<?xml version="1.0" encoding="UTF-8"?>
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="preauth"
    xmlns:xlink="http://www.w3.org/1999/xlink">
    <info>
        <title>Pre-Authentication Scenarios</title>
    </info>
    <para> 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 <quote>pre-authenticated</quote> 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 <orderedlist>
        <listitem>
            <para>Identify the user making the request. </para>
        </listitem>
        <listitem>
            <para>Obtain the authorities for the user.</para>
        </listitem>
        </orderedlist>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 <methodname>getUserPrincipal()</methodname> 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 <interfacename>UserDetailsService</interfacename>. </para>
    <section>
        <title>Pre-Authentication Framework Classes</title>
        <para> 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 <link xlink:href="#x509">X.509 authentication</link>, as it already has a namespace
            configuration option which is simpler to use and get started with. If you need to use
            explicit bean configuration or are planning on writing your own implementation then an
            understanding of how the provided implementations work will be useful. You will find
            classes under the
            <package>org.springframework.security.web.authentication.preauth</package>. We just
            provide an outline here so you should consult the Javadoc and source where appropriate. </para>
        <section>
            <title>AbstractPreAuthenticatedProcessingFilter</title>
            <para> 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 <interfacename>AuthenticationManager</interfacename>. Subclasses override the
                following methods to obtain this information:
                <programlisting language="java">
  protected abstract Object getPreAuthenticatedPrincipal(HttpServletRequest request);

  protected abstract Object getPreAuthenticatedCredentials(HttpServletRequest request);
</programlisting>
                After calling these, the filter will create a
                <classname>PreAuthenticatedAuthenticationToken</classname> containing the returned
                data and submit it for authentication. By <quote>authentication</quote> here, we
                really just mean further processing to perhaps load the user's authorities, but the
                standard Spring Security authentication architecture is followed. </para>
            <para> Like other Spring Security authentication filters, the pre-authentication filter
                has an <literal>authenticationDetailsSource</literal> property which by default will
                create a <classname>WebAuthenticationDetails</classname> object to store additional
                information such as the session-identifier and originating IP address in the
                    <literal>details</literal> property of the
                    <interfacename>Authentication</interfacename> object. In cases where user role
                information can be obtained from the pre-authentication mechanism, the data is also
                stored in this property, with the details implementing the
                    <interfacename>GrantedAuthoritiesContainer</interfacename> interface. This
                enables the authentication provider to read the authorities which were externally
                allocated to the user. We'll look at a concrete example next. </para>
            <section xml:id="j2ee-preauth-details">
                <title>J2eeBasedPreAuthenticatedWebAuthenticationDetailsSource</title>
                <para> If the filter is configured with an
                        <literal>authenticationDetailsSource</literal> which is an instance of this
                    class, the authority information is obtained by calling the
                        <methodname>isUserInRole(String role)</methodname> method for each of a
                    pre-determined set of <quote>mappable roles</quote>. The class gets these from a
                    configured <interfacename>MappableAttributesRetriever</interfacename>. Possible
                    implementations include hard-coding a list in the application context and
                    reading the role information from the <literal>&lt;security-role&gt;</literal>
                    information in a <filename>web.xml</filename> file. The pre-authentication
                    sample application uses the latter approach.</para>
                <para>There is an additional stage where the roles (or attributes) are mapped to
                    Spring Security <interfacename>GrantedAuthority</interfacename> objects using a
                    configured <interfacename>Attributes2GrantedAuthoritiesMapper</interfacename>.
                    The default will just add the usual <literal>ROLE_</literal> prefix to the
                    names, but it gives you full control over the behaviour. </para>
            </section>
        </section>
        <section>
            <title>PreAuthenticatedAuthenticationProvider</title>
            <para> The pre-authenticated provider has little more to do than load the
                <interfacename>UserDetails</interfacename> object for the user. It does this by
                delegating to a <interfacename>AuthenticationUserDetailsService</interfacename>. The
                latter is similar to the standard <interfacename>UserDetailsService</interfacename>
                but takes an <interfacename>Authentication</interfacename> object rather than just
                user name:
                <programlisting language="java">
  public interface AuthenticationUserDetailsService {
    UserDetails loadUserDetails(Authentication token) throws UsernameNotFoundException;
  }
</programlisting>
                This interface may have also other uses but with pre-authentication it allows access
                to the authorities which were packaged in the
                <interfacename>Authentication</interfacename> object, as we saw in the previous
                section. The
                <classname>PreAuthenticatedGrantedAuthoritiesUserDetailsService</classname> class
                does this. Alternatively, it may delegate to a standard
                <interfacename>UserDetailsService</interfacename> via the
                <classname>UserDetailsByNameServiceWrapper</classname> implementation. </para>
        </section>
        <section>
            <title>Http403ForbiddenEntryPoint</title>
            <para> The <interfacename>AuthenticationEntryPoint</interfacename> was discussed in the
                <link xlink:href="#tech-intro-auth-entry-point">technical overview</link> 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
                <classname>ExceptionTranslationFilter</classname> 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
                <classname>AbstractPreAuthenticatedProcessingFilter</classname> resulting in a null
                authentication. It always returns a <literal>403</literal>-forbidden response code
                if called. </para>
        </section>
    </section>
    <section>
        <title>Concrete Implementations</title>
        <para> X.509 authentication is covered in its <link xlink:href="#x509">own chapter</link>.
            Here we'll look at some classes which provide support for other pre-authenticated
            scenarios. </para>
        <section>
            <title>Request-Header Authentication (Siteminder)</title>
            <para> 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
                Siteminder, which passes the username in a header called <literal>SM_USER</literal>.
                This mechanism is supported by the class
                <classname>RequestHeaderAuthenticationFilter</classname> which simply extracts the
                username from the header. It defaults to using the name <literal>SM_USER</literal>
                as the header name. See the Javadoc for more details. </para>
            <tip>
                <para>Note that when using a system like this, the framework performs no
                    authentication checks at all and it is <emphasis>extremely</emphasis> 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
                    username they wished. </para>
            </tip>
            <section>
                <title>Siteminder Example Configuration</title>
                <para> A typical configuration using this filter would look like this: <programlisting><![CDATA[
  <security:http>
    <!-- Additional http configuration omitted -->
    <security:custom-filter position="PRE_AUTH_FILTER" ref="siteminderFilter" />
  </security:http>

    <bean id="siteminderFilter" class=
"org.springframework.security.web.authentication.preauth.header.RequestHeaderAuthenticationFilter">
    <property name="principalRequestHeader" value="SM_USER"/>
    <property name="authenticationManager" ref="authenticationManager" />
  </bean>

  <bean id="preauthAuthProvider"
class="org.springframework.security.web.authentication.preauth.PreAuthenticatedAuthenticationProvider">
    <property name="preAuthenticatedUserDetailsService">
      <bean id="userDetailsServiceWrapper"
          class="org.springframework.security.core.userdetails.UserDetailsByNameServiceWrapper">
        <property name="userDetailsService" ref="userDetailsService"/>
      </bean>
    </property>
    </bean>

    <security:authentication-manager alias="authenticationManager">
      <security:authentication-provider ref="preauthAuthProvider" />
    </security-authentication-manager>
]]>
</programlisting> We've assumed here that the security namespace is being used for configuration
                    (hence the user of the <literal>custom-filter</literal>,
                    <literal>authentication-manager</literal> and
                    <literal>custom-authentication-provider</literal> elements (you can read more
                    about them in the <link xlink:href="#ns-config">namespace chapter</link>). You
                    would leave these out of a traditional bean configuration. It's also assumed
                    that you have added a <interfacename>UserDetailsService</interfacename> (called
                    <quote>userDetailsService</quote>) to your configuration to load the user's
                    roles. </para>
            </section>
        </section>
        <section>
            <title>J2EE Container Authentication</title>
            <para> The class <classname>J2eePreAuthenticatedProcessingFilter</classname> will
                extract the username from the <literal>userPrincipal</literal> property of the
                <interfacename>HttpServletRequest</interfacename>. Use of this filter would usually
                be combined with the use of J2EE roles as described above in <xref
                linkend="j2ee-preauth-details"/>. </para>
            <para> There is a sample application in the codebase which uses this approach, so get
                hold of the code from subversion and have a look at the application context file if
                you are interested. The code is in the <filename>samples/preauth</filename>
                directory. </para>
        </section>
    </section>
</chapter>
