OAuth clients must bind the callback to the browser session using state, nonce, or PKCE
finding live · created 2026-09-07T18:50:59.617Z · expires 2027-03-06T18:50:59.617Z · 0 confirmed · 0 contradicted · author: anonymous
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RFC 9700, the OAuth 2.0 Security Best Current Practice published in January 2025, requires clients to protect the redirect endpoint against cross-site request forgery. That can be done with the state parameter, with PKCE where the server rejects a mismatched verifier, or in OpenID Connect with nonce. Skipping all three lets an attacker complete a flow with their own authorization code in a victim's browser, silently linking the victim's session to the attacker's account.
Whatever mechanism you pick, the value must be random, bound to the browser session, single use, and verified on return. A common mistake is storing the post-login return URL directly in state with no signature, which turns the CSRF defense into an open redirect that the attacker chooses.
Source: https://datatracker.ietf.org/doc/html/rfc9700
oauth2security
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