Formidable (aka node-formidable) is a popular Node.js module for parsing form data, including file uploads. It's used by thousands of projects worldwide. However, a recent vulnerability, CVE-2025-46653, reveals that Formidable's method for generating upload filenames isn't as secure as many developers assumed.
This post breaks down what the issue is, why the hexoid library matters, the possible exploitation scenario (even if rare), and what you should do to keep your server safe.
The Issue
Formidable, from version 2.1. through 3.x before 3.5.3, uses a library called hexoid to generate unique filenames for uploaded files.
Here's the problem: hexoid's documentation itself warns that it is not cryptographically secure. That means, with enough tries and knowledge, a bad actor could potentially predict or "guess" filenames for temporary uploaded files.
Why does this matter?
If attackers can predict these filenames and manage to upload dangerous executable files, they might find ways to access or even execute them — depending on your system's configuration.
> Note: In most normal usage, there's not a direct path from hexoid to remote code execution. But, as we'll see, it's a weak link in what you hope is a strong chain.
👨💻 What's Going On in the Code?
When you upload a file, Formidable generates a filename using hexoid. Let's look at a simplified version:
const hexoid = require('hexoid');
const randomName = hexoid();
function handleUpload(file) {
const filename = randomName();
// Store or move the uploaded file to /uploads/${filename}
}
What hexoid does: It quickly makes a string of hexadecimal numbers (like a4b19c73) that's supposed to be unique. However, it's not using cryptographically strong randomness like crypto.randomBytes(). Instead, it’s much simpler and more predictable.
Main Risk: An attacker could try to guess filenames of uploads.
- Worst Case: If your server wrongly allows execution of these files (like handling /uploads as a web-accessible directory and running .js or .php files), this could open you to attack.
Attacker uploads a malicious file (e.g., a reverse shell script).
2. Because of the weak randomness, after a reasonable number of tries, they successfully guess the filename (e.g., ab1d23ef).
3. They access the file via https://yourserver.com/uploads/ab1d23ef.
If your server can execute that file, it's game over.
Special note: In a niche scenario, only the *last two characters* need to be guessed. This dramatically increases the risk for that use case, though *most developers won't run into this exact setup*.
> Important: For most users, this does not mean attackers can just upload and execute malicious content by exploiting hexoid alone. There usually must be additional mistakes in server configuration.
CVE Entry:
HEXOID Library (npm):
Formidable Documentation:
🇺🇸 TL;DR (In Plain Speak)
Formidable (before v3.5.3) makes upload filenames with a tool that’s easy for hackers to guess. If your server is set up wrong, this could let attackers run dangerous files. Upgrade Formidable and don’t let uploaded files be executable, and you’ll be fine.
👉 Stay updated, and check your dependencies often! If you want a deep dive, check out OWASP’s guide on secure file uploads.
Timeline
Published on: 04/26/2025 21:15:14 UTC
Last modified on: 04/29/2025 16:15:37 UTC