Bug 2517569 (CVE-2026-18165)

Summary: CVE-2026-18165 @fastify/oauth2: @fastify/oauth2: Login CSRF via predictable OAuth state cookies
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in @fastify/oauth2, an OAuth 2.0 plugin for Fastify. This vulnerability allows a remote attacker to perform a Login Cross-Site Request Forgery (CSRF) attack. By planting predictable OAuth state and code verifier cookies, an attacker can trick a victim's browser into completing an attacker-controlled OAuth flow. This results in the victim being silently signed into the attacker's account, though it does not expose the victim's own credentials or tokens.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-08-17 19:59:54 UTC 
@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.