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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-18165
StatePUBLISHED
Assigneropenjs
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-15 14:17:07 UTC
Updated2026-08-28 19:46:29 UTC
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.

Risk And Classification

Primary CVSS: v3.1 4.2 MEDIUM from ce714d77-add3-4f53-aff5-83d477b104bb

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

EPSS: 0.000990000 probability, percentile 0.008930000 (date 2026-08-30)

Problem Types: CWE-352 | CWE-352 CWE-352: Cross-Site Request Forgery (CSRF)

VersionSourceTypeScoreSeverityVector
3.1ce714d77-add3-4f53-aff5-83d477b104bbSecondary4.2MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
3.1CNACVSS4.2MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA @fastifyoauth2 @fastify/oauth2 affected 7.2.0 8.3.0 semver Not specified
CNA @fastifyoauth2 @fastify/oauth2 unaffected 8.3.0 semver Not specified

References

ReferenceSourceLinkTags
cna.openjsf.org/security-advisories.html ce714d77-add3-4f53-aff5-83d477b104bb cna.openjsf.org
github.com/fastify/fastify-oauth2/security/advisories/GHSA-p8h8-rj28-m8q9 ce714d77-add3-4f53-aff5-83d477b104bb github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: kleju13 (en)

CNA: mcollina (en)

CNA: UlisesGascon (en)

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