A critical security vulnerability—CVE-2025-4918—was found in Mozilla Firefox and Thunderbird affecting how JavaScript Promise objects are handled. This flaw lets attackers perform out-of-bounds (OOB) read/write operations, potentially allowing data exposure or remote code execution. This long-read will explain the vulnerability in simple terms, present code demonstration, and share how attackers might exploit it. We’ll also point to relevant links for further reading.


## What is an Out-of-Bounds Read/Write?

An out-of-bounds read/write happens when a program accesses memory outside the area it's supposed to use. In C/C++, this often happens with arrays. In Javascript engines (like those in Firefox), these bugs sometimes appear inside powerful features—like Promises—deep under the hood where C++ meets JS.

How Does It Work?

A Promise combines complex memory management and concurrency features. When some property of a Promise gets mishandled—maybe because of bad length checking or a race condition—a malicious script can access or tamper with memory outside its intended boundary.

Code Example: Triggering the Vulnerability

Below is a simplified demonstration (it won’t crash a modern, patched browser—don't worry!) that shows how attackers craft Promises to mess with memory boundaries.

// This is a reproduction pattern, not an actual exploit
for (let i = ; i < 10000; i++) {
    Promise.resolve().then(function() {
        let arr = [1.1, 2.2, 3.3];
        // Simulate poorly checked Promise internals
        // In reality, a bug here in the JS engine might let arr outlive its bounds
        // Attacker could now try to read or write outside 'arr'
    });
}

In case of a vulnerable engine, an attacker can carefully manipulate the state so that the array or another object overlaps with engine internals, leading to OOB reads or writes.

Actual exploits use far more complex code and tools to ‘spray’ memory and ‘catch’ data or to hijack the program counter.

1. Heap Spraying

Attackers fill the heap (browser memory) with known data using lots of Promises and crafted objects.

2. Triggering OOB

They trigger the Promise bug, causing an out-of-bounds access.

3. Exfiltrating Data

With out-of-bounds read, attackers leak secrets (like pointers, or other objects’ values).

4. Arbitrary Write

In some rare cases, they write to an address of their choice (write-what-where).

5. Remote Code Execution

Attackers modify a piece of browser code or point browser code to their own instructions.

Thunderbird 138..2 or above

Disable JavaScript if you cannot update (not practical for most users).

References

- Mozilla Security Advisory 2025-4918
- NVD entry for CVE-2025-4918
- What is an Out-of-Bounds Vulnerability?

Final Thoughts

CVE-2025-4918 is a high-impact vulnerability that let attackers poke into browser memory through a flaw in JavaScript Promise handling in Firefox and Thunderbird. If you haven’t already, update your browser and mail client now—because sometimes, all it takes is a web page with a little too much Promise.

Timeline

Published on: 05/17/2025 22:15:19 UTC
Last modified on: 05/28/2025 14:05:35 UTC