Architecture

This section discusses Spring Security’s high level architecture within Servlet based applications. We build on this high level understanding within Authentication, Authorization, Protection Against Exploits sections of the reference.

A Review of Filters

Spring Security’s Servlet support is based on Servlet Filters, so it is helpful to look at the role of Filters generally first. The picture below shows the typical layering of the handlers for a single HTTP request.

Figure 1. FilterChain

The client sends a request to the application, and the container creates a FilterChain which contains the Filters and Servlet that should process the HttpServletRequest based on the path of the request URI. In a Spring MVC application the Servlet is an instance of DispatcherServlet (opens new window). At most one Servlet can handle a single HttpServletRequest and HttpServletResponse. However, more than one Filter can be used to:

• Prevent downstream Filters or the Servlet from being invoked. In this instance the Filter will typically write the HttpServletResponse.

• Modify the HttpServletRequest or HttpServletResponse used by the downstream Filters and Servlet

The power of the Filter comes from the FilterChain that is passed into it.

Example 1. FilterChain Usage Example

Java

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
	// do something before the rest of the application
    chain.doFilter(request, response); // invoke the rest of the application
    // do something after the rest of the application
}

Kotlin

fun doFilter(request: ServletRequest, response: ServletResponse, chain: FilterChain) {
    // do something before the rest of the application
    chain.doFilter(request, response) // invoke the rest of the application
    // do something after the rest of the application
}

Since a Filter only impacts downstream Filters and the Servlet, the order each Filter is invoked is extremely important.

DelegatingFilterProxy

Spring provides a Filter implementation named DelegatingFilterProxy (opens new window) that allows bridging between the Servlet container’s lifecycle and Spring’s ApplicationContext. The Servlet container allows registering Filters using its own standards, but it is not aware of Spring defined Beans.DelegatingFilterProxy can be registered via standard Servlet container mechanisms, but delegate all the work to a Spring Bean that implements Filter.

Here is a picture of how DelegatingFilterProxy fits into the Filters and the FilterChain.

Figure 2. DelegatingFilterProxy

DelegatingFilterProxy looks up Bean Filter₀ from the ApplicationContext and then invokes Bean Filter₀. The pseudo code of DelegatingFilterProxy can be seen below.

Example 2. DelegatingFilterProxy Pseudo Code

Java

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
	// Lazily get Filter that was registered as a Spring Bean
	// For the example in DelegatingFilterProxy delegate is an instance of Bean Filter0
	Filter delegate = getFilterBean(someBeanName);
	// delegate work to the Spring Bean
	delegate.doFilter(request, response);
}

Kotlin

fun doFilter(request: ServletRequest, response: ServletResponse, chain: FilterChain) {
	// Lazily get Filter that was registered as a Spring Bean
	// For the example in DelegatingFilterProxy delegate is an instance of Bean Filter0
	val delegate: Filter = getFilterBean(someBeanName)
	// delegate work to the Spring Bean
	delegate.doFilter(request, response)
}

Another benefit of DelegatingFilterProxy is that it allows delaying looking Filter bean instances. This is important because the container needs to register the Filter instances before the container can startup. However, Spring typically uses a ContextLoaderListener to load the Spring Beans which will not be done until after the Filter instances need to be registered.

FilterChainProxy

Spring Security’s Servlet support is contained within FilterChainProxy.FilterChainProxy is a special Filter provided by Spring Security that allows delegating to many Filter instances through SecurityFilterChain. Since FilterChainProxy is a Bean, it is typically wrapped in a DelegatingFilterProxy.

Figure 3. FilterChainProxy

SecurityFilterChain

SecurityFilterChain (opens new window) is used by FilterChainProxy to determine which Spring Security Filters should be invoked for this request.

Figure 4. SecurityFilterChain

The Security Filters in SecurityFilterChain are typically Beans, but they are registered with FilterChainProxy instead of DelegatingFilterProxy.FilterChainProxy provides a number of advantages to registering directly with the Servlet container or DelegatingFilterProxy. First, it provides a starting point for all of Spring Security’s Servlet support. For that reason, if you are attempting to troubleshoot Spring Security’s Servlet support, adding a debug point in FilterChainProxy is a great place to start.

Second, since FilterChainProxy is central to Spring Security usage it can perform tasks that are not viewed as optional. For example, it clears out the SecurityContext to avoid memory leaks. It also applies Spring Security’s HttpFirewall to protect applications against certain types of attacks.

In addition, it provides more flexibility in determining when a SecurityFilterChain should be invoked. In a Servlet container, Filters are invoked based upon the URL alone. However, FilterChainProxy can determine invocation based upon anything in the HttpServletRequest by leveraging the RequestMatcher interface.

In fact, FilterChainProxy can be used to determine which SecurityFilterChain should be used. This allows providing a totally separate configuration for different slices of your application.

Figure 5. Multiple SecurityFilterChain

In the Multiple SecurityFilterChain Figure FilterChainProxy decides which SecurityFilterChain should be used. Only the first SecurityFilterChain that matches will be invoked. If a URL of /api/messages/ is requested, it will first match on SecurityFilterChain<sub>0</sub>'s pattern of /api/**, so only SecurityFilterChain<sub>0</sub> will be invoked even though it also matches on SecurityFilterChain<sub>n</sub>. If a URL of /messages/ is requested, it will not match on SecurityFilterChain<sub>0</sub>'s pattern of /api/**, so FilterChainProxy will continue trying each SecurityFilterChain. Assuming that no other, SecurityFilterChain instances match SecurityFilterChain<sub>n</sub> will be invoked.

Notice that SecurityFilterChain<sub>0</sub> has only three security Filters instances configured. However, SecurityFilterChain<sub>n</sub> has four security Filters configured. It is important to note that each SecurityFilterChain can be unique and configured in isolation. In fact, a SecurityFilterChain might have zero security Filters if the application wants Spring Security to ignore certain requests.

Security Filters

The Security Filters are inserted into the FilterChainProxy with the SecurityFilterChain API. The order of Filters matters. It is typically not necessary to know the ordering of Spring Security’s Filters. However, there are times that it is beneficial to know the ordering

Below is a comprehensive list of Spring Security Filter ordering:

• ChannelProcessingFilter

• WebAsyncManagerIntegrationFilter

• SecurityContextPersistenceFilter

• HeaderWriterFilter

• CorsFilter

• CsrfFilter

• LogoutFilter

• OAuth2AuthorizationRequestRedirectFilter

• Saml2WebSsoAuthenticationRequestFilter

• X509AuthenticationFilter

• AbstractPreAuthenticatedProcessingFilter

• CasAuthenticationFilter

• OAuth2LoginAuthenticationFilter

• Saml2WebSsoAuthenticationFilter

• UsernamePasswordAuthenticationFilter

• OpenIDAuthenticationFilter

• DefaultLoginPageGeneratingFilter

• DefaultLogoutPageGeneratingFilter

• ConcurrentSessionFilter

• DigestAuthenticationFilter

• BearerTokenAuthenticationFilter

• BasicAuthenticationFilter

• RequestCacheAwareFilter

• SecurityContextHolderAwareRequestFilter

• JaasApiIntegrationFilter

• RememberMeAuthenticationFilter

• AnonymousAuthenticationFilter

• OAuth2AuthorizationCodeGrantFilter

• SessionManagementFilter

• ExceptionTranslationFilter

• FilterSecurityInterceptor

• SwitchUserFilter

Handling Security Exceptions

The ExceptionTranslationFilter (opens new window) allows translation of AccessDeniedException (opens new window) and AuthenticationException (opens new window) into HTTP responses.

ExceptionTranslationFilter is inserted into the FilterChainProxy as one of the Security Filters.

• First, the ExceptionTranslationFilter invokes FilterChain.doFilter(request, response) to invoke the rest of the application.

• If the user is not authenticated or it is an AuthenticationException, then Start Authentication.

  • The SecurityContextHolder is cleared out

  • The HttpServletRequest is saved in the RequestCache (opens new window). When the user successfully authenticates, the RequestCache is used to replay the original request.

  • The AuthenticationEntryPoint is used to request credentials from the client. For example, it might redirect to a log in page or send a WWW-Authenticate header.

• Otherwise if it is an AccessDeniedException, then Access Denied. The AccessDeniedHandler is invoked to handle access denied.

	If the application does not throw an AccessDeniedException or an AuthenticationException, then ExceptionTranslationFilter does not do anything.

The pseudocode for ExceptionTranslationFilter looks something like this:

ExceptionTranslationFilter pseudocode

try {
	filterChain.doFilter(request, response); (1)
} catch (AccessDeniedException | AuthenticationException ex) {
	if (!authenticated || ex instanceof AuthenticationException) {
		startAuthentication(); (2)
	} else {
		accessDenied(); (3)
	}
}

1	You will recall from A Review of Filters that invoking FilterChain.doFilter(request, response) is the equivalent of invoking the rest of the application. This means that if another part of the application, (i.e. FilterSecurityInterceptor or method security) throws an AuthenticationException or AccessDeniedException it will be caught and handled here.
2	If the user is not authenticated or it is an AuthenticationException, then Start Authentication.
3	Otherwise, Access Denied

Getting StartedAuthentication