oauth2-authentication

OAuth2 authentication uses the Authorization Code flow to securely delegate user login to an identity provider. Implement by redirecting users to the provider's authorization endpoint, capturing the returned authorization code, exchanging it for access tokens, and validating tokens to grant application access. This protects user credentials from your application.

PKCE (Proof Key for Code Exchange) prevents authorization code interception attacks in public clients like SPAs and mobile apps. It requires generating a code verifier, hashing it into a code challenge, sending the challenge during authorization, and proving possession of the verifier when exchanging the code for tokens. Required for public clients without a secure backend.

Access tokens grant temporary API access and should be short-lived; refresh tokens obtain new access tokens without re-authentication. Store refresh tokens securely (HTTP-only cookies or encrypted storage), validate access tokens on each API request, and rotate tokens regularly. Implement token expiration and revocation mechanisms to limit exposure if tokens are compromised.

Yes, the Client Credentials flow enables server-to-server OAuth2 authentication. The client application authenticates directly to the authorization server using its credentials, receives an access token, and uses it to access protected APIs. This flow suits backend services, microservices, and automated processes that don't involve end users.

OpenID Connect extends OAuth2 by adding an ID token containing user identity claims and a UserInfo endpoint for additional profile data. After obtaining an authorization code, exchange it for ID and access tokens, validate the ID token's signature and claims, and call UserInfo to retrieve user attributes. This provides both authentication and user information in one flow.

Use HTTPS for all endpoints, validate state parameters to prevent CSRF attacks, implement token expiration and rotation, validate all token signatures and issuer claims, and store secrets securely. For SPAs and mobile apps, use PKCE. Implement proper error handling without leaking sensitive information, and regularly audit authorization logic for compliance with OAuth2 security specifications.