Spring-Security-Oauth2: Full authentication is required to access this resource

The client_id and client_secret, by default, should go in the Authorization header, not the form-urlencoded body.

1. Concatenate your client_id and client_secret, with a colon between them: [email protected]:12345678.
2. Base 64 encode the result: YWJjQGdtYWlsLmNvbToxMjM0NTY3OA==
3. Set the Authorization header: Authorization: Basic YWJjQGdtYWlsLmNvbToxMjM0NTY3OA==

By default Spring OAuth requires basic HTTP authentication. If you want to switch it off with Java based configuration, you have to allow form authentication for clients like this:

@Configuration
@EnableAuthorizationServer
protected static class OAuth2Config extends AuthorizationServerConfigurerAdapter {
  @Override
  public void configure(AuthorizationServerSecurityConfigurer oauthServer) throws Exception {
    oauthServer.allowFormAuthenticationForClients();
  }
}