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Vulnerability Assessment

Weakness
CWE
OWASP Top 10:2025
A01:2021 - Broken Access Control
ASVS 5.0
V4 - Access Control Authorization enforcement, RBAC validation
MITRE ATT&CK

2 techniques across 2 tactics: Discovery, Collection

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

Impact

High risk (EPSS 94.3%). The exploit works in the Office preview pane (no document opening required in some configurations). Widely adopted by APT groups and ransomware operators after zero-day exploitation was disclosed.

Exploit ScenarioDerived

A phishing email delivers a .docx file. When opened (or previewed in Explorer), the embedded MHTML reference loads the MSHTML engine, which processes the malicious ActiveX control. The control downloads a CAB file from the attacker's server, extracts and executes a DLL, providing a backdoor on the victim's system.

Act Now Prioritize within 24-48h. Why: In CISA KEV with patch available.
Exploited in the wild (CISA KEV) Patch available 94.3% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

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.

Earlier versions of this analysis (1)
v1 Mar 26, 2026 - 11:19 CVSS 8.8

A remote code execution vulnerability in MSHTML (the IE rendering engine) allows attackers to craft malicious ActiveX controls embedded in Office documents for code execution. Exploited as a zero-day in August-September 2021.

RemediationAI

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.

Affects your stack

Find out whether the stacks and servers you run are on an affected version.

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

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.

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  • Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

Vendor (microsoft) PRIMARY 8.8 HIGH
AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L

Primary rating from Vendor (microsoft) · only source for this CVE.

CVSS VectorVendor: microsoft

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
High
Availability
Low

Lifecycle Timeline

4
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

DescriptionCVE.org

<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>

Executive ViewAI

Why It Matters

CVE-2021-40444 presents significant security risk, a path traversal vulnerability rated CVSS 8.8. This could allow attackers to access or modify files outside intended directories, potentially exposing sensitive configuration or credentials. This vulnerability is actively exploited in the wild (KEV-listed) and requires immediate remediation. A patch is available and should be applied as part of regular vulnerability management.

Business Impact: Severe
Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Restrict file upload functionality via WAF; (2) Implement allowlist-based file type validation; (3) Use chroot/containerization to limit filesystem access.

Threat Intelligence Overview

Threat Score
9.6/10
Composite threat intelligence
EPSS Score
94.3%
Top 0% exploitable
Threat Intel
2
1 sources
Exploit
POC
Public exploit code exists
Data Quality
6 sources
Confidence: High

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV Yes
Public POC Yes
EPSS 94.3%
Patch available Yes
Active threat Yes
Deciding rule
In CISA KEV with patch available
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Immediate
Exploitation· derived
Active
System Exposure· derived
Open
Automatable· derived
No
Human Impact
Very High

Priority Score

233
Low Medium High Critical
KEV: +50
EPSS: +94
CVSS: +44
PoC: +20
Weaponized: +15
Threat intel: +10
Gov advisory: 0

Attack Technique

Path traversal exploits occur when applications use user-controlled input to construct file system paths without proper validation.

Remote Code Execution represents the critical moment when an attacker successfully runs arbitrary code on a target system without physical access.

Affected Products (CPE)CVE.org

cpe:2.3:o:microsoft:windows_10_1507:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_1909:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_2004:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_20h2:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_21h1:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_7:-:sp1:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_8.1:-:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_rt_8.1:-:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2004:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2008:-:sp2:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2008:r2:sp1:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_server_2012:-:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2012:-:r2:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2016:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2022:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_20h2:*:*:*:*:*:*:*:*
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Threat IntelligenceMISP

Threat Actors
EXOTIC LILY
ATT&CK Techniques (observed)
Sources: MITRE ATT&CK, MISP Galaxies (CC0), Metasploit/Nuclei

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