# Pre-Authentication Scenarios

There are situations where you want to use Spring Security for authorization, but the user has already been reliably authenticated by some external system prior to accessing the application. We refer to these situations as pre-authenticated scenarios. Examples include X.509, Siteminder and authentication by the J2EE container in which the application is running. When using pre-authentication, Spring Security has to

1. Identify the user making the request.

2. Obtain the authorities for the user.

The details will depend on the external authentication mechanism. A user might be identified by their certificate information in the case of X.509, or by an HTTP request header in the case of Siteminder. If relying on container authentication, the user will be identified by calling the getUserPrincipal() method on the incoming HTTP request. In some cases, the external mechanism may supply role/authority information for the user but in others the authorities must be obtained from a separate source, such as a UserDetailsService.

## Pre-Authentication Framework Classes

Because most pre-authentication mechanisms follow the same pattern, Spring Security has a set of classes which provide an internal framework for implementing pre-authenticated authentication providers. This removes duplication and allows new implementations to be added in a structured fashion, without having to write everything from scratch. You don't need to know about these classes if you want to use something like X.509 authentication, as it already has a namespace configuration option which is simpler to use and get started with. If you need to use explicit bean confiuration or are planning on writing your own implementation then an understanding of how the provided implementations work will be useful. You will find the web related classes under the org.springframework.security.ui.preauth package and the backend classes under org.springframework.security.providers.preauth. We just provide an outline here so you should consult the Javadoc and source where appropriate.

### AbstractPreAuthenticatedProcessingFilter

This class will check the current contents of the security context and, if empty, it will attempt to extract user information from the HTTP request and submit it to the AuthenticationManager. Subclasses override the following methods to obtain this information:

      protected abstract Object getPreAuthenticatedPrincipal(HttpServletRequest request);

      protected abstract Object getPreAuthenticatedCredentials(HttpServletRequest request);

After calling these, the filter will create a PreAuthenticatedAuthenticationToken containing the returned data and submit it for authentication. By authentication here, we really just mean further processing to perhaps load the user's authorities, but the standard Spring Security authentication architecture is followed.

### AbstractPreAuthenticatedAuthenticationDetailsSource

Like other Spring Security authentication filters, the pre-authentication filter has an `authenticationDetailsSource` property which by default will create a WebAuthenticationDetails object to store additional information such as the session-identifier and originating IP address in the `details` property of the Authentication object. In cases where user role information can be obtained from the pre-authentication mechanism, the data is also stored in this property. Subclasses of AbstractPreAuthenticatedAuthenticationDetailsSource use an extended details object which implements the GrantedAuthoritiesContainer interface, thus enabling the authentication provider to read the authorities which were externally allocated to the user. We'll look at a concrete example next.

#### J2eeBasedPreAuthenticatedWebAuthenticationDetailsSource

If the filter is configured with an `authenticationDetailsSource` which is an instance of this class, the authority information is obtained by calling the isUserInRole(String role) method for each of a pre-determined set of mappable roles. The class gets these from a configured MappableAttributesRetriever. Possible implementations include hard-coding a list in the application context and reading the role information from the `<security-role>` information in a `web.xml` file. The pre-authentication sample application uses the latter approach.

There is an additional stage where the roles (or attributes) are mapped to Spring Security GrantedAuthority objects using a configured Attributes2GrantedAuthoritiesMapper. The default will just add the usual `ROLE_` prefix to the names, but it gives you full control over the behaviour.

### PreAuthenticatedAuthenticationProvider

The pre-authenticated provider has little more to do than load the UserDetails object for the user. It does this by delegating to a AuthenticationUserDetailsService. The latter is similar to the standard UserDetailsService but takes an Authentication object rather than just user name:

      public interface AuthenticationUserDetailsService {
        UserDetails loadUserDetails(Authentication token) throws UsernameNotFoundException;
      }

This interface may have also other uses but with pre-authentication it allows access to the authorities which were packaged in the Authentication object, as we saw in the previous section. The PreAuthenticatedGrantedAuthoritiesUserDetailsService class does this. Alternatively, it may delegate to a standard UserDetailsService via the UserDetailsByNameServiceWrapper implementation.

### PreAuthenticatedProcessingFilterEntryPoint

The AuthenticationEntryPoint was discussed in the technical overview chapter. Normally it is responsible for kick-starting the authentication process for an unauthenticated user (when they try to access a protected resource), but in the pre-authenticated case this doesn't apply. You would only configure the ExceptionTranslationFilter with an instance of this class if you aren't using pre-authentication in combination with other authentication mechanisms. It will be called if the user is rejected by the AbstractPreAuthenticatedProcessingFilter resulting in a null authentication. It always returns a `403`-forbidden response code if called.

## Concrete Implementations

X.509 authentication is covered in its own chapter. Here we'll look at some classes which provide support for other pre-authenticated scenarios.

### Request-Header Authentication (Siteminder)

An external authentication system may supply information to the application by setting specific headers on the HTTP request. A well known example of this is is Siteminder, which passes the username in a header called `SM_USER`. This mechanism is supported by the class RequestHeaderPreAuthenticatedProcessingFilter which simply extracts the username from the header. It defaults to using the name `SM_USER` as the header name. See the Javadoc for more details.

Note that when using a system like this, the framework performs no authentication checks at all and it is extremely important that the external system is configured properly and protects all access to the application. If an attacker is able to forge the headers in their original request without this being detected then they could potentially choose any userame they wished.

#### Siteminder Example Configuration

A typical configuration using this filter would look like this:

    	<bean id="siteminderFilter"
          class="org.springframework.security.ui.preauth.header.RequestHeaderPreAuthenticatedProcessingFilter">
        <security:custom-filter position="PRE_AUTH_FILTER" />
        <property name="principalRequestHeader" value="SM_USER"/>
        <property name="authenticationManager" ref="authenticationManager" />
      </bean>

      <bean id="preauthAuthProvider"
          class="org.springframework.security.providers.preauth.PreAuthenticatedAuthenticationProvider">
        <security:custom-authentication-provider />
        <property name="preAuthenticatedUserDetailsService">
          <bean id="userDetailsServiceWrapper"
                class="org.springframework.security.userdetails.UserDetailsByNameServiceWrapper">
            <property name="userDetailsService" ref="userDetailsService"/>
          </bean>
        </property>
    	</bean>

    	<security:authentication-manager alias="authenticationManager" />

We've assumed here that the security namespace is being used for configuration (hence the user of the `custom-filter`, `authentication-manager` and `custom-authentication-provider` elements (you can read more about them in the namespace chapter). You would leave these out of a traditional bean configuration. It's also assumed that you have added a UserDetailsService (called userDetailsService) to your configuration to load the user's roles.

### J2EE Container Authentication

The class J2eePreAuthenticatedProcessingFilter will extract the username from the `userPrincipal` property of the HttpServletRequest. use of this filter would usually be combined with the use of J2EE roles as described above in .