In early 2024, security researchers highlighted a new vulnerability tracked as CVE-2024-1673 affecting Google Chrome’s Accessibility component. Sitting at the heart of this bug is a classic *use-after-free* flaw, which, when exploited, can lead to heap corruption, opening the door for attackers to run arbitrary code or crash the browser. This post walks you through the technical background, how the bug can be triggered, and what threat actors might do with it – in everyday language, for easy understanding.

References

- Chromium Security Advisory
- CVE Details Page
- Chromium Issue Tracker #1516522 *(restricted access)*

How Does This Bug Work?

A *use-after-free* happens when a program continues to use a piece of memory after it’s been released (freed). In Accessibility code, this could happen when a DOM element is deleted but Chrome's Accessibility code still tries to use it. If an attacker is running code with renderer privileges (e.g., exploiting another bug that gives JavaScript code advanced control), they can force Chrome to use the freed memory in a carefully timed way, corrupting the heap.

Technical Breakdown

1. Object Allocation: Chrome creates Accessibility-related objects (like AXNode or similar) when building or updating the page’s accessibility tree.
2. Freeing Object: Through specific JavaScript-driven DOM manipulations and UI gestures (user actions like focus, keyboard navigation, or screen reader events), an attacker can force Chrome to free those objects while still in use.
3. Dangling Pointer Usage: If Chrome tries to access those freed Accessibility objects, it may crash (best case) or allow code execution (worst case) if the attacker can control the contents of the memory that was just freed and possibly re-allocated.

Proof of Concept (PoC)

Here’s a generic demonstration of how a use-after-free bug might be triggered (for educational purposes only). Real exploit code would need detailed knowledge of Chrome’s internals and is not public due to responsible disclosure policies.

// Pseudo-code representation of triggering the bug
let targetElem = document.createElement('input');
document.body.appendChild(targetElem);

targetElem.focus(); // Activate Accessibility routines

targetElem.addEventListener('focus', () => {
  // Remove the element just as it's being accessed
  document.body.removeChild(targetElem);
  
  // Force garbage collection and memory reuse (in real exploitation, would need precise timing)
  // window.gc(); // Not exposed in browsers
});

setTimeout(() => {
  targetElem.focus(); // Trigger the event handler
}, 100);

Note: The above is illustrative only. Exploiting the real bug would require a renderer compromise — a browser process already under some attacker control.

Exploit Path & Real-World Impact

To actually exploit CVE-2024-1673, an attacker must compromise the renderer process first. This is often done by chaining vulnerabilities: for instance, exploiting another Chrome bug (like a JavaScript engine flaw) to run code in the renderer, then using this bug to bypass sandboxing or further escalate privileges.

How was it Fixed?

Google’s fix, released in version 122..6261.57, involved improving the lifetime management of Accessibility objects, ensuring they are not used after being freed.

Update your browser to the latest version – Chrome auto-updates for most users, so verify you’re running a version newer than 122..6261.57.

Release Note:
> Chrome 122 Stable Update:
> “[$TBD][1516522] Medium CVE-2024-1673: Use after free in Accessibility. Reported by David Erceg on 2024-02-14”

Responsible Disclosure

The researcher, David Erceg, reported the bug responsibly, preventing public exploit until users had time to update. The Chromium team silently fixed the underlying object management issues.

Always keep your browser updated – fixes arrive silently but make a big difference

- Researchers play a crucial role in reporting and fixing these issues before attackers can exploit them

Stay safe, and patch your browsers! If you’re interested in bug hunting, check out the Chromium Security page to learn more.

Timeline

Published on: 02/21/2024 04:15:08 UTC
Last modified on: 08/01/2024 13:46:09 UTC