SandboxJS is a JavaScript sandboxing library. Prior to 0.8.29, a sandbox escape is possible by shadowing hasOwnProperty on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to __proto__ and other blocked prototype properties, enabling host Object.prototype pollution and persistent cross-sandbox impact. This vulnerability is fixed in 0.8.29.

Affected: npm: @nyariv/sandboxjs

## Summary A sandbox escape is possible by shadowing `hasOwnProperty` on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to `__proto__` and other blocked prototype properties, enabling **host `Object.prototype` pollution** and persistent cross-sandbox impact.

The issue was reproducible on Node `v23.9.0` using the project’s current build output. The bypass works with default `Sandbox` configuration and does not require custom globals or whitelists.

## Root Cause `prototypeAccess` uses `a.hasOwnProperty(b)` directly, which can be attacker‑controlled if the sandboxed object shadows `hasOwnProperty`. When this returns `true`, the whitelist checks are skipped.

- [src/executor.ts:348](https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L348) `const prototypeAccess = isFunction || !(a.hasOwnProperty(b) || typeof b === 'number');`

<img width="1030" height="593" alt="image" src="https://github.com/user-attachments/assets/0fa0807e-81cc-45b5-be13-bd839c974a4f" />

- [src/executor.ts:367-399](https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L367) prototype whitelist enforcement only happens when `prototypeAccess` is true.

<img width="929" height="345" alt="image" src="https://github.com/user-attachments/assets/27cff24d-b892-4d56-9f59-1e5fd32ef471" />

- [src/executor.ts:220-233](https://github.com/nyariv/SandboxJS/b…