Vulnerability · Published Aug 15, 2026 · Assigned by openjs
CVE-2026-18165@fastify/oauth2 vulnerable to Login CSRF via plantable OAuth state cookies
Medium 4.2
In plain English
A medium-severity cross-site request forgery in @fastify/oauth2 from @fastify/oauth2. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 2% of all known vulnerabilities).
Official 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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| @fastify/oauth2 | @fastify/oauth2 | 7.2.0 – before 8.3.0 |
Weakness: CWE-352 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N