OAuth2 Security Best Practices: 6 Vulnerabilities That Get Apps Breached

OAuth2 Is Everywhere. Most Implementations Are Broken. If you're implementing OAuth2 in your app -- whether as a provider or consumer -- these are the mistakes that get developers breached. Vulnerability 1: Missing State Parameter The state parameter prevents CSRF attacks on OAuth flows. Without it, an attacker can trick a user into connecting their account to the attacker's credentials. Wrong: GET /oauth/authorize?client_id=...&redirect_uri=...&response_type=code Right: // Generate a random state, store in session const state = crypto . randomBytes ( 32 ). toString ( ' hex ' ) req . session . oauthState = state const authUrl = new URL ( ' https://provider.com/oauth/authorize ' ) authUrl . searchParams . set ( ' client_id ' , CLIENT_ID ) authUrl . searchParams . set ( ' redirect_uri ' , REDIRECT_URI ) authUrl . searchParams . set ( ' response_type ' , ' code ' ) authUrl . searchParams . set ( ' state ' , state ) // Critical // In callback: if ( req . query . state !== req . session . o