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

CVE-2021-40444 is a remote code execution vulnerability in Microsoft MSHTML, the component Windows uses to render web content in applications such as Office and the browser engine. CISA describes it as an unspecified flaw that allows remote code execution. It matters because successful abuse can let an attacker run code on a user’s system, and this CVE has been associated with known ransomware use. Defenders should treat it as a high-priority patch item and confirm all version and configuration details against the vendor advisory.

How it works

The weakness is classified as CWE-22 (improper limitation of a pathname to a restricted directory, often called path traversal). In products that embed MSHTML, this class of flaw can allow crafted input to influence how the component resolves or loads resources, ultimately leading to remote code execution when a user opens or previews malicious content. Public detail on the exact trigger and exploit mechanics is limited in the provided record; treat the attack surface as document- or content-driven abuse of the MSHTML rendering path. An attacker typically needs to deliver a specially crafted file or content that the victim opens in an application that invokes MSHTML. Do not rely on incomplete public write-ups—confirm behavior and affected configurations against Microsoft’s advisory.

Am I affected? How to find it in your systems

MSHTML is part of the Windows platform and is used by Microsoft Office and other applications that render HTML or related content. It typically appears on Windows endpoints and servers where Office or browser-related components are installed.

• Inventory Windows workstations and servers, especially those with Microsoft Office or other software that embeds MSHTML.
• Use your software asset management, SCCM/Intune, or vulnerability scanner to identify systems missing the updates Microsoft released for this CVE; exact build and version ranges must be confirmed against the vendor advisory.
• Check group policy and application control settings that govern ActiveX, preview panes, and protected view, as these often intersect with MSHTML usage.
• For exploitation signs, review endpoint detection logs for unusual child processes spawned from Office or browser-related hosts, unexpected network connections after document open, and alerts tied to this CVE or MSHTML abuse. Correlate with email gateway and web proxy logs for delivery of suspicious documents. Specific IOCs are not provided here—use vendor and CISA guidance.

How to remediate

Patch first. CISA’s required action is to apply updates per vendor instructions. Deploy the Microsoft security updates that address CVE-2021-40444 across all affected Windows and Office environments as soon as your change process allows, and verify installation with your patch-management reporting.

• Prioritize internet-facing and high-risk user populations (users who open external email and documents).
• After patching, validate that the relevant updates are present and that MSHTML-dependent applications behave normally.
• Harden the class of risk: enforce Protected View and similar sandboxing for files from the internet, disable unnecessary ActiveX and legacy rendering features where business-compatible, and keep Office and Windows fully updated on a recurring cadence.
• Ensure EDR/AV signatures and behavioral rules covering document-based RCE are current.

If you can't patch immediately

If you cannot complete patching at once, reduce exposure with compensating controls until updates are installed.

• Segment and restrict: limit outbound connectivity from high-risk endpoints where practical, and isolate systems that handle untrusted documents.
• Virtual patching / email and web controls: block or sandbox suspicious Office and HTML-related attachments at the mail gateway; use content disarm and reconstruction if available; apply WAF or proxy rules only where they meaningfully cover the delivery path (confirm efficacy—document-based client flaws are not fully mitigated by network WAF alone).
• Disable or constrain the affected feature path: tighten policies that control preview panes, ActiveX, and automatic rendering of untrusted HTML/Active content in Office; prefer opening untrusted files in hardened viewers or isolated environments.
• Increase monitoring: alert on Office or MSHTML-related processes launching shells, script hosts, or unusual network activity; retain and review endpoint and email logs for related activity. Revisit these controls once the vendor updates are applied.

If your data may have been exposed

Actively exploited vulnerabilities, including those with known ransomware use, can lead to account takeover, malware installation, and data theft. If you suspect compromise, isolate affected hosts, preserve logs, and follow your incident-response process, including credential resets and ransomware containment steps as appropriate. You can run a free exposure scan of your email addresses against known breach data to see whether associated credentials or personal data have appeared in prior breaches, then prioritize password changes and monitoring for any matches.