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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Craft malicious HTML document
  2. Delivery
    Send to target via email or web
  3. Exploit
    User opens document in browser
  4. Execution
    MSHTML parsing bypasses protection mechanism
  5. Impact
    Execute arbitrary code with user privileges

Vulnerability Assessment

Weakness
CWE
CWE-693
OWASP Top 10:2025
A06:2025 - Insecure Design
ASVS 5.0
V15 - Secure Coding and Architecture Protection mechanism completeness
Exploitation Conditions

User interaction required — target must open attacker-controlled HTML document in a browser using MSHTML rendering engine. No authentication needed; affects default MSHTML Framework installations.
Impact

CVSS 8.8 (High). KEV-listed — actively exploited. EPSS 4.8%. Remote exploitation over network. MSHTML still widely used in Windows ecosystem (Outlook, Office, third-party apps). Common attack vector for phishing emails with HTML content.
Exploit ScenarioDerived

An attacker sends a phishing email containing crafted HTML that exploits the MSHTML bypass. When the email is previewed in Outlook (which uses MSHTML for HTML rendering), the malicious content executes without the security restrictions normally applied to HTML email content. Alternatively, the attacker can embed MSHTML-rendered content in Office documents or deliver it through other applications that use MSHTML for HTML display.
Act Now Prioritize within 24-48h. Why: In CISA KEV with patch available.
Exploited in the wild (CISA KEV) Patch available 4.8% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 12, 2026

MSHTML Framework contains a protection mechanism failure (CVE-2026-21513, CVSS 8.8) allowing remote attackers to bypass security features over a network. KEV-listed, this vulnerability in the legacy HTML rendering engine (still used by many Windows applications and email clients) enables execution of malicious content by circumventing the browser's security sandbox and content restrictions.

Earlier versions of this analysis (1)
v1 Mar 12, 2026 - 22:02 CVSS 8.8

MSHTML Framework (Trident) contains a protection mechanism failure (CWE-693) that allows remote attackers to bypass security features, enabling execution of malicious HTML content without normal security restrictions.

RemediationAI

Apply Microsoft security update immediately. Consider disabling HTML email rendering in Outlook (plain text mode). Enable ASR rules for MSHTML-based attacks. Monitor for exploitation indicators in email processing logs.

Affected Packages

microsoft windows_10_version_1607_10_0_14393_8868 fix: Windows 10 Version 1607 for 32-bit Systems 10.0.14393.8868, Windows 10 Version 1607 for x64-based Systems 10.0.14393.8868
microsoft windows_10_version_1809_10_0_17763_8389 fix: Windows 10 Version 1809 for 32-bit Systems 10.0.17763.8389, Windows 10 Version 1809 for x64-based Systems 10.0.17763.8389
microsoft windows_10_version_21h2_10_0_19044_6937 fix: Windows 10 Version 21H2 for 32-bit Systems 10.0.19044.6937, Windows 10 Version 21H2 for ARM64-based Systems 10.0.19044.6937, Windows 10 Version 21H2 for x64-based Systems 10.0.19044.6937
microsoft windows_10_version_22h2_10_0_19045_6937 fix: Windows 10 Version 22H2 for 32-bit Systems 10.0.19045.6937, Windows 10 Version 22H2 for ARM64-based Systems 10.0.19045.6937, Windows 10 Version 22H2 for x64-based Systems 10.0.19045.6937
microsoft windows_11_version_23h2_10_0_22631_6649 fix: Windows 11 Version 23H2 for ARM64-based Systems 10.0.22631.6649, Windows 11 Version 23H2 for x64-based Systems 10.0.22631.6649
microsoft windows_11_version_24h2_10_0_26100_7840 fix: Windows 11 Version 24H2 for ARM64-based Systems 10.0.26100.7840, Windows 11 Version 24H2 for x64-based Systems 10.0.26100.7840
microsoft windows_11_version_25h2_10_0_26200_7840 fix: Windows 11 Version 25H2 for ARM64-based Systems 10.0.26200.7840, Windows 11 Version 25H2 for x64-based Systems 10.0.26200.7840
microsoft windows_11_version_26h1_10_0_28000_1575 fix: Windows 11 Version 26H1 for ARM64-based Systems 10.0.28000.1575, Windows 11 version 26H1 for x64-based Systems 10.0.28000.1575
microsoft windows_server_2012_6_2_9200_25923 fix: Windows Server 2012 6.2.9200.25923, Windows Server 2012 (Server Core installation) 6.2.9200.25923
microsoft windows_server_2012_r2_6_3_9600_23022 fix: Windows Server 2012 R2 6.3.9600.23022, Windows Server 2012 R2 (Server Core installation) 6.3.9600.23022
microsoft windows_server_2016_10_0_14393_8868 fix: Windows Server 2016 10.0.14393.8868, Windows Server 2016 (Server Core installation) 10.0.14393.8868
microsoft windows_server_2019_10_0_17763_8389 fix: Windows Server 2019 10.0.17763.8389, Windows Server 2019 (Server Core installation) 10.0.17763.8389
microsoft windows_server_2022_10_0_20348_4773 fix: Windows Server 2022 10.0.20348.4773, Windows Server 2022 (Server Core installation) 10.0.20348.4773
microsoft windows_server_2022_23h2_edition_10_0_25398_2149 fix: Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.2149
microsoft windows_server_2025_10_0_26100_32370 fix: Windows Server 2025 10.0.26100.32370, Windows Server 2025 (Server Core installation) 10.0.26100.32370

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 15 packages in microsoft

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Technical ContextAI · show details

MSHTML (Trident) is the legacy HTML rendering engine used by Internet Explorer and many Windows applications that render HTML content (Outlook email preview, help files, many third-party apps). Despite IE's deprecation, MSHTML remains a critical attack surface because it's still used for rendering HTML in non-browser contexts. This bypass allows malicious content to execute outside the normal security restrictions applied to rendered HTML.

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Severity by source

NVD PRIMARY 8.8 HIGH
AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

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

Lifecycle Timeline

2
Added to CISA KEV
Mar 30, 2026 - 13:28 cisa
CISA KEV
CVE Published
Feb 10, 2026 - 18:16 nvd
HIGH 8.8

DescriptionCVE.org

Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network.

Executive ViewAI

Why It Matters

CVE-2026-21513 is a critical vulnerability in MSHTML Framework that is currently being actively exploited in the wild. This flaw allows attackers to bypass security controls and gain unauthorized access to systems, potentially leading to data breaches, malware installation, or system compromise across any organization running affected Windows environments.

Business Impact: Severe
Compensating Controls

Disable MSHTML rendering where possible and switch to alternative browsers (Chromium-based); implement strict network segmentation to limit lateral movement; deploy WAF rules blocking known exploitation signatures; restrict email with embedded objects; enforce application whitelisting; increase endpoint detection and response (EDR) monitoring; disable Office macro execution where feasible; apply Windows SmartScreen protections; monitor for suspicious PowerShell and script execution

Threat Intelligence Overview

EPSS Score
4.8%
Top 11% exploitable
Data Quality
5 sources
Confidence: Medium
NVD CVE.org CISA KEV EPSS VulDB

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 No
EPSS 4.8%
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

99
Low Medium High Critical
KEV: +50
EPSS: +5
CVSS: +44
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Authentication Bypass CRITICAL

Authentication bypass attacks exploit flaws in the verification mechanisms that control access to systems and applications.

Affected Products (CPE)CVE.org

cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_11_23h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_23h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_11_24h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_24h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_11_25h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_25h2:*:*:*:*:*:*: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:*:*:*:*:-:*:x64:*
cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:-:*:x64:*
+ 3 more

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Same weakness CWE-693 – Protection Mechanism Failure

View all
CVE-2013-2465 CRITICAL POC
9.8
CVE-2024-21412 HIGH POC
8.1
CVE-2019-1003030 CRITICAL POC
9.9
CVE-2013-0431 LOW POC
3.7
CVE-2025-40536 HIGH POC
8.1
CVE-2024-38217 MEDIUM POC
5.4
CVE-2026-21510 HIGH POC
8.8
CVE-2026-32202 MEDIUM POC
4.3

Same technique Authentication Bypass

View all
CVE-2015-3306 CRITICAL POC
10.0
CVE-2023-22515 CRITICAL POC
9.8
CVE-2012-4681 CRITICAL POC
9.8
CVE-2011-3544 CRITICAL POC
9.8
CVE-2022-40684 CRITICAL POC
9.8
CVE-2025-61882 CRITICAL POC
9.8
CVE-2024-0012 CRITICAL POC
9.3

References

Advisories & patches
ADVISORY https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21513 ADVISORY https://vuldb.com/?id.345290 REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21513 REF https://www.vicarius.io/vsociety/posts/cve-2026-21513-detection-script-security-feature-bypass-vulnerability-in-mshtml-framework REF https://www.vicarius.io/vsociety/posts/cve-2026-21513-mitigation-script-security-feature-bypass-vulnerability-in-mshtml-framework

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