In May 2024, security researchers identified a significant vulnerability—CVE-2024-4947—affecting Google Chrome’s JavaScript engine, V8. This bug, present in Chrome versions before 125..6422.60, is a Type Confusion flaw that allows attackers to run arbitrary code within Chrome’s sandbox just by getting a user to visit a specially crafted web page.

In this long read, we’ll walk through what a Type Confusion is, how this bug works, and how a potential exploit could unfold. You’ll also find code snippets, references, and practical insights to help you understand why patching your browser promptly is critical.

What is Type Confusion in V8?

V8 is the JavaScript engine powering Chrome and many other apps. Type Confusion bugs happen when code assumes an object is of one type, but it’s actually another. This lets attackers break memory safety, sometimes opening doors to get arbitrary code running inside the browser process.

If the attacker gets code running inside Chrome’s renderer process (the sandbox), they may chain further bugs to escape the sandbox and compromise the whole system.

The security bulletin describes it as

> Type Confusion in V8 in Google Chrome prior to 125..6422.60 allowed a remote attacker to execute arbitrary code inside a sandbox via a crafted HTML page. (Chromium security severity: High)

Impact: Remote code execution in browser sandbox.

- Attack Vector: Malicious website (crafted HTML/JS).

How Type Confusion Works – A Simple Analogy

Suppose you have a JavaScript array of numbers. Due to optimization, V8 might treat this array differently than an array of objects. But what happens if, for some reason, V8 gets confused and treats a number as an object or vice versa?

This can let an attacker trick V8 into writing or reading to memory locations they shouldn’t control, sometimes letting them run custom machine code.

Example Code that Could Trigger the Bug

Here’s a simplified snippet (illustrative, not the actual exploit code!) of how attackers often target V8 type confusions:

function confuse(obj) {
    // Suppose V8 mistakenly believes obj is of a certain internal type
    return obj.someProperty; // Underlying memory confusion!
}

let numArr = [1.1, 2.2, 3.3];
let objArr = [{}, {}, {}];

// JIT optimization gets tricked after many runs:
for (let i = ; i < 10000; ++i)
    confuse(numArr);

// Now, call with an object array:
confuse(objArr); // Type confusion can happen here!

In real exploits, attackers use techniques like structural spraying), manipulation of inline caches, and other advanced tricks to massage V8’s memory layout.

With CVE-2024-4947, the bug was in V8's code that failed to check object types correctly before using them, so malicious JS could unexpectedly access memory they shouldn't, and eventually achieve code execution.

The page runs specially crafted JavaScript targeting the Type Confusion bug.

3. The attacker achieves out-of-bounds read/write in memory.
4. Shellcode is sprayed into memory, and the out-of-bounds write hijacks control flow (like overwriting a function pointer).

A simplified part of a real exploit chain might look like

// 1. Spray heap with shellcode
let shellcode = new Uint8Array([...]); // attacker-supplied bytes

// 2. Achieve arbitrary read/write using confusion
let addr = get_memory_address_of(shellcode); // hypothetical utility

// 3. Overwrite a function pointer
write_memory(addr, evil_function_pointer);

In reality, the steps are more complex and involve chaining several V8 internals, but this is the general direction.

Is a Sandbox Escape Possible?

By default, Chrome’s sandbox restricts what this shellcode can access. However, attackers often try to combine this bug with a *second* vulnerability—a sandbox escape—to fully compromise the system.

How to Stay Safe

- Update Chrome: Fixed in 125..6422.60 – Update instructions here.

References & Further Reading

- Official Chromium Security Advisory
- NIST NVD CVE-2024-4947
- Understanding V8 Exploitation (Project Zero)
- What is Type Confusion?
- Chrome CVE List

Final Thoughts

CVE-2024-4947 is a classic example of why complex JavaScript engines like V8 need relentless scrutiny. Even with sandboxing, a single bug can hand attackers the keys to your browser, opening the door for further system exploits if chained with other bugs.

Timeline

Published on: 05/15/2024 21:15:09 UTC
Last modified on: 05/23/2024 19:54:02 UTC