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.
Published: 2026-08-15
Score: 4.2 Medium
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

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Remediation

No vendor fix or workaround currently provided.

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Tracking

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History

Mon, 17 Aug 2026 20:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Mon, 17 Aug 2026 11:30:00 +0000

Type Values Removed Values Added
First Time appeared Fastify
Fastify oauth2
Vendors & Products Fastify
Fastify oauth2

Sat, 15 Aug 2026 14:00:00 +0000

Type Values Removed Values Added
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.
Title @fastify/oauth2 vulnerable to Login CSRF via plantable OAuth state cookies
Weaknesses CWE-352
References
Metrics cvssV3_1

{'score': 4.2, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N'}


cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-08-17T19:41:55.649Z

Reserved: 2026-07-28T20:21:36.198Z

Link: CVE-2026-18165

cve-icon Vulnrichment

Updated: 2026-08-17T19:41:49.769Z

cve-icon NVD

Status : Received

Published: 2026-08-15T14:17:07.290

Modified: 2026-08-17T20:16:41.040

Link: CVE-2026-18165

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-17T10:59:23Z

Weaknesses