Vulnerability Description
@fastify/oauth2 is an OAuth 2.0 plugin for Fastify. In versions from 7.2.0 up to but not including 8.3.0, the plugin validates the OAuth state, and with PKCE the code verifier, by comparing the callback query parameter against an unprefixed, predictable cookie, with no server-side binding to the browser that began the flow. Any party able to write a cookie for the application's host, such as a sibling subdomain under the same registrable domain, can plant matching state and verifier cookies and complete an attacker-owned OAuth flow inside a victim's browser, silently signing the victim in to the attacker's account (login CSRF). It does not expose the victim's own account, credentials, or tokens. The issue is fixed in @fastify/oauth2 8.3.0, which adds an opt-in hostPrefixedCookies option. Users should upgrade to 8.3.0 and enable it, or bind state to a server-side session.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify\/Oauth2 | >= 7.2.0, < 8.3.0 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlVendor Advisory
- https://github.com/fastify/fastify-oauth2/security/advisories/GHSA-p8h8-rj28-m8qMitigationVendor Advisory
FAQ
What is CVE-2026-18165?
CVE-2026-18165 is a vulnerability with a CVSS score of 4.2 (MEDIUM). @fastify/oauth2 is an OAuth 2.0 plugin for Fastify. In versions from 7.2.0 up to but not including 8.3.0, the plugin validates the OAuth state, and with PKCE the code verifier, by comparing the callba...
How severe is CVE-2026-18165?
CVE-2026-18165 has been rated MEDIUM with a CVSS base score of 4.2/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-18165?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify\/Oauth2.