OAuth 2.0

OAuth 2.0 is the most widely adopted authorization protocol on the internet, used by major providers such as Google, Facebook, GitHub, and Microsoft. It works by issuing access tokens to third-party clients after the resource owner grants permission, allowing those clients to interact with protected resources on behalf of the user.

The protocol defines four primary grant types: Authorization Code, Implicit, Resource Owner Password Credentials, and Client Credentials. Each grant type is designed for different use cases and threat models. The Authorization Code grant with PKCE (Proof Key for Code Exchange) is considered the most secure option for public clients such as single-page applications and mobile apps.

Common security concerns with OAuth 2.0 implementations include token leakage through redirect URI manipulation, insufficient scope validation, cross-site request forgery attacks on the authorization endpoint, and improper token storage on the client side. Developers must validate redirect URIs strictly, use short-lived access tokens with refresh tokens, enforce the principle of least privilege when defining scopes, and always transmit tokens over TLS. When implemented correctly, OAuth 2.0 provides a robust and flexible authorization layer for modern API-driven architectures.