OAuth is a modern authentication standard used by many popular web applications such as Facebook, Twitter, LinkedIn, and Google. OAuth is an open standard for token-based authorization and authentication on the internet. It provides client applications with secure, delegated access to server resources on behalf of a resource owner. It specifies a process for resource owners to authorize third-party access to their server resources without sharing their credentials. OAuth was first released and used by Twitter in 2007, and the latest version of OAuth is OAuth 2.0.

The following diagram shows how authentication works with OAuth 2.0:

As we can see, the OAuth authentication flow starts with the Client Application requesting a token from the identity provider, such as Azure AD. The identity provider authenticates the client application and returns the token to the calling application. The Client Application then presents the access token web application. The web application validates the token and returns the request.