1. Introduction
Are you confused about the type of OAuth2 you should be using? Or do you not even know that there are several grant types in the OAuth2 protocol? Here’s a quick guide to help you understand the differences between them and determine which one you should use.
2. 4 Authorization Grant Types
- Authorization Code Grant
The authorization code grant is the most common, secure, and recommended grant type. After the resource owner grants permission, the authorization server redirects them to a callback URL with an authorization code in the query string. The client then exchanges this code for an access token.
Why is this secure? Because the resource owner only authenticates with the authorization server, meaning their credentials are never shared with the client. Additionally, the access token is transmitted by an authenticated client rather than through the user-agent, reducing exposure to malicious applications.
- Implicit Grant
The implicit grant is a simplified version of the authorization code grant. Instead of receiving an authorization code first, the client immediately gets an access token after the resource owner’s consent.
This grant type is commonly used for client-side applications where securely storing client credentials isn’t possible. However, since the access token is transmitted in the query string, it is more vulnerable to exposure. Due to these security concerns, this grant type is considered obsolete. Even for public clients, it’s recommended to use the authorization code grant with additional security enhancements, such as PKCE.
- Resource Owner Password Credentials Grant
With this grant type, the resource owner directly shares their credentials (e.g., username and password) with the client, which then uses them to request an access token.
This approach is inherently insecure since it requires trusting the client not to store or misuse the credentials. As a result, it should only be used in legacy applications where other grant types are not feasible.
- Client Credentials Grant
The client credentials grant is used for server-to-server authentication, where a client (such as a backend service) needs to access protected resources without user involvement. The client authenticates itself with its own credentials to obtain an access token.
This grant type is only suitable for machine-to-machine communication.
3. Conclusion
OAuth2 offers four grant types. The authorization code grant remains the most secure and recommended option. The implicit grant is now considered obsolete due to security risks. The password credentials grant should only be used in legacy systems, while the client credentials grant is best suited for machine-to-machine communication.
What do you think about these grant types? Let me know in the comments!