows 10 1507 from Microsoft, published on 15 September 2021. It can be attacked from anywhere on the internet without logging in, but a person has to take an action first. If it works, the attacker can change data. ### How serious is CVE-2021-40444? It scores 8.8 out of 10 on the CVSS severity scale, which is high. Our risk score for CVE-2021-40444 is 79 out of 100, which is High. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. ### Is CVE-2021-40444 being exploited? Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 98%. ### Which versions does CVE-2021-40444 affect? For Windows 10 1507 from Microsoft: before 10.0.10240.19060. The record lists 17 products in all, each with its versions, under "Who is affected" on this page. ### How do I fix CVE-2021-40444? Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in. ## Does CVE-2021-40444 affect you? Precursor Intelligence keeps checking for you: - **Tell us what you run.** Pick your software from a list, or let us work it out from your website. - **We match it to every flaw.** Over 400,000 published flaws, checked against your list. - **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it. - **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools. 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