piscina is a node.js worker pool implementation. Prior to 6.0.0-rc.2, 5.2.0, and 4.9.3, piscina's constructor and run() paths read the filename option via plain member access. 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 the inherited value flows to worker_threads.Worker import and the attacker's .mjs runs in the worker. This vulnerability is fixed in 6.0.0-rc.2, 5.2.0, and 4.9.3.

Affected: npm: piscina

## Summary

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

```js // 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 precondi…