CVE-2021-40444 | Microsoft MSHTML Remote Code Execution Vulnerability

Exp

<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>

Published: 2021-09-15 Last update: 2025-10-30 Assigner: [email protected] Source: [email protected]

Conclusion & alert: CVE-2021-40444 is rated Critical Active Threat (95.1/100): CVSS High severity, with high exploitation likelihood (EPSS 94.33%, 100th percentile). Core evidence: CISA KEV confirms active exploitation (added 2021-11-03) affecting Microsoft / MSHTML. a weakness (CWE-22) Unauthenticated remote administrative access may be possible. Mandatory action: The CISA remediation deadline has passed—treat as an emergency patch priority.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

CISA KEV Record for CVE-2021-40444

Name: Microsoft MSHTML Remote Code Execution Vulnerability · CISA KEV detail

Exploit added: 2021-11-03

Action due: 2021-11-17

Required action: Apply updates per vendor instructions.

Public exploit references (Exploit-DB) for CVE-2021-40444

EDB-ID Source Kind Published Link
nvd_ref exploit_tag Exploit-DB ↗
nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-40444

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-03-23 94.38% 94.33% -0.05%
2 2026-03-21 94.33% 94.38% +0.05%
3 2025-11-21 94.33%

Full EPSS history (36 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-40444

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
8.8 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:L)
Might cause slowdowns, glitches, or partial disruption—not a full brick.
2.8 5.3 [email protected]
7.8 3.1 HIGH
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:L)
They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
1.8 5.9 [email protected]
6.8 2.0 MEDIUM
AV:N/AC:M/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
8.6 6.4 [email protected]

Weakness enumeration for CVE-2021-40444

Affected software / configurations for CVE-2021-40444

Vendor Product Version Raw CPE
microsoft windows_10_1507 < 10.0.10240.19060 cpe:2.3:o:microsoft:windows_10_1507:*:*:*:*:*:*:*:*
microsoft windows_10_1607 < 10.0.14393.4651 cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:*:*
microsoft windows_10_1809 < 10.0.17763.2183 cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:*:*
microsoft windows_10_1909 < 10.0.18363.1801 cpe:2.3:o:microsoft:windows_10_1909:*:*:*:*:*:*:*:*
microsoft windows_10_2004 < 10.0.19041.1237 cpe:2.3:o:microsoft:windows_10_2004:*:*:*:*:*:*:*:*
microsoft windows_10_20h2 < 10.0.19042.1237 cpe:2.3:o:microsoft:windows_10_20h2:*:*:*:*:*:*:*:*
microsoft windows_10_21h1 < 10.0.19043.1237 cpe:2.3:o:microsoft:windows_10_21h1:*:*:*:*:*:*:*:*
microsoft windows_7 cpe:2.3:o:microsoft:windows_7:-:sp1:*:*:*:*:*:*
microsoft windows_8.1 cpe:2.3:o:microsoft:windows_8.1:-:*:*:*:*:*:*:*
microsoft windows_rt_8.1 cpe:2.3:o:microsoft:windows_rt_8.1:-:*:*:*:*:*:*:*
microsoft windows_server_2004 < 10.0.19041.1237 cpe:2.3:o:microsoft:windows_server_2004:*:*:*:*:*:*:*:*
microsoft windows_server_2008 cpe:2.3:o:microsoft:windows_server_2008:-:sp2:*:*:*:*:*:*
microsoft windows_server_2008 r2 cpe:2.3:o:microsoft:windows_server_2008:r2:sp1:*:*:*:*:x64:*
microsoft windows_server_2012 cpe:2.3:o:microsoft:windows_server_2012:-:*:*:*:*:*:*:*
microsoft windows_server_2012 cpe:2.3:o:microsoft:windows_server_2012:-:r2:*:*:*:*:*:*
microsoft windows_server_2016 < 10.0.14393.4651 cpe:2.3:o:microsoft:windows_server_2016:*:*:*:*:*:*:*:*
microsoft windows_server_2019 < 10.0.17763.2183 cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:*:*:*:*
microsoft windows_server_2022 < 10.0.20348.230 cpe:2.3:o:microsoft:windows_server_2022:*:*:*:*:*:*:*:*
microsoft windows_server_20h2 < 10.0.19042.1237 cpe:2.3:o:microsoft:windows_server_20h2:*:*:*:*:*:*:*:*

References for CVE-2021-40444

cvelogic Threat Intelligence