CVE-2024-7971 - Exploiting Type Confusion in V8 (Chrome) – Full Technical Deep Dive

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In June 2024, the Chrome security community announced a critical vulnerability — CVE-2024-7971. This bug affected the V8 JavaScript engine in Google Chrome versions prior to 128..6613.84. It’s a high severity issue because remote, untrusted web pages could exploit a *type confusion bug* to corrupt the browser’s heap and potentially execute arbitrary code.

This post gives an exclusive, plain-English explanation of how this bug works, how it can be exploited, and what code is involved.

What is Type Confusion in V8?

Type confusion is a bug class common in JavaScript engines like V8 (which powers Chrome). In V8, variables and objects are internally tagged with types. Type confusion happens when the engine misidentifies an object’s type — for example, treating an object as an array when it isn't — leading to unsafe memory access.

With type confusion, if an attacker can control the type and data, they can often corrupt memory, read/write to other objects, or even jump to and execute native code.

The Vulnerability: CVE-2024-7971

- Affected engine: V8 JavaScript Engine

Impact: Heap corruption via webpage, possible RCE

- Official reference: Chromium Issue Tracker: 334952006

Technical Details

*V8 optimizes lots of inline JavaScript code. Sometimes, during these optimizations, the engine assumes a variable is always of a particular type. If that assumption proves wrong, memory can get corrupted.*

The bug lies in how V8 handles JIT (Just-In-Time) optimized code, especially with arrays and objects that get dynamically modified or replaced.

Consider this simplified, illustrative version

function confuse(arr, idx, val) {
  arr[idx] = val;
  return arr[idx];
}

let intArray = [1.1, 2.2, 3.3]; // Array with double elements

for (let i = ; i < 10000; i++) {
  confuse(intArray, , 1.1); // JIT optimizes under type assumption
}

// Now pass an object in place of a float -- triggers type confusion
let fake_obj = {fake: true};
confuse(intArray, , fake_obj);

In versions prior to the patch, the last call confuses the engine, as it assumes it operates only on doubles. Suddenly, the underlying memory may interpret the object pointer as a double, or vice versa.

If this happens in the right context — and the attacker can further manipulate objects — it becomes possible to control what memory addresses are read from or written to.

Attackers chain together several *primitives*

- Out-of-bounds (OOB) Read/Write: Gain ability to read/write memory outside the bounds of an object or array.

Fake Object Construction: Fabricate JavaScript objects to point to attacker-controlled memory.

- Arbitrary Code Execution: Use fake objects/pointers to corrupt browser memory, hijack code execution.

Here’s a fake *exploit* flow (based on public reports and prior V8 exploits, slightly modified for this post):

let arr = [1.1, 2.2, 3.3];
let objarr = [{}, {}, {}];
let victim = {"a": 1};

function trigger() {
  for (let i = ; i < 10000; i++) {
    arr[] = 1.1; // Train type feedback in JIT
  }
  arr[] = victim; // Now confuse type
}

trigger(); // After this, if buggy, arr[] points to a JS object but is handled as if it's a double

// With more bugs, this can be used to craft arbitrary read/write primitives

In a real exploit, additional steps and objects would be used to manipulate ArrayBuffers, DataViews, and pointers.

Real-World Impact

- Remote Code Execution: If an attacker gets a victim to visit a crafted website, they may be able to run code on the victim’s machine, escaping the browser sandbox.

References and Further Reading

- Chromium Security Advisory
- Chromium CVE-2024-7971 Bug Tracker *(Details may be restricted until users have updated)*
- V8 Blog *(General JIT and bug classes)*
- Project Zero: Exploiting Type Confusion in V8 (similar issue)

Closing Thoughts

CVE-2024-7971 is another serious reminder about the risks of complex JavaScript engines and the importance of browser updates. These bugs are not just theoretical — attackers *do* exploit them in the wild. For users, the takeaway is easy: Update Chrome right now.

For researchers and defenders, reviewing past and present V8/JIT bugs gives key insights into exploit trends and prevention strategies.


*Stay safe. Keep Chrome up-to-date. For more deep-dives like this, follow our blog!*

Timeline

Published on: 08/21/2024 21:15:09 UTC
Last modified on: 08/27/2024 01:00:02 UTC