[[recipe-securing-a-client-application-with-oauth]]
= Recipe: Securing a Client Application with OAuth

This section describes how to create an OAuth client application.

NOTE: We use "`OAuth`" and "`OAuth2`" interchangeably. Spring Security has moved away from the first version of OAuth.

[[understanding-oauth]]
== Understanding OAuth

Another way to secure an application is with OAuth. According to one https://oauth.net/[OAuth web site], OAuth is "`An open protocol to allow secure authorization in a simple and standard method from web, mobile and desktop applications.`"

To understand OAuth, you should first understand the difference between authentication and authorization.
Authentication is the process of proving that someone is who they say they are.
Authentication is typically done with a login screen that prompts for a user name and password (though it can be done by other means, such as fingerprint readers).
Authorization is the process of granting permission to do something.
Authorization does not (with one notable exception, which we cover later in this section) involve authentication.

In the typical OAuth authorization scenario, a third-party application wants authorization to do some action on another application, and it wants authorization to do whatever that action may be.
Consider the following common scenario. An application wants to add content (perhaps news about a certain subject) to your Facebook feed.
The application that wants to add news stories is the third-party application, and the end user can authorize Facebook to let that happen.
In this scenario (and many similar scenarios), we see three roles:

* User: The person who wants to get news stories added to their Facebook page.
* Client: The third-party application that has the news stories.
* Resource Server: Facebook's API that can grant authorization.

Another part of the process is the authorization server, which is the resource server's user interface that lets the Facebook user say OK to those news stories being added.
For small applications, the resource server and the authentication server may be the same thing. For larger applications, the authorization server is a separate application.

To make things more clear, consider the flow of actions in the process to which we have so far alluded in this section:

. The user finds a web application that offers to add news stories to their Facebook page and thinks that is a fine idea.
. The user clicks a button to make that happen.
. The news application (the client in this scenario) redirects to Facebook's authorization server.
The user may have to log in to the resource server (Facebook, in this case) at this point, to verify their identity.
. The user clicks an Accept (or similar) button to tell Facebook to grant permission to the news application.
. The user returns to the news application, which now has permission to add news stories to the user's Facebook page.

The whole process is not complicated, but it is substantially different than the security scenarios we have covered in other recipes, notably logging in may not be required.
Again, we deal with authorization rather than authentication.

=== OAuth and Redirect URIs

OAuth applications do not accept redirects from just anywhere.
Before such a redirect can work, the third-party (client) application must register with the resource server application (Facebook in our example).
Then the resource server knows that redirects from the client application are OK.

=== The Client ID and the Secret

When the user in the example we described earlier authorizes the client (the news service in our example), the resource server (Facebook in our example) sends a client ID to the client and may also send a secret to the client.
The client ID is public and identifies the user for the client application (the news service).
The secret (if it is sent) makes it easy for the client to complete transactions (sending news stories to the user's Facebook page).

==== When to Not Send Secrets

If the client cannot keep a secret, the resource server should not send a secret.
That happens for single-page web applications (such as the Angular application from the <<recipe-securing-an-angular-web-application>> recipe) and for mobile applications.
Because bad actors can get the secret in those situations, the resource server should not send a secret to mobile and single-page applications.

Instead, the resource server and client can use a secret that is generated for each request.
One standard for doing so is https://oauth.net/2/pkce/[PKCE] (Proof Key for Code Exchange).
PKCE exists "`to prevent certain attacks and to be able to securely perform the OAuth exchange from public clients.`"

For the resource server, the trick is knowing when to send a secret and when to use PKCE.
This can be determined when the client registers with the resource server.

== Writing the Client Application

NOTE: This section, including the application, was adapted from a Spring Security example at https://github.com/spring-projects/spring-security/tree/master/samples/boot/oauth2login. You can find many other Spring Security examples in the project that contains this application.

To create a client application that accesses Facebook, you must:

. <<oauth-adding-a-new-application,Add a new application>>

. Configure application.yml

. Boot up the application

[[oauth-adding-a-new-application]]
=== Adding a New Application

To use Facebook’s OAuth 2.0 authentication system for login, you must first https://developers.facebook.com/apps[Add a New App].

To do so from Facebook's New App page:

. Select *Create a New App*
+
The "Create a New App ID" page appears.

. Enter the *Display Name*, *Contact Email*, and *Category*
+
NOTE: The selection for the *Category* field is not relevant, but the field is requred. Select "Local" for its value.

. Click *Create App ID*.
+
The next page to appear is "Product Setup".

. Click the *Get Started* button for the Facebook Login product.

. In the left sidebar, under *Products → Facebook Login*, select *Settings*.

. For the field Valid OAuth redirect URIs, enter `http://localhost:8080/login/oauth2/code/facebook`.
+
For a real application, you would almost certainly have a URI that did not use `localhost`.
However the sample shown in this recipe uses localhost, so that you can run it locally.

. Click *Save Changes*.

The OAuth redirect URI is the path in the application that the end-user’s user-agent is redirected back to after they have authenticated with Facebook and have granted access to the application on the Authorize application page.

TIP: The default redirect URI template is `{baseUrl}/login/oauth2/code/{registrationId}`. The `registrationId` is a unique identifier for the `ClientRegistration`.

IMPORTANT: If the application runs behind a proxy server, you should check https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#appendix-proxy-server[Proxy Server Configuration] to ensure that the application is correctly configured. Also, see the supported https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#oauth2Client-auth-code-redirect-uri[URI template variables] for `redirect-uri`.

==== Configuring the `application.yml`

Now that you have created a new application with Facebook, you need to configure the sample application to use the application for the authentication flow. To do so:

. Go to `application.yml` (in the `resources` directory) and set the following configuration:
+
====
----
spring:
  security:
    oauth2:
      client:
        registration:	<1>
          facebook:		<2>
            client-id: facebook-client-id
            client-secret: facebook-client-secret
----
<1> `spring.security.oauth2.client.registration` is the base property prefix for OAuth client properties.
<2> The base property prefix follows the ID for the `ClientRegistration` -- in this case, `facebook`.
====

. Replace the values in the client-id and client-secret property with the OAuth 2.0 credentials you created earlier.

==== Creating the Application

The application consists of two small classes: a class with a main method that is annotated with `@SpringBootApplication` to create a Spring Boot application and an OAuth2 login controller. The following listing shows the application class:

====
[source,java]
----
package sample;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

/**
 * @author Joe Grandja
 */
@SpringBootApplication
public class OAuth2LoginApplication {

	public static void main(String[] args) {
		SpringApplication.run(OAuth2LoginApplication.class, args);
	}
}
----
====

The following listing shows the OAuth2 login controller class:

====
[source,java]
----
package sample.web;

import org.springframework.security.core.annotation.AuthenticationPrincipal;
import org.springframework.security.oauth2.client.OAuth2AuthorizedClient;
import org.springframework.security.oauth2.client.annotation.RegisteredOAuth2AuthorizedClient;
import org.springframework.security.oauth2.core.user.OAuth2User;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;

/**
 * @author Joe Grandja
 * @author Rob Winch
 */
@Controller
public class OAuth2LoginController {

	@GetMapping("/")
	public String index(Model model,
						@RegisteredOAuth2AuthorizedClient OAuth2AuthorizedClient authorizedClient,
						@AuthenticationPrincipal OAuth2User oauth2User) {
		model.addAttribute("userName", oauth2User.getName());
		model.addAttribute("clientName", authorizedClient.getClientRegistration().getClientName());
		model.addAttribute("userAttributes", oauth2User.getAttributes());
		return "index";
	}
}
----
====

You can find the original source for these classes plus test classes in the Spring Security samples at https://github.com/spring-projects/spring-security/tree/master/samples/boot/oauth2login.

== OAuth Resources

OAuth2 is defined by https://tools.ietf.org/html/rfc6749[IETF RFC (Request for Comment) 6749].
Two highly regarded and closely related web sites offer more detail.
Those sites are https://oauth.net/ and https://oauth.com/.
https://oauth.net/ is organized as a wiki. https://oauth.com/ is organized as a book.
Both are worth reading if you need to understand OAuth2 in depth.
