OAuth account takeover doesn't need leaked tokens. No state param = CSRF to forced account linking. Loose redirect_uri matching = code theft via open redirect chains. Implicit flow puts tokens in browser history and Referer headers. PKCE bypass when not enforced server-side. SSRF via OpenID dynamic client registration. Six patterns, all with labs. https://www.kayssel.com/newsletter/issue-43/ #OAuth #BugBounty #Pentesting #websecurity #Offsec #InfoSec