CVE-2023-4046 - Deep Dive Into the Firefox WASM JIT Global Variable Vulnerability

In July 2023, Mozilla patched a notable security flaw in Firefox's handling of WebAssembly (WASM) code. Tracked as CVE-2023-4046, the vulnerability stems from how the JIT (Just-In-Time) compiler handled global variables in some circumstances. If the bug was triggered, it could cause incorrect code execution and, potentially, crash the browser. Worse, a clever attacker could exploit this to run malicious code.

This long read explains what went wrong, how it can be exploited, and how it was fixed. If you're a developer, cybersecurity professional, or just a curious web user, read on for an exclusive, easy-to-understand breakdown.

What Exactly is CVE-2023-4046?

Summary:  
A stale value (an out-of-date value) for a WebAssembly global variable could be used during certain JIT analysis steps in Firefox. This meant the JIT compiler might generate incorrect machine code, leading to security vulnerabilities, including browser crashes or, in some cases, arbitrary code execution.

Firefox ESR before 115.1

Original Mozilla Security Advisory:  
MFSA 2023-30  
Bugzilla Issue #1848321

WebAssembly, JIT, and Global Variables: The Basics

- WebAssembly (WASM): A binary format for code that runs in browsers at near-native speed. It's sandboxed and fast.

JIT Compiler: Converts WASM into native machine code *as* it's needed, for speed.

- Global Variables: Variables accessible anywhere within the WASM module. Used for sharing state between functions.

Firefox's WASM engine analyzes and optimizes WebAssembly code as it runs. But in its analysis, a bug allowed an *old* value of a global variable to be used when generating code for *new* accesses. This could cause the JIT to "miscompile" the WASM, making it buggy or dangerous.

What Went Wrong? Technical Explanation

The bug was in how the JIT compiler “tracked” changes to global variables. If a global variable was changed, but the analysis failed to "see" the new value, the JIT would generate code assuming the global was still set to its previous value. This stale information taints further analysis.

Mini Example

Let's say a WASM module defines a mutable global variable and changes it. If the JIT doesn't update its record, the wrong value is used, which might lead to invalid memory access or code paths.

Vulnerable Pseudocode

// Sample WASM with a mutable global
(module
  (global $g (mut i32) (i32.const ))
  (func $setGlobal (param $val i32)
    (global.set $g (local.get $val)))
  (func $getGlobal (result i32)
    (global.get $g))
  (export "setGlobal" (func $setGlobal))
  (export "getGlobal" (func $getGlobal))
)

When $g is changed, the JIT should "know" it and use the new value. But with CVE-2023-4046, it might use the *old*  instead.

Exploitation: Why is This Dangerous?

Attackers could use this bug to confuse the JIT, tricking it into writing buggy native code. In a browser, this might let them crash the content process. Worse, with enough creativity, a crash can become a security exploit—like running their code or reading memory they shouldn't.

Leveraging the miscompilation for privilege escalation or sandbox escape.

Proof of concept:  
Public exploits are not available (as of writing), but the bug’s logic allows for similar exploitation patterns found in other JIT-related vulnerabilities: repeatedly mutate globals, trigger recompilation, and observe misbehavior.

Code Snipplet: Demonstrating the Issue

Below is a simple WebAssembly JavaScript interaction. While this doesn't exploit CVE-2023-4046 *directly*, it illustrates how quick global changes are possible:

const wasm_code = new Uint8Array([
  // ... WASM bytecode for the module above ...
]);

WebAssembly.instantiate(wasm_code).then(obj => {
  obj.instance.exports.setGlobal(42);
  let current = obj.instance.exports.getGlobal();
  console.log("Global is: ", current); // Should log 42
});

If Firefox’s JIT had the stale-value bug, the console.log might print the wrong value (like ). With creative code, this confusion can be leveraged for exploitation.

References

- CVE-2023-4046
- Mozilla Security Advisory MFSA 2023-30
- Mozilla Bugzilla Issue #1848321
- WebAssembly Reference

Fixed in: Firefox 116, Firefox ESR 102.14, ESR 115.1

Mozilla quickly patched the bug after private reporting. If you use Firefox, update right away!

Conclusion

CVE-2023-4046 shows how small bugs in JIT analysis can have big security consequences. It’s another reminder that WASM, while powerful, needs careful handling in browsers.

Be wary of suspicious sites and WASM demos.

*Security is a process—bugs like CVE-2023-4046 remind us why transparent reporting and quick patching matter.*


Sources:  
- Mozilla Security Advisory MFSA 2023-30  
- Bug #1848321  
- CVE database  

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Timeline

Published on: 08/01/2023 15:15:00 UTC
Last modified on: 08/09/2023 21:15:00 UTC