In December 2022, a critical vulnerability (CVE-2022-4178) was discovered in Google Chrome's Mojo interface. This bug is a classic use-after-free issue that can let a remote attacker who has control over Chrome's renderer process exploit heap corruption. That means visiting a malicious webpage—without downloads or warnings—could give attackers a stepping stone to take over your browser or even execute code on your computer. This article explains what happened, how an exploit might work, and how to protect yourself.

> Chrome Security Severity: High
> Affected Versions: Prior to 108..5359.71
> Component: Mojo (Chromium IPC)
> References:
> Chromium issue tracker
> NIST NVD page

What is Mojo, and What Went Wrong?

Mojo is the internal message passing system Google Chrome uses so that different parts of the browser (renderers, GPU process, etc.) can talk to each other securely. Think of it like a “mailman” delivering packages (data/messages) inside Chrome.

A use-after-free (UAF) bug happens when a program continues to use “pointers” (memory references) after freeing that memory. Imagine reading from a page that has been torn out of a book—the results are unpredictable and exploitable.

In CVE-2022-4178, certain Mojo messages could cause Chrome to reuse memory after freeing it, especially if that memory allocation gets refilled with attacker-controlled data quickly enough. By doing this, a skillful attacker can gain control over what the browser code does next.

1. Compromise the Renderer Process

The attacker first needs to run code in the renderer process. In practice, this is very doable—JavaScript from a malicious webpage does exactly this.

2. Trigger Use-After-Free via Crafted HTML

A malicious page with specially designed JavaScript triggers the UAF in the browser’s Mojo code. It tries to free a Mojo object, and immediately allocate the same area with attacker-controlled contents.

3. Corrupt the Heap

If attackers win the "race" (timing is important!), they can overwrite critical data and cause Chrome to behave unpredictably. This can end up in a browser crash, malware delivery, or even running the attacker’s code.

Dissection: Simple Exploit Example

While actual exploit code is advanced, here's a distilled, educational example showing how use-after-free works in Chrome’s renderer process context. (This uses simplified pseudo-JavaScript and comments to highlight the concept.)

Simplified Exploit Code

// Step 1: Create lots of DOM objects to fill the memory ("heap spraying")
let arr = [];
for (let i = ; i < 10000; i++) {
  let div = document.createElement('div');
  div.id = 'testDiv' + i;
  arr.push(div);
  document.body.appendChild(div);
}

// Step 2: Trigger a vulnerable Mojo interface call
function triggerUAF() {
  // This hypothetical function triggers the UAF by freeing a Mojo object
  navigator.someMojoVulnerableAPI.releaseObject();

  // Step 3: Allocate new objects to reclaim freed memory with attacker data
  for(let i = ; i < 10000; i++) {
    let fake = { controlled: true, data: x41414141 }; // Fill with 'A'
    // In the real exploit, this would overwrite freed Mojo object
  }
}

// Launch exploit after a short delay
setTimeout(triggerUAF, 100);

We fill up memory with HTML elements (heap spray).

2. We call a function that triggers the UAF (the real bug was in Mojo, not JavaScript, but this mimics the timing).
3. We flood memory again to fill the spot just freed, hoping our fake data is used by the freed pointer.

Possibly escape Chrome’s sandbox to run code on your machine (with further tricks)

This is why Google rates this high severity and urges everyone to update as soon as fixes come out.

Patched in Chrome 108..5359.71 and later

- Official security advisory
- Chromium bug tracker report (public shortly after fix)

Consider site isolation features:

Chrome’s Site Isolation helps prevent exploits from spreading between tabs.

Conclusion

CVE-2022-4178 shines a light on how complex browser security really is. Even a single use-after-free can allow highly skilled attackers to breach the browser’s defenses. Chrome’s engineers patched this fast—so make sure you are running the latest version.

*If you want to dive deeper, see the Chromium NVD Entry for CVE-2022-4178. For developers, look into safe memory management practices and use automatic tools like AddressSanitizer to catch these bugs early.*

Timeline

Published on: 11/30/2022 00:15:00 UTC
Last modified on: 05/03/2023 12:16:00 UTC