Gängige OAuth2-Grant-Typen

Welcome! In OAuth2, a Grant Type (or 'Authorization Grant') is a method an application uses to get an access token from an authorization server.

Think of it as the specific procedure or negotiation protocol for obtaining permission to access protected resources.

You might wonder why there isn't just one way to get a token. Different applications have different security needs and capabilities:

• Web applications with a backend
• Single-page applications (SPAs) in a browser
• Mobile applications
• Command-line tools
• Server-to-server communication

Each scenario requires a tailored, secure approach.

The Authorization Code Grant is the most common and recommended grant type for confidential clients, especially traditional web applications with a backend.

It's considered the most secure because the access token is never exposed directly in the user's browser.

Here's how the Authorization Code flow typically starts:

1. The user clicks 'Login with X' on your app.
2. Your app redirects the user's browser to the Authorization Server (e.g., Google, GitHub).
3. The user logs in and grants permission to your app.
4. The Authorization Server then redirects the user's browser back to your app with a temporary authorization code.

After receiving the authorization code:

5. Your app's backend receives the authorization code.
6. It then securely exchanges this code (along with its own client ID and client secret) directly with the Authorization Server's token endpoint. This is a server-to-server communication.
7. The Authorization Server validates the code and client credentials, then issues an access token (and often a refresh token).

The Client Credentials Grant is used for machine-to-machine communication where there is no end-user involved.

The client (your application or service) acts on its own behalf, authenticating itself directly to the Authorization Server to get an access token.

The flow for Client Credentials is simpler:

1. Your client application (e.g., a background service) sends its client ID and client secret directly to the Authorization Server's token endpoint.
2. The Authorization Server verifies these credentials.
3. If valid, the Authorization Server directly issues an access token to your client.

No user interaction or browser redirects are needed.

Imagine a backend service that needs to query an external API to fetch data. It doesn't need a user to log in; it just needs access as 'itself'.

It would use the Client Credentials flow to get an access token:

The Device Code Grant is designed for input-constrained devices like smart TVs, IoT devices, or command-line tools that cannot easily host a web browser or accept redirects.

It separates the authorization process, allowing the user to authorize the device on a separate, more capable device (like a smartphone or computer).

Here's a simplified Device Code flow:

1. The device requests a device code and a user verification URI from the Authorization Server.
2. The device displays the URI and a short code to the user.
3. The user goes to the URI on their phone/PC, logs in, and enters the code to grant access.
4. Meanwhile, the device repeatedly polls the Authorization Server until authorization is confirmed, then it receives the access token.

We've explored key OAuth2 grant types: Authorization Code for secure web applications with user interaction, Client Credentials for server-to-server communication, and Device Code for input-constrained devices.

Each grant type addresses specific security and usability needs, ensuring secure authorization flows for different application scenarios.