OAuth2 is the industry-standard protocol for authorization, enabling applications to securely delegate access without sharing credentials. However, choosing the right OAuth2 grant type can be a challenge, as each one is tailored to different use cases. In this post, we’ll explore the four primary OAuth2 grant types, explain when to use each, and provide practical code examples with Java and Spring Boot to give you a clearer understanding.

The Four Main OAuth2 Grant Types

Authorization Code Grant

The most common and secure grant type, the authorization code grant is ideal for server-side applications that can store secrets securely. This flow involves the client app obtaining an authorization code from the authorization server and then exchanging it for an access token. It’s well-suited for web apps and APIs that require strong security.

When to use:

• You have a backend server.
• You need to authenticate users and interact with an external API.
• Your application can securely store a client secret.

Java with Spring Boot Example:

Here’s a basic example of configuring OAuth2 Authorization Code Grant using Spring Security:

@Configuration
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.oauth2Login()
            .clientRegistrationRepository(clientRegistrationRepository())
            .authorizedClientService(authorizedClientService());
    }
    @Bean
    public ClientRegistrationRepository clientRegistrationRepository() {
        return new InMemoryClientRegistrationRepository(googleClientRegistration());
    }
    private ClientRegistration googleClientRegistration() {
        return ClientRegistration.withRegistrationId("google")
            .clientId("your-client-id")
            .clientSecret("your-client-secret")
            .redirectUriTemplate("{baseUrl}/login/oauth2/code/{registrationId}")
            .authorizationGrantType(AuthorizationGrantType.AUTHORIZATION_CODE)
            .scope("openid", "profile", "email")
            .authorizationUri("<https://accounts.google.com/o/oauth2/auth>")
            .tokenUri("<https://oauth2.googleapis.com/token>")
            .userInfoUri("<https://www.googleapis.com/oauth2/v3/userinfo>")
            .build();
    }
}

Implicit Grant

The implicit grant is an optimized flow designed for single-page applications (SPAs) or mobile apps, where redirecting through a server backend is impractical. In this flow, the client receives the access token directly from the authorization server without exchanging an authorization code.

When to use:

• You’re building a client-side application (SPA or mobile app).
• The access token needs to be obtained in the browser without server-side interaction.
• Security risks like token leakage are less concerning (considered less secure than authorization code).

Important Note:

• The implicit grant is being deprecated due to security vulnerabilities, so it’s recommended to use the Authorization Code Grant with PKCE in modern applications.

Password Grant

The password grant allows the user to provide their credentials (username and password) directly to the application, which then exchanges them for an access token. This flow is only suitable for first-party applications that have complete trust with the user.

When to use:

• The client application is a first-party app, and the user fully trusts it with their credentials.
• It’s not recommended for third-party apps, as this flow involves sharing credentials.

Java with Spring Boot Example:

Spring Boot can support password grant by enabling Resource Owner Password Credentials flow in your OAuth2 configuration:

spring.security.oauth2.client.provider.myprovider.token-uri=https://example.com/oauth/token
spring.security.oauth2.client.registration.myclient.client-id=your-client-id
spring.security.oauth2.client.registration.myclient.client-secret=your-client-secret
spring.security.oauth2.client.registration.myclient.authorization-grant-type=password
spring.security.oauth2.client.registration.myclient.scope=read,write

In this flow, you’d need to explicitly send the user’s credentials via an HTTP POST request to the token endpoint.

Client Credentials Grant

This grant is used when the client itself needs to authenticate without any user interaction, often when the app is acting on its own behalf. For instance, this is useful for machine-to-machine communications, like calling an external API to fetch resources.

When to use:

• Your application needs to authenticate itself with the authorization server.
• There is no end user, such as in backend microservices or daemon processes.

Java with Spring Boot Example:

Here’s a simplified Spring Boot configuration using the client credentials grant:

spring.security.oauth2.client.registration.myclient.client-id=your-client-id
spring.security.oauth2.client.registration.myclient.client-secret=your-client-secret
spring.security.oauth2.client.registration.myclient.authorization-grant-type=client_credentials
spring.security.oauth2.client.registration.myclient.scope=read,write

The backend service can now request access tokens using its client credentials.

Which OAuth2 Grant Type Should You Use?

To decide the most suitable grant type for your application, consider the following:

• Authorization Code Grant: Best for web apps and server-side applications where you need to act on the user’s behalf.
• Implicit Grant: Ideal for SPAs and mobile apps, but should be avoided due to security concerns. Use Authorization Code with PKCE instead.
• Password Grant: Use only for first-party apps, where the user fully trusts the client.
• Client Credentials Grant: Perfect for backend services or machine-to-machine communication.

Conclusion

Choosing the right OAuth2 grant type is crucial for balancing security, performance, and ease of integration. While Authorization Code with PKCE is the go-to for modern applications, the client credentials and password grants still have their place in specific scenarios. By understanding the nuances of each grant type and leveraging the power of Spring Boot’s OAuth2 support, you can implement secure and efficient authorization in your projects.