CVE-2026-18165

@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 v3 4.2 MEDIUM

4.2^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 1.6 / Impact : 2.5

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required NONE

User Interaction REQUIRED

Confidentiality Impact LOW

Integrity Impact LOW

Availability Impact NONE

Scope UNCHANGED

References

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

Configurations

Configuration 1 (hide)

cpe:2.3:a:fastify:fastify\/oauth2:*:*:*:*:*:node.js:*:*

History

No history.

Information

Published : 2026-08-15 14:17

Updated : 2026-09-04 13:16

NVD link : CVE-2026-18165

Mitre link : CVE-2026-18165

CVE.ORG link : CVE-2026-18165

JSON object : View

Products Affected

fastify

fastify\/oauth2

CWE

CWE-352

Cross-Site Request Forgery (CSRF)

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