Description

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:

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
    }
  }...
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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');...
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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]...
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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]
}

Mitigation

Aliases

References