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Introduction to OAuth 2.0

What is OAuth 2.0?

OAuth 2.0 is a simple authorization (access delegation) protocol that allows a third-party application to obtain limited access to a secured resource, on behalf of the resource owner without sharing the password.

Why OAuth 2.0?

One of the key reasons for an enterprise to use OAuth 2.0 is for the access delegation, where a resource owner can give required permissions to a third-party application to access a resource located in a resource server.

It decouples authentication from authorization and supports multiple use cases addressing different client types such as server-to-server apps, browser-based apps, mobile/native apps, and consoles/TVs.

This protocol which replaces the password-sharing anti-pattern and encourages Federated Identity, is more secure and usable. When working with the tokens it guarantees API security by relying on Secure Sockets Layer (SSL) that ensures data between the web server and browsers remain private. If a user can log in to a single OAuth 2.0 provider then they can log in to other web services without logging in to those web services differently.

Which OAuth 2.0 grant type to choose?

OAuth 2.0 supports different grant types.

  • Authorization code
  • Client credential
  • Implicit

These different grant types target different application types.

Flow Usage
Authorization code grant This is mostly recommended for web applications and single-page applications (SPA). This method is very safe in web applications, as tokens are passed directly to the web server and stored there without exposing the tokens to the resource owners. For single-page applications (JavaScript clients), it is recommended to use this grant type with PKCE.
Client credential grant Recommended for machine to machine communications (CLIs), if a machine needs to access a resource without human interaction.
Implicit grant This is used for single-page applications (SPA) to retrieve the access tokens directly without spending several round trips. However, there is a security consideration, as the access token is exposed to the client-side.

How does it work?

The exact flow differs based on the OAuth 2.0 grant type. However, the abstract protocol flow is as follows.

OAuth 2.0 flow

  1. The application requests the authorization from the end-user (resource owner).
  2. The resource owner grants the authorization to the application.
  3. The application grants the authorization to the authorization server (OAuth 2.0 provider).
  4. The server generates the access tokens and passes them back to the client (relying party).
  5. The client passes the access token to the resource server.
  6. The resource server provides access to the protected resource.