<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="anonymous"
    xmlns:xlink="http://www.w3.org/1999/xlink">
    <info>
        <title>Anonymous Authentication</title>
    </info>
    <section xml:id="anonymous-overview">
        <info>
            <title>Overview</title>
        </info>
        <para>It's generally considered good security practice to adopt a
            <quote>deny-by-default</quote> where you explicitly specify what is allowed and disallow
            everything else. Defining what is accessible to unauthenticated users is a similar
            situation, particularly for web applications. Many sites require that users must be
            authenticated for anything other than a few URLs (for example the home and login pages).
            In this case it is easiest to define access configuration attributes for these specific
            URLs rather than have for every secured resource. Put differently, sometimes it is nice
            to say <literal>ROLE_SOMETHING</literal> is required by default and only allow certain
            exceptions to this rule, such as for login, logout and home pages of an application. You
            could also omit these pages from the filter chain entirely, thus bypassing the access
            control checks, but this may be undesirable for other reasons, particularly if the pages
            behave differently for authenticated users.</para>
        <para>This is what we mean by anonymous authentication. Note that there is no real
            conceptual difference between a user who is <quote>anonymously authenticated</quote> and
            an unauthenticated user. Spring Security's anonymous authentication just gives you a
            more convenient way to configure your access-control attributes. Calls to servlet API
            calls such as <methodname>getCallerPrincipal</methodname>, for example, will still
            return null even though there is actually an anonymous authentication object in the
            <classname>SecurityContextHolder</classname>.</para>
        <para>There are other situations where anonymous authentication is useful, such as when an
            auditing interceptor queries the <classname>SecurityContextHolder</classname> to
            identify which principal was responsible for a given operation. Classes can be authored
            more robustly if they know the <classname>SecurityContextHolder</classname> always
            contains an <interfacename>Authentication</interfacename> object, and never
            <literal>null</literal>.</para>
    </section>
    <section xml:id="anonymous-config">
        <info>
            <title>Configuration</title>
        </info>
        <para>Anonymous authentication support is provided automatically when using the HTTP
            configuration Spring Security 3.0 and can be customized (or disabled) using the
            <literal>&lt;anonymous></literal> element. You don't need to configure the beans
            described here unless you are using traditional bean configuration.</para>
        <para>Three classes that together provide the anonymous authentication feature.
            <literal>AnonymousAuthenticationToken</literal> is an implementation of
            <interfacename>Authentication</interfacename>, and stores the
            <interfacename>GrantedAuthority</interfacename>s which apply to the anonymous principal.
            There is a corresponding <literal>AnonymousAuthenticationProvider</literal>, which is
            chained into the <literal>ProviderManager</literal> so that
            <literal>AnonymousAuthenticationToken</literal>s are accepted. Finally, there is an
            <classname>AnonymousAuthenticationFilter</classname>, which is chained after the normal
            authentication mechanisms and automatically adds an
            <literal>AnonymousAuthenticationToken</literal> to the
            <classname>SecurityContextHolder</classname> if there is no existing
            <interfacename>Authentication</interfacename> held there. The definition of the filter
            and authentication provider appears as follows:</para>
        <para> <programlisting language="xml">
<![CDATA[
<bean id="anonymousAuthFilter"
    class="org.springframework.security.web.authentication.AnonymousAuthenticationFilter">
  <property name="key" value="foobar"/>
  <property name="userAttribute" value="anonymousUser,ROLE_ANONYMOUS"/>
</bean>

<bean id="anonymousAuthenticationProvider"
    class="org.springframework.security.authentication.AnonymousAuthenticationProvider">
  <property name="key" value="foobar"/>
</bean>]]>
    </programlisting> </para>
        <para>The <literal>key</literal> is shared between the filter and authentication provider,
            so that tokens created by the former are accepted by the latter<footnote>
            <para>The use of the <literal>key</literal> property should not be regarded as providing
                any real security here. It is merely a book-keeping exercise. If you are sharing a
                <classname>ProviderManager</classname> which contains an
                <classname>AnonymousAuthenticationProvider</classname> in a scenario where it is
                possible for an authenticating client to construct the
                <interfacename>Authentication</interfacename> object (such as with RMI invocations),
                then a malicious client could submit an
                <classname>AnonymousAuthenticationToken</classname> which it had created itself
                (with chosen username and authority list). If the <literal>key</literal> is
                guessable or can be found out, then the token would be accepted by the anonymous
                provider. This isn't a problem with normal usage but if you are using RMI you would
                be best to use a customized <classname>ProviderManager</classname> which omits the
                anonymous provider rather than sharing the one you use for your HTTP authentication
                mechanisms.</para>
            </footnote>. The <literal>userAttribute</literal> is expressed in the form of
            <literal>usernameInTheAuthenticationToken,grantedAuthority[,grantedAuthority]</literal>.
            This is the same syntax as used after the equals sign for
            <literal>InMemoryDaoImpl</literal>'s <literal>userMap</literal> property.</para>
        <para>As explained earlier, the benefit of anonymous authentication is that all URI patterns
            can have security applied to them. For example:</para>
        <para> <programlisting language="xml">
<![CDATA[
<bean id="filterSecurityInterceptor"
    class="org.springframework.security.web.access.intercept.FilterSecurityInterceptor">
  <property name="authenticationManager" ref="authenticationManager"/>
  <property name="accessDecisionManager" ref="httpRequestAccessDecisionManager"/>
  <property name="securityMetadata">
    <security:filter-security-metadata-source>
      <security:intercept-url pattern='/index.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/hello.htm' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/logoff.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/login.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/**' access='ROLE_USER'/>
    </security:filter-security-metadata-source>" +
  </property>
</bean>]]>
    </programlisting> </para>
    </section>
    <section xml:id="anonymous-auth-trust-resolver">
        <title><interfacename>AuthenticationTrustResolver</interfacename></title>
        <para> Rounding out the anonymous authentication discussion is the
            <interfacename>AuthenticationTrustResolver</interfacename> interface, with its
            corresponding <literal>AuthenticationTrustResolverImpl</literal> implementation. This
            interface provides an <literal>isAnonymous(Authentication)</literal> method, which
            allows interested classes to take into account this special type of authentication
            status. The <classname>ExceptionTranslationFilter</classname> uses this interface in
            processing <literal>AccessDeniedException</literal>s. If an
            <literal>AccessDeniedException</literal> is thrown, and the authentication is of an
            anonymous type, instead of throwing a 403 (forbidden) response, the filter will instead
            commence the <interfacename>AuthenticationEntryPoint</interfacename> so the principal
            can authenticate properly. This is a necessary distinction, otherwise principals would
            always be deemed <quote>authenticated</quote> and never be given an opportunity to login
            via form, basic, digest or some other normal authentication mechanism. </para>
        <para> You will often see the <literal>ROLE_ANONYMOUS</literal> attribute in the above
            interceptor configuration replaced with <literal>IS_AUTHENTICATED_ANONYMOUSLY</literal>,
            which is effectively the same thing when defining access controls. This is an example of
            the use of the <classname>AuthenticatedVoter</classname> which we will see in the <link
            xlink:href="#authz-authenticated-voter">authorization chapter</link>. It uses an
            <interfacename>AuthenticationTrustResolver</interfacename> to process this particular
            configuration attribute and grant access to anonymous users. The
            <classname>AuthenticatedVoter</classname> approach is more powerful, since it allows you
            to differentiate between anonymous, remember-me and fully-authenticated users. If you
            don't need this functionality though, then you can stick with
            <literal>ROLE_ANONYMOUS</literal>, which will be processed by Spring Security's standard
            <classname>RoleVoter</classname>. </para>
    </section>
</chapter>
