CVE-2024-1555 - Bypassing SameSite Cookies in Firefox Using `firefox://` Protocol Handler

*CVE-2024-1555 is a significant vulnerability discovered in Mozilla Firefox browsers before version 123. It involves how the browser mistakenly handles SameSite cookie restrictions when routing website loads through the internal firefox:// protocol handler. In this detailed post, we'll unpack what this means, give a real example, and discuss how an attacker might exploit it.*

What Is CVE-2024-1555?

In short, when a web page is opened using the firefox:// protocol handler (an internal Firefox mechanism), Firefox didn't properly enforce SameSite cookie rules. Normally, SameSite cookies are designed to limit cross-site request forgery (CSRF) and protect user authentication by restricting or blocking cookies that are sent with requests initiated from other sites. With this bug, SameSite restrictions could be bypassed—your browser could send cookies that should have been blocked.

Mozilla’s Security Advisory:
- Mozilla Foundation Security Advisory 2024-12
- CVE Details Reference

Why Does This Matter?

SameSite cookie attributes (Strict or Lax) are crucial defenses against specific classes of attacks, chiefly CSRF. If they're not honored, it’s a lot easier for bad actors to trick your browser into sending cookies that allow access to your accounts or personal information.

1. Setting the Stage

Let’s say you’re logged into a bank site, https://www.safe-bank.com, which uses Set-Cookie: sessionid=secure; SameSite=Strict. Normally, any requests to the bank from another domain wouldn’t send this cookie.

2. Malicious Webpage

An attacker creates a malicious web page at https://www.attacker.com and uses the following code snippet to try and open a page on the bank’s origin—but through the firefox:// protocol handler:

<!-- Attacker's Page: https://www.attacker.com -->
<a href="firefox://open-url?url=https://www.safe-bank.com/transfer-money">Click me!</a>

Or, with JavaScript, to auto-trigger

window.location = 'firefox://open-url?url=https://www.safe-bank.com/transfer-money';

3. The Bypass in Action

When a victim clicks the link (or the JavaScript runs), Firefox (before version 123) opens the target site (https://www.safe-bank.com/transfer-money) using its internal handler rather than a traditional navigation. Here’s the problem: Firefox fails to apply the SameSite restrictions, and the sessionid=secure cookie is sent even though it shouldn’t be.

This allows the attacker to perform actions on behalf of the user, like tricking the bank into moving money.

Suppose the bank site logs cookies it receives

# Flask example for demonstration
from flask import Flask, request
app = Flask(__name__)

@app.route('/transfer-money')
def transfer_money():
    session_id = request.cookies.get('sessionid')
    print(f"Session ID received: {session_id}")
    # In real-life, initiate money transfer if session valid!
    return "Transfer initiated!"

If that page receives the sessionid cookie, it means the attack is working; otherwise, SameSite is holding strong.

Browsers Affected: Firefox < 123 (Desktop)

- Attack Vector: Malicious website sending you to a firefox:// URL

Update Firefox: Always ensure you’re running Firefox 123 or later.

- Be Wary of firefox:// Links: Don’t click suspicious protocol handler links or popups.
- Web Developers: Make sure to also use CSRF tokens in your apps, not just rely on SameSite cookies.

References

- Mozilla Foundation Security Advisory 2024-12
- CVE Details: CVE-2024-1555
- MDN: SameSite cookies explained
- CSRF Protection Using SameSite Cookies

Conclusion

CVE-2024-1555 might sound technical, but it boils down to cookie security being accidentally disabled through a creative browser quirk. If you use Firefox, update now! If you build websites, don’t rely only on SameSite for security—layer up your defenses. With flaws like this, attackers will always look for new, sneaky ways in.

Timeline

Published on: 02/20/2024 14:15:09 UTC
Last modified on: 11/15/2024 21:35:03 UTC