CVE-2026-18165
The CVE Severity Digest turns the National Vulnerability Database feed into a prioritized security briefing. It groups affected software into practical product families, distinguishes patch and mitigation references from general citations, and ranks the vulnerabilities most likely to matter to technology operators while preserving direct links to the official record.
- CVE
- CVE-2026-18165
- 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
- 08/15/2026
- Severity
- MEDIUM
- CVSS
- 4.2
- Priority score
- 47.558