OAuth 2.0 and OpenID Connect are widely used in modern applications, but they are often implemented without a full understanding of their roles. As a result, incorrect implementations and security vulnerabilities appear.

In this article, we explain how OAuth 2.0 and OpenID Connect really work.

OAuth 2.0 – An Authorization Protocol

OAuth 2.0 is used for delegating access to protected resources. It is not an authentication protocol.

Typical OAuth use cases include:

• application access to APIs,
• integrations with external systems,
• mobile applications and SPAs.

Grant Types

The most commonly used flow today is the Authorization Code Flow with PKCE, especially in frontend applications.

OpenID Connect – The Identity Layer

OpenID Connect (OIDC) extends OAuth 2.0 by adding user authentication. It provides:

• an ID Token,
• user identity information,
• standardized login mechanisms.

OIDC answers the question: who the user is, not just whether they have access.

Common Implementation Mistakes

• using OAuth for login without OIDC,
• missing token validation,
• insecure token storage.

OAuth and OIDC in Practice

A correct implementation requires:

• short-lived access tokens,
• secure storage,
• refresh token rotation,
• validation of signatures and audience claims.

Summary

OAuth 2.0 and OpenID Connect solve different problems but are often confused. Proper understanding of both is essential for building secure and scalable authentication systems.