[[anonymous]]
= Anonymous Authentication


[[anonymous-overview]]
== Overview
It is generally considered good security practice to adopt a "`deny-by-default`" stance, 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 that case, it is easiest to define access configuration attributes for these specific URLs rather than for every secured resource.
Put differently, sometimes it is nice to say `ROLE_SOMETHING` is required by default and allow only 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.

This is what we mean by anonymous authentication.
Note that there is no real conceptual difference between a user who is "`anonymously authenticated`" 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 `getCallerPrincipal`, still return null, even though there is actually an anonymous authentication object in the `SecurityContextHolder`.

There are other situations where anonymous authentication is useful, such as when an auditing interceptor queries the `SecurityContextHolder` to identify which principal was responsible for a given operation.
Classes can be authored more robustly if they know the `SecurityContextHolder` always contains an `Authentication` object and never contains `null`.


[[anonymous-config]]
== Configuration
Anonymous authentication support is provided automatically when you use the HTTP configuration (introduced in Spring Security 3.0).
You can customize (or disable) it by using the `<anonymous>` element.
You need not configure the beans described here unless you are using traditional bean configuration.

Three classes work together to provide the anonymous authentication feature.
`AnonymousAuthenticationToken` is an implementation of `Authentication` and stores the `GrantedAuthority` instances that apply to the anonymous principal.
There is a corresponding `AnonymousAuthenticationProvider`, which is chained into the `ProviderManager` so that `AnonymousAuthenticationToken` instances are accepted.
Finally, an `AnonymousAuthenticationFilter` is chained after the normal authentication mechanisms and automatically adds an `AnonymousAuthenticationToken` to the `SecurityContextHolder` if there is no existing `Authentication` held there.
The filter and authentication provider is defined as follows:

[source,xml]
----
<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>
----



The `key` is shared between the filter and authentication provider, so that tokens created by the former are accepted by the latter

[NOTE]
====
The use of the `key` property should not be regarded as providing any real security here.
It is merely a book-keeping exercise.
If you share a `ProviderManager` that contains an `AnonymousAuthenticationProvider` in a scenario where it is possible for an authenticating client to construct the `Authentication` object (such as with RMI invocations), then a malicious client could submit an `AnonymousAuthenticationToken` that it had created itself (with the chosen username and authority list).
If the `key` is guessable or can be found out, the token would be accepted by the anonymous provider.
This is not a problem with normal usage. However, if you use RMI, you should use a customized `ProviderManager` that omits the anonymous provider rather than sharing the one you use for your HTTP authentication mechanisms.
====

The `userAttribute` is expressed in the form of `usernameInTheAuthenticationToken,grantedAuthority[,grantedAuthority]`.
The same syntax is used after the equals sign for the `userMap` property of `InMemoryDaoImpl`.

As explained earlier, the benefit of anonymous authentication is that all URI patterns can have security applied to them, as the following example shows:

[source,xml]
----
<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>
----

[[anonymous-auth-trust-resolver]]
== AuthenticationTrustResolver
Rounding out the anonymous authentication discussion is the `AuthenticationTrustResolver` interface, with its corresponding `AuthenticationTrustResolverImpl` implementation.
This interface provides an `isAnonymous(Authentication)` method, which allows interested classes to take into account this special type of authentication status.
The `ExceptionTranslationFilter` uses this interface in processing `AccessDeniedException` instances.
If an `AccessDeniedException` is thrown and the authentication is of an anonymous type, instead of throwing a 403 (forbidden) response, the filter, instead, commences the `AuthenticationEntryPoint` so that the principal can authenticate properly.
This is a necessary distinction. Otherwise, principals would always be deemed "`authenticated`" and never be given an opportunity to login through form, basic, digest, or some other normal authentication mechanism.

We often see the `ROLE_ANONYMOUS` attribute in the earlier interceptor configuration replaced with `IS_AUTHENTICATED_ANONYMOUSLY`, which is effectively the same thing when defining access controls.
This is an example of the use of the `AuthenticatedVoter`, which we cover in the xref:servlet/authorization/architecture.adoc#authz-authenticated-voter[authorization chapter].
It uses an `AuthenticationTrustResolver` to process this particular configuration attribute and grant access to anonymous users.
The `AuthenticatedVoter` approach is more powerful, since it lets you differentiate between anonymous, remember-me, and fully authenticated users.
If you do not need this functionality, though, you can stick with `ROLE_ANONYMOUS`, which is processed by Spring Security's standard `RoleVoter`.

[[anonymous-auth-mvc-controller]]
== Getting Anonymous Authentications with Spring MVC

https://docs.spring.io/spring-framework/docs/current/reference/html/web.html#mvc-ann-arguments[Spring MVC resolves parameters of type `Principal`] using its own argument resolver.

This means that a construct like this one:

[tabs]
======
Java::
+
[source,java,role="primary"]
----
@GetMapping("/")
public String method(Authentication authentication) {
	if (authentication instanceof AnonymousAuthenticationToken) {
		return "anonymous";
	} else {
		return "not anonymous";
	}
}
----

Kotlin::
+
[source,kotlin,role="secondary"]
----
@GetMapping("/")
fun method(authentication: Authentication?): String {
    return if (authentication is AnonymousAuthenticationToken) {
        "anonymous"
    } else {
        "not anonymous"
    }
}
----
======

will always return "not anonymous", even for anonymous requests.
The reason is that Spring MVC resolves the parameter using `HttpServletRequest#getPrincipal`, which is `null` when the request is anonymous.

If you'd like to obtain the `Authentication` in anonymous requests, use `@CurrentSecurityContext` instead:

.Use CurrentSecurityContext for Anonymous requests
[tabs]
======
Java::
+
[source,java,role="primary"]
----
@GetMapping("/")
public String method(@CurrentSecurityContext SecurityContext context) {
	return context.getAuthentication().getName();
}
----

Kotlin::
+
[source,kotlin,role="secondary"]
----
@GetMapping("/")
fun method(@CurrentSecurityContext context : SecurityContext) : String =
		context!!.authentication!!.name
----
======
