In June 2024, a subtle but serious vulnerability shook the JavaScript world: CVE-2024-7652. This bug, tied to a flaw in the ECMA-262 specification's handling of Async Generators, could lead to type confusion, memory corruption, and potentially even remote code execution. This article unpacks the vulnerability, demonstrates a simplified exploit scenario, and serves as a guide for users and developers concerned about the affected products—Firefox < 128, Firefox ESR < 115.13, and Thunderbird < 115.13 and < 128.

1. What is CVE-2024-7652?

CVE-2024-7652 is a flaw in how Mozilla's SpiderMonkey engine (the core JavaScript engine for Firefox and Thunderbird) implements Async Generators as defined by ECMA-262. The error resides in a situation where the engine mishandles the type of values yielded in certain async generator scenarios. This can confuse internal memory management, potentially allowing malicious Javascript to corrupt memory or crash the browser.

Affected Software

| Product | Versions Vulnerable |
|--------------|-----------------------------|
| Firefox | All before 128 |
| Firefox ESR | All before 115.13 |
| Thunderbird | All before 115.13 & 128 |

2. Understanding Async Generators and the Spec Error

JavaScript Async Generators mix asynchronous and generator functions to allow yielding asynchronously produced values, like this:

async function* numbers() {
  yield 1;
  yield 2;
  await new Promise(r => setTimeout(r, 100));
  yield 3;
}

The ECMA-262 specification defines exactly how the "generator state" changes internally. A mismatch or ambiguity here—like assuming a value is always an object when it might not be—can lead to type confusion.

Type confusion errors happen when the program misinterprets the type of a value—thinking it’s a string, for example, when it’s a pointer. This can lead to writing or reading arbitrary memory, paving the way to memory corruption or code execution.

3. How Could the Exploit Work?

The vulnerable code path involves async generators returning unexpected or malformed values, breaking the expectations of the engine's internal structures.

Proof-of-Concept

_Note: This snippet is a simplified demonstration and is benign. Do not use this code for malicious purposes. It demonstrates the kind of logic that could be abused._

async function* exploitedGenerator() {
  // Yield a crafted value that will confuse the engine's expectations
  let evilObj = { toString: () => { 
    // Possible manipulation when object's value is coerced
    // In a real exploit, you might try to tamper with memory here
    return "123"; 
  }};
  yield evilObj;

  // Return a value that triggers an edge case in the spec
  return 42;
}

(async () => {
  const gen = exploitedGenerator();
  for await (const value of gen) {
    // Process the value, which could be unexpectedly crafted
    console.log(value);
  }
})();
// In a real attack, the generator’s output could trick
// the engine into an unsafe operation, causing a crash or corruption.

How Attackers Could Leverage This

If the internal type expectation is broken (say, the engine treats a primitive as an internal pointer), an attacker could:

In some scenarios, execute arbitrary code

But the exploitability depends on specific security mitigations in place (like heap isolation and process sandboxing).

4. Official References

- Mozilla Foundation Security Advisory 2024-24
- Bugzilla Report on CVE-2024-7652 (replace XXXXX with CVE number or search)
- ECMA-262, §25.5 AsyncGenerator Objects

What Should You Do?

- Upgrade Firefox and Thunderbird immediately: New versions fix the broken logic that caused the confusion.
- If running a vulnerable version, avoid untrusted JavaScript or sketchy websites/mail until patched.

Vendor Patch Example (C++ Internal)

Mozilla’s fix included tightening internal checks on what types can be returned/yielded from async generators.

// Pseudocode for type sanity check in SpiderMonkey
if (!actualValue.isExpectedType()) {
  throw new TypeError("Unexpected type in Async Generator");
}

6. Conclusion

CVE-2024-7652 is a reminder that even small specification ambiguities in language runtimes can lead to serious security risks—especially around new features like async generators. If you use the affected software, upgrade now. Developers: always check for updated specifications, secure coding guides, and respect the complex dance of Javascript’s event loop, types, and memory model.

Stay safe, and keep your browsers and mail clients patched!

References

- Mozilla Security Advisory
- ECMA-262 Specification
- Async Generators on MDN


*Exclusively written for educational purposes. Share responsibly!*

Timeline

Published on: 09/06/2024 19:15:12 UTC
Last modified on: 09/12/2024 15:48:01 UTC