Description

vm2's Bridge Proxy set trap ignores receiver parameter, enabling host object property injection via prototype chain

Summary

The BaseHandler.set trap in bridge.js (line 1231) ignores the receiver parameter and unconditionally writes to the host target object. Per the Proxy set trap specification, when receiver !== proxy (e.g., when a child object inherits from the proxy via Object.create), the property assignment should create an own property on the receiver, not on the proxy target. The current implementation always calls otherReflectSet(object, key, value) against the host target, causing all inherited property writes to leak through to the host object.

This bug provides an alternative attack vector for writing dangerous cross-realm Symbol keys (e.g., nodejs.util.promisify.custom) to host objects, bypassing any future per-trap isDangerousCrossRealmSymbol guard on the direct set path.

Vulnerable Code

// bridge.js:1231-1260
set(target, key, value, receiver) {
    validateHandlerTarget(this, target);
    const object = getHandlerObject(this);
    if (isProtectedHostObject(object)) throw new VMError(OPNA);
    // ...
    try {
        value = otherFromThis(value);...
Show more

Impact

Sandbox code can write arbitrary properties (including dangerous Symbol-keyed properties) to any host object it holds a reference to, by creating a prototype-inheriting child:

// Sandbox code
const child = Object.create(hostObj);
child.injectedProp = 'attacker-value';
// hostObj now has 'injectedProp' on the HOST side

Combined with the Symbol.for coverage gap, this enables semantic confusion attacks:

const kCustom = Symbol.for('nodejs.util.promisify.custom');
const child = Object.create(hostFunction);
child[kCustom] = function() {
    return Promise.resolve('attacker-controlled');
};
// Host: util.promisify(hostFunction)() returns 'attacker-controlled'

Reproduction

const { VM } = require('vm2');
const util = require('util');

const vm = new VM();
const hostFn = function api(cb) { cb(null, 'ok'); };
vm.setGlobal('hostFn', hostFn);

vm.run(`...
Show more

Suggested Fix

set(target, key, value, receiver) {
    validateHandlerTarget(this, target);
    const object = getHandlerObject(this);
    if (isProtectedHostObject(object)) throw new VMError(OPNA);
    if (isDangerousCrossRealmSymbol(key)) throw new VMError(OPNA);
    if (key === '__proto__' && !thisOtherHasOwnProperty(object, key)) {
        return this.setPrototypeOf(target, value);
    }...
Show more

Mitigation

Aliases

References