CVE-2025-1933 - JIT WASM Return Value Memory Corruption in Firefox and Thunderbird (<136)

A new and significant vulnerability, CVE-2025-1933, has been discovered in Mozilla’s products, specifically targeting the Just-In-Time (JIT) compiler for WebAssembly (WASM) on 64-bit CPUs. This flaw has far-reaching consequences, potentially allowing attackers to read, leak, or manipulate memory in unexpected ways. This post gives a clear explanation, proof-of-concept code, and practical guidance for users and developers.

What Is CVE-2025-1933?

At its core, the vulnerability comes from how the JIT compiler handles WASM i32 return values on 64-bit CPUs. When a WASM function returns a 32-bit integer, the JIT compiler sometimes copies (or "leaks") leftover bits from memory. These extra bits could include sensitive information from previous operations, or even let the attacker craft a value that gets misinterpreted as a different, potentially more dangerous type.

How Does It Happen?

Under normal circumstances, when you return an i32 from WASM, the upper 32 bits of the 64-bit register *should* be zeroed out. Due to CVE-2025-1933, the WASM JIT compiler sometimes doesn’t clear these, allowing bits from older data in memory, which might hold sensitive information—think crypto keys, pointers, or data processed earlier—to be attached to the return value.

Suppose you have this WASM function

(func $leaky (result i32)
  ;; Some operations
  i32.const 42
  ;; return 42
)

When compiled without this bug, you’d get x0000002A_00000000 in a 64-bit register (rax), and only the lower 32-bits (x2A) would be used.

With the bug

The upper 32-bits might still contain old, potentially sensitive data.

Proof-of-Concept Exploit

Here’s how an attacker can potentially exploit this bug using JavaScript and WASM. The attacker crafts a situation where the upper 32 bits of a register retain old memory from a sensitive operation, then reads them back:

const wasmCode = new Uint8Array([
  // minimal wasm code to trigger leaky i32 return on 64-bit CPU
  x00,x61,x73,x6d, // WASM binary magic
  x01,x00,x00,x00, // WASM binary version
  // ... rest of your WASM code
]);

WebAssembly.instantiate(wasmCode).then(obj => {
  // Call the leaky function
  let result = obj.instance.exports.leaky();
  // On CVE-2025-1933-vulnerable browser, result may contain leftover bits
  console.log("Returned value:", result);
});

Depending on system architecture and luck, the attacker may observe the value as not simply 42 but with extra upper bits set, which might encode leaked secrets.

Information Disclosure: Attackers may mine process memory for secrets or pointers.

- Type Confusion: Corrupted return values can trick the browser into treating data as a different type—potentially a stepping stone to more severe bugs or code execution.
- Sandbox Bypass: In advanced scenarios, this might help break out of WASM or JavaScript sandboxes.

Update Thunderbird to 136 or 128.8 or later

Always keep your browser and email client up to date!

References and More Reading

- Mozilla Foundation Security Advisory 2025-13
- CVE-2025-1933 MITRE Entry
- JIT Security: Lessons from the Trenches (Blog)
- WebAssembly Specification

Conclusion

CVE-2025-1933 is a clear reminder that low-level memory handling—even in modern, safe languages like WebAssembly running inside a browser—can still be a security minefield if JIT compilers leave old data behind. If you work with web apps or handle sensitive information in Firefox or Thunderbird, update your software now.

Timeline

Published on: 03/04/2025 14:15:38 UTC
Last modified on: 03/25/2025 14:15:27 UTC