Background
MSHTML (also known as Trident) is Microsoft’s legacy HTML rendering engine, originally built into Internet Explorer and still present in Windows for rendering HTML content in applications, mail clients, and Office documents. Despite Internet Explorer’s retirement, MSHTML remains a Windows component used by legacy applications, ActiveX controls, and certain email rendering scenarios.
CVE-2026-21513 is a protection mechanism failure (CWE-693) in MSHTML that allows an unauthorised attacker to bypass a security feature over the network. CISA added this to the KEV catalog on 2026-02-10, coinciding with February 2026 Patch Tuesday, confirming active exploitation.
MSHTML security bypasses have a significant exploitation history — they have been used to circumvent Mark-of-the-Web (MotW) protections, Protected Mode, and other IE/Windows security boundaries that prevent malicious web content from executing code or accessing local resources.
Technical Mechanism
CWE-693 (Protection Mechanism Failure) in MSHTML typically involves a flaw that allows content parsed by the MSHTML engine to escape a sandboxing or filtering mechanism. This could include bypassing MotW checks, escaping the Protected Mode sandbox, or circumventing content security policies applied to HTML content rendered by MSHTML.
<!-- Conceptual MSHTML security bypass technique -->
<!-- Zone identifier bypass: MotW normally blocks script execution from downloaded content -->
<html>
<head>
<!-- Crafted MSHTML-specific directives that bypass zone checks -->
<meta http-equiv="X-UA-Compatible" content="IE=edge">
</head>
<body>
<!-- Content that exploits the protection mechanism failure
to execute beyond expected security boundaries -->
<object classid="..." data="..."></object>
</body>
</html>The specific bypass mechanism allows a remote attacker to deliver content that executes with higher trust or broader capabilities than the MSHTML security model should permit when accessed over the network.
Real-World Exploitation Evidence
CISA’s KEV listing confirms active exploitation. MSHTML security bypasses are useful in phishing and drive-by download campaigns because they let attackers serve content that executes despite Windows security controls. MSHTML remains present in Windows email clients and Office applications, meaning malicious email attachments or linked content can trigger MSHTML processing in contexts where users may not expect browser-level security to apply.
Impact Assessment
- Bypass of Windows MSHTML security controls including MotW or zone restrictions
- Potential for code execution when combined with other vulnerabilities
- Security boundary circumvention affecting applications using MSHTML for content rendering
- Email client and Office application attack surface if MSHTML processes content from those
- Network-deliverable: no local access or elevated privileges required to exploit
Affected Versions
| Product | Affected | Fixed |
|---|---|---|
| Windows 10 | Affected MSHTML versions | February 2026 Patch Tuesday |
| Windows 11 | Affected MSHTML versions | February 2026 Patch Tuesday |
| Windows Server 2019/2022 | Affected MSHTML versions | February 2026 Patch Tuesday |
Remediation Steps
- Apply February 2026 Patch Tuesday updates immediately.
- Enable automatic Windows Update on all systems.
- Disable MSHTML rendering in applications that do not require it via Group Policy.
- Configure Protected Mode and Enhanced Protected Mode in any IE/MSHTML-using applications.
- Deploy email filtering to block HTML content from external senders in Outlook.
Detection Guidance
Log Sources: Windows Event Logs, Sysmon, EDR process telemetry.
IOCs: MSHTML (mshtml.dll) loading in unexpected process contexts; processes spawned from mshta.exe or iexplore.exe; unusual network connections following HTML rendering events.
Sigma rule:
title: MSHTML Security Bypass Exploitation CVE-2026-21513
logsource:
product: windows
category: image_load
detection:
selection:
ImageLoaded|endswith: '\mshtml.dll'
Image|endswith:
- '\outlook.exe'
- '\winword.exe'
- '\excel.exe'
condition: selection
level: mediumTimeline
| Date | Event |
|---|---|
| 2026-02-10 | CVE-2026-21513 disclosed (February Patch Tuesday) |
| 2026-02-10 | CISA adds to KEV catalog; active exploitation confirmed |