piscina is a node.js worker pool implementation. Prior to 4.9.4, 5.3.2, and 6.0.0-rc.5, Piscina stores ThreadPool.options in src/index.ts as a plain object that inherits from Object.prototype. Applications with a separate prototype-pollution primitive can therefore supply inherited values for security-sensitive options that do not have own defaults. An inherited execArgv value is passed to the Node.js Worker constructor and can preload attacker-controlled code in worker threads, an inherited loadBalancer function can execute during task scheduling, and inherited env values can alter worker environments. This issue is fixed in versions 4.9.4, 5.3.2, and 6.0.0-rc.5.

Affected: npm: piscina

### Summary

A prototype-pollution gadget in `ThreadPool.options` allows an attacker who can pollute `Object.prototype` to execute arbitrary code in Piscina worker threads, invoke arbitrary functions during task scheduling, or inject environment variables into workers. The root cause is that `ThreadPool.options` is created as a plain object inheriting from `Object.prototype`, so any option without an explicit default in `kDefaultOptions` can be supplied via the prototype chain.

### Details

In `src/index.ts` the `ThreadPool` constructor builds the resolved options object as a plain object:

```ts this.options = { ...kDefaultOptions, ...options, filename, maxQueue: 0 } ```

Because this object has `Object.prototype` as its prototype, reads for properties that are not own properties of the object and are not present in `kDefaultOptions` fall back to `Object.prototype`. This means a prototype-pollution primitive (e.g. from a vulnerable `merge()` or `JSON.parse` merge elsewhere in the application) can inject values for `execArgv`, `env`, `loadBalancer`, `argv`, `workerData`, `resourceLimits`, `niceIncrement`, `closeTimeout`, `recordTiming`, `stricterFIFO`, `workerHistogram`, and `trackUnmanagedFds`.

The most serious gadget is `execArgv`, which is passed directly to `new Worker(..., { execArgv })`. An attacker can set `Object.prototype.execArgv = ['--require', '/tmp/attacker.js']`, causing every worker to preload and execute the attacker-controlled module on startup.

This issu…