Spring Boot (1.5) OAuth2 Server in Enterprise environment

The problem

If we want to have an array of microservices and support user interaction through delegated authorization, this implementation would be one of the options to consider or at least review. We have to first understand the differences between OAuth2 and e.g. OIDC before we continue explaining how to achieve OAuth2 implementation in Spring Boot in a way that is stateless and integrated so many microservices can rely on this implementation.  OAuth 2.0 is the industry-standard protocol for authorization where OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It allows Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server, as well as to obtain basic profile information about the End-User in an interoperable and REST-like manner. This does not mean that OAuth2 cannot do authorization, it is just that OIDC is better suited for this work.

We are going to focus on OAuth2 implementation in this article. There are several good references for reading that you can look for before venturing to create your own implementation (one, two, three, four, five, six). My reasons to do this were several, like not having OAuth2 service available, need for microservices in our architecture, uncertain structure of the client-server architecture and several other. Since we already worked with Spring Boot implementing this solution was the next best thing we could do to move our project further towards our goals.

There are two ways we can handle tokens in OAuth2 and those are plain token and JWT token. For our purposes we chose plain token implementation. The difference was that plain token needs to be verified by OAuth2 service every time it is accessed and JWT can be stored in the resource and verified by the public keys provided. Either way we can have a working solution but implementing plain tokens was a simpler and faster way to go.

Our requirements were to provide solution for the stateless authentication/authorization for the we client, server had to have a small footprint, had to be scalable, we had to see the tokens/users that we generated, we had to have revoke token capability, we had to provide automated solution to obtain the token to integration testing, microservices had to be able to both authenticate themselves and take user authenticated tokens, we had to have ability to connect to LDAP or database and ability to later support SSO from third party provider. There is always an opportunity to implement different solutions but this was something that could potentially play well into the future banking architecture and plans.

Server configuration

To start off we chose Spring Boot OAuth2 and created spring boot application. There were several configurations that we needed to implement to make our server to perform authorization and authentication.

• WebSecurityConfigurerAdapter (to define /login, /logout, swagger, filters - we also can use @EnableOAuth2Client to additionally configure SSO client)
• GlobalAuthenticationConfigurerAdapter (to define user details service, BCrypt password encoder and init method to distinguish between LDAP and database source). This adapter was needed as there are several filters to read users depending on the flow invoked. 
• auth.ldapAuthentication() was starting point for LDAP
• auth.userDetailsService(...) was starting point for user details service and password encoding
• LdapAuthoritiesPopulator bean for LDAP custom authorities (used repository to load authorities based on user authentication)
• AuthorizationServerConfigurerAdapter (to define OAuth2 server infrastrusture, including custom SQL queries (as we needed DB access for stateless solution across servers). This included tables like oauth_access_token, oauth_refresh_token, oauth_code and oauth_client_details. Tables are used depending on the flow invoked. Action involved overriding TokenStore, ClientDetailsService, AuthorizationCodeServices, configure(AuthorizationServerEndpointsConfigurer endpoints), configure(AuthorizationServerSecurityConfigurer security), DefaultTokenServices and configure(ClientDetailsServiceConfigurer clients) - with @EnableAuthorizationServer.
• ResourceServerConfigurerAdapter (to define adapter that will server as entry point and configuration for any custom APIs) - with @EnableResourceServer.
• We also needed to expose API for the user verification where we publish Principal object (this will be used by the microservices to obtain user details)

It is very important to note that adapter ordering is extremely important and that you may loose a lot of time investigating why something is not working just because of this. The order (lowest to the highest) should be Web (needed for authorization_code and implicit and stateful - because of the login page and authentication) -> OAuth2 (needed for all grants but stateless for password, refresh_token and client_credentials grants) -> Resource (needed for APIs).

We opted to use authorization_code without secret and refresh token for user authentication, client_credentials for the server and microservices that needed to authenticate themselves and password grant for the integration test cases (as this is the easiest way to obtain the token for specific user). Our client_credentials added a default role for the client and the rest added a default role for the user. This way, every authenticated client/user will have a default role to start with. This was a sure way to distinguish between human user and server API. The one problem that we still needed to solve for is propagation of the tokens in the layers of services. It is not a good practice to propagate same token between different horizontal layers due to the loss of the identification of the service doing the authorization.

Client configuration

Before we start with the microservices it is important to say that all services are defined as resources (using @EnableResourceServer annotation). This automatically means that we can for one identify a resource and enable its usage to the client for the OAuth2 configuration, and second, we can setup verification URL for the token. In order for any microservice to identify itself, we have two options for the configuration. First in the application.yml and the other programmatic (in case we need to obtain the token itself). For the first option it is useful to use it on any service that needs to implement verify_token, that is, whenever we receive the token our API will send the request to validate the token and populate user details in the SecurityContext in the spring. This is achieved in the security.oauth2.client and security.oauth2.resource entries. There we have to specify our given client id, secret, verify_token URL, resource id, user details URL and a few other parameters. The second option is to obtain token programatically, for example in the spring integration layer, where declarative approach might be difficult, non existent or depending on the extensive logic. In this case the approach is to create a client code that is annotated by the @EnableOAuth2Client. In our case this was done in the ClientHttpRequestFactory. Obtaining the token is achieved by the OAuthClient and OAuthClientRequest to our authorization server using client_credentials grant. 

In the end the goal we had was achieved and all flows are now functional and serving the purpose. The next thing we need to worry about is switching the framework to the OIDC coupled with OAuth2. This will, perhaps, be a good topic for one of the next blogs.