OAuth 2.0 and OpenID Connect: a practical guide for fullstack developers

OAuth 2.0 and OpenID Connect are the foundation of modern authentication and authorization. OAuth 2.0 handles authorization granting access to resources. OpenID Connect builds on OAuth 2.0 to handle authentication verifying who the user is. Understanding both is essential for building secure applications.

OAuth 2.0 defines several grant types for different scenarios. The Authorization Code grant is the most secure for web applications. The client redirects the user to the authorization server, which authenticates the user and redirects back with an authorization code. The client exchanges the code for an access token on the server side.

OpenID Connect adds an ID token to the OAuth 2.0 flow. The ID token contains claims about the user: their name, email, profile picture, and authentication time. Your application can decode and verify the ID token to identify the user without making additional API calls.

Always use PKCE with the Authorization Code grant. PKCE prevents authorization code interception attacks by requiring the client to prove it initiated the flow. Mobile apps and single-page applications should always use PKCE. It's become the recommended practice even for server-side applications.

Validate tokens on every request. Check the signature using the authorization server's public keys from the JWKS endpoint. Verify the token hasn't expired, the issuer matches your authorization server, and the audience includes your application.

Use a reputable OAuth/OIDC provider rather than building your own. Auth0, Clerk, Supabase Auth, and AWS Cognito handle the complex parts token management, session handling, MFA, and social login. Building your own OAuth server is extremely difficult to get right.

Store tokens securely. Access tokens should be short-lived. Refresh tokens should be stored in HTTP-only, Secure, SameSite cookies. Never expose tokens in URLs, logs, or error messages. Implement token rotation so a compromised refresh token is only usable once.

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Rizwan Saleem | https://rizwansaleem.co