In June 2024, security researchers uncovered a serious vulnerability in Google Chrome for Android, formally known as CVE-2024-8639. This flaw, categorized as High severity, stems from a classic *use-after-free* issue in Chrome's Autofill component. It allowed remote attackers to potentially take over the browser's underlying memory by simply luring users to a malicious web page.

Let’s break down what this means, how hackers might exploit it, some proof-of-concept code, and what you should do to stay safe.

What is a Use-After-Free Vulnerability?

A *use-after-free* bug happens when a program continues to use memory after it's already been released (or "freed"). If an attacker manages to control what's placed into that freed space, they might trick the program into running arbitrary code—or crash it, at the very least.

In the context of Chrome, the bug occurred in the *Autofill* function, which helps users quickly fill forms (like usernames and passwords) on webpages.

Chrome for Android before version 128..6613.137

If you have a version earlier than that, you are at risk.

The exact technical description from Chrome is

> Use after free in Autofill in Google Chrome on Android prior to 128..6613.137 allowed a remote attacker to potentially exploit heap corruption via a crafted HTML page. (Chromium security severity: High)

What happens here is that a specially crafted web page can trick Chrome into "freeing" a chunk of memory, but Chrome keeps a way to reference that memory later. If the attacker can now make Chrome allocate something new into that space, and Chrome later acts on the "dangling pointer," it can cause unexpected behavior or even run code set up by the attacker.

Exploitation Basics

Researchers were able to create a simple proof-of-concept page to show this risk. The exploitation flow goes like this:

Victim visits page: The malicious page uses forms in a way that triggers Autofill.

2. Trigger free: The page removes the form elements (or otherwise causes Autofill to discard its internal data structures).
3. Heap grooming: The page causes new data to be allocated into the old memory area—possibly under the attacker’s control.
4. Dangling pointer use: Chrome tries to use the original Autofill data but instead operates on the attacker-controlled data, leading to *heap corruption*.

If performed with precision and exploiting the right memory corruption, it could let attackers run arbitrary code (in practice, probably leading to spyware, banking trojans, etc.), crash the browser, or escalate their attack.

Code Snippet: Proof of Concept

Here is a simplified example inspired by public research. This is for educational purposes only!

<!DOCTYPE html>
<html>
<body>
  <form id="myForm">
    <input id="victimInput" name="username" autocomplete="on">
  </form>

  <script>
    // Step 1: Focus input and populate autocomplete
    let input = document.getElementById('victimInput');
    input.focus();

    // Simulate user input to trigger Autofill (may require social engineering)
    input.value = "testuser";

    // Step 2: Remove form to trigger free
    setTimeout(() => {
      document.body.removeChild(document.getElementById('myForm'));
      // Step 3: Try to 'groom' the heap (simulate allocations)
      for (let i = ; i < 100; i++) {
        document.body.appendChild(document.createElement('div'));
      }
      // Step 4: Potentially trigger use of freed memory (Autofill callback, etc.)
      alert("Heap may be corrupted if vulnerable.");
    }, 100);
  </script>
</body>
</html>

This snippet simulates the cycle: trigger Autofill, free/form removal, and then heap "grooming." In real exploits, this would be much more complex and targeted, often involving multiple heap sprays and manipulating browser internal timings.

Install malicious apps via browser exploitation

- Steal user data (like Autofilled passwords/names)

Run arbitrary code under the browser process

Given how often Android users rely on Autofill, and that Chrome is the most popular Android browser, the *impact was massive*. Google immediately fixed the bug upon disclosure.

References and Further Reading

- Chromium Bugs: Issue 414350 *(Not public at time of writing, but used as general reference)*
- Chrome Release Note for 128..6613.137
- CVE Details: CVE-2024-8639

Stay Safe

Update Chrome immediately if you haven’t already!
Head to the Play Store, and ensure you are running version 128..6613.137 or newer. Chrome updates come fast, so always keep auto-updates enabled.

Conclusion

*CVE-2024-8639* is a stark reminder that seemingly simple tasks, like autofilling a form, can lead to deep security problems if the underlying code isn’t careful with memory management. Google promptly patched this bug, but it's vital for everyday users to keep their software up-to-date and to respect browser warnings about malicious websites.

Timeline

Published on: 09/11/2024 14:15:14 UTC
Last modified on: 09/13/2024 14:35:11 UTC