Which OAuth 2.0 Flow Should I Use?
The OAuth 2.0 Authorization Framework supports several different flows (or grants). Flows are ways of retrieving an Access Token. Deciding which one is suited for your use case depends mostly on your application type, but other parameters weigh in as well, like the level of trust for the client, or the experience you want your users to have.
OAuth 2.0 terminology
Resource Owner: Entity that can grant access to a protected resource. Typically, this is the end-user.
Client: Application requesting access to a protected resource on behalf of the Resource Owner.
Resource Server: Server hosting the protected resources. This is the API you want to access.
Authorization Server: Server that authenticates the Resource Owner and issues Access Tokens after getting proper authorization. In this case, Auth0.
User Agent: Agent used by the Resource Owner to interact with the Client (for example, a browser or a native application).
Is the Client the Resource Owner?
The first decision point is about whether the party that requires access to resources is a machine. In the case of machine-to-machine authorization, the Client is also the Resource Owner, so no end-user authorization is needed. An example is a cron job that uses an API to import information to a database. In this example, the cron job is the Client and the Resource Owner since it holds the Client ID and Client Secret and uses them to get an Access Token from the Authorization Server.
If this case matches your needs, then to learn how this flow works and how to implement it, see Client Credentials Flow.
Is the Client a web app executing on the server?
If the Client is a regular web app executing on a server, then the Authorization Code Flow is the flow you should use. Using this the Client can retrieve an Access Token and, optionally, a Refresh Token. It's considered the safest choice since the Access Token is passed directly to the web server hosting the Client, without going through the user's web browser and risking exposure.
If this case matches your needs, then to learn how this flow works and how to implement it, see Authorization Code Flow.
Is the Client absolutely trusted with user credentials?
This decision point may result in the Resource Owner Password Credentials Grant. In this flow, the end-user is asked to fill in credentials (username/password), typically using an interactive form. This information is sent to the backend and from there to Auth0. It is therefore imperative that the Client is absolutely trusted with this information.
This grant should only be used when redirect-based flows (like the Authorization Code Flow) are not possible. If this is your case, then to learn about how this flow works and how to implement it, see Resource Owner Password Flow.
Is the Client a Single-Page App?
To learn more about how this flow works and how to implement it, see Authorization Code Flow with Proof Key for Code Exchange (PKCE). The Auth0 Single-Page App SDK provides high-level API for implementing Authorization Code Flow with PKCE in SPAs.
If your SPA doesn't need an Access Token, you can use the Implicit Flow with Form Post. To learn more about how this flow works and how to implement it, see Implicit Flow with Form Post.
Is the Client a Native/Mobile App?
If the Application is a native app, then use the Authorization Code Flow with Proof Key for Code Exchange (PKCE).
To learn more about how this flow works and how to implement it, see Authorization Code Flow with Proof Key for Code Exchange (PKCE).