Reference page — cumulative record through Sunday, October 4, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2026-18165

CVE-2026-18165MEDIUM

@fastify/oauth2 vulnerable to Login CSRF via plantable OAuth state cookies

  AV  AC  PR  UI  S  C  I  A   CVSS    EPSS   %ile   KEV
   N   L   N   R  U  L  L  N    5.4   .0010    0.8     —

AFFECTED
  Product          Versions  Fixed
  @fastify/oauth2  7.2.0 –   8.3.0

TIMELINE
  Jul 28  Reserved by openjs
  Aug 15  Published (CNA: openjs)
  Sep 4   RESCORED — CVE-2026-18165 (@fastify/oauth2). CVSS 4.2 → 5.4 (NVD).

CWE-352 · CNA: openjs · CVSS v3.1 · 2 references · NVD status: Analyzed

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.

Lifecycle

Complete event history — 3 events, chronological
Date	Event	Detail
July 28, 2026	Reserved	Reserved by openjs
August 15, 2026	Published	Published (CNA: openjs)
September 4, 2026	RESCORED	RESCORED — CVE-2026-18165 (@fastify/oauth2). CVSS 4.2 → 5.4 (NVD).

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
@fastify/oauth2	@fastify/oauth2	—	7.2.0	8.3.0

Weaknesses

CWE-352

References (2)

https://cna.openjsf.org/security-advisories.html [Vendor Advisory]
https://github.com/fastify/fastify-oauth2/security/advisories/GHSA-p8h8-rj28-m8q9 [Mitigation, Vendor Advisory]

Authoritative record: CVE-2026-18165 at cve.org

Vendors: @fastify/oauth2

Weaknesses: CWE-352

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-18165 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Sunday, October 4, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.