How do you use refresh tokens with different types of OAuth 2.0 clients and scopes?

1 Authorization Code Flow

The authorization code flow is the most common and secure way to obtain access and refresh tokens from an authorization server, making it suitable for web applications that can securely store the client credentials and communicate with the server via back-end channels. This process involves four steps: the client redirects the user to the authorization server, requests an authorization code with the desired scope and a refresh token, user authenticates and consents to the scope, and the server returns an authorization code to the client via a redirect URI. Then, the client exchanges the authorization code for an access token and a refresh token using its credentials and code. Finally, the client uses the access token to access the protected resources until it expires, and then uses the refresh token to request a new access token.

2 Implicit Flow

The implicit flow is a simplified version of the authorization code flow which does not require the client to exchange an authorization code for an access token. This flow is suitable for single-page applications or mobile apps which cannot store the client credentials or use back-end channels. It consists of two steps: first, the client redirects the user to the authorization server and requests an access token with the desired scope; second, after the user authenticates and consents to the scope, the server returns an access token to the client via a redirect URI. However, this flow does not support refresh tokens as it would expose them to the browser or app, which are not secure environments. As a result, the client must either rely on the access token lifetime or prompt the user to authenticate again when the token expires.

3 Resource Owner Password Credentials Flow

The resource owner password credentials flow is a way to obtain access and refresh tokens by directly sending the user's username and password to the authorization server. It is suitable for trusted applications that have a high degree of control over the user's credentials, such as legacy systems or native apps. The flow involves one step:

- The client sends the user's username and password, along with its credentials and the desired scope, to the authorization server and requests an access token and a refresh token.

The client uses the access token to access the protected resources until it expires, and then uses the refresh token to request a new access token. This flow is less secure than the other flows, because it requires the user to share their credentials with the client, and it bypasses the user's consent.

4 Client Credentials Flow

The client credentials flow is a way to obtain access tokens by using only the client's credentials, without involving the user. It is suitable for server-to-server applications that need to access protected resources that belong to the client itself, such as APIs or microservices. The flow involves one step:

- The client sends its credentials and the desired scope to the authorization server and requests an access token.

The client uses the access token to access the protected resources until it expires. The client credentials flow does not support refresh tokens, because there is no user involved in the process. Therefore, the client has to request a new access token when the previous one expires.

5 Benefits and Risks of Refresh Tokens

Refresh tokens offer several advantages for OAuth 2.0 clients and users, such as reducing the frequency of user authentication, allowing the client to maintain long-lived sessions with the protected resources, and enabling the client to request different scopes for different access tokens. However, there are some risks to consider—such as increasing the attack surface, requiring more storage and management, and introducing more complexity and potential errors. As a result, refresh tokens should be used with care and only when necessary, adhering to best practices and recommendations of the OAuth 2.0 specification.