[[servlet-architecture]] = Architecture :figures: servlet/architecture This section discusses Spring Security's high level architecture within Servlet based applications. We build on this high level understanding within xref:servlet/authentication/index.adoc#servlet-authentication[Authentication], xref:servlet/authorization/index.adoc#servlet-authorization[Authorization], xref:servlet/exploits/index.adoc#servlet-exploits[Protection Against Exploits] sections of the reference. // FIXME: Add links to other sections of architecture [[servlet-filters-review]] == A Review of ``Filter``s Spring Security's Servlet support is based on Servlet ``Filter``s, so it is helpful to look at the role of ``Filter``s generally first. The picture below shows the typical layering of the handlers for a single HTTP request. .FilterChain [[servlet-filterchain-figure]] image::{figures}/filterchain.png[] The client sends a request to the application, and the container creates a `FilterChain` which contains the ``Filter``s 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 {spring-framework-reference-url}web.html#mvc-servlet[`DispatcherServlet`]. At most one `Servlet` can handle a single `HttpServletRequest` and `HttpServletResponse`. However, more than one `Filter` can be used to: * Prevent downstream ``Filter``s 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 ``Filter``s and `Servlet` The power of the `Filter` comes from the `FilterChain` that is passed into it. .`FilterChain` Usage Example ==== .Java [source,java,role="primary"] ---- 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 [source,kotlin,role="secondary"] ---- 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 ``Filter``s and the `Servlet`, the order each `Filter` is invoked is extremely important. [[servlet-delegatingfilterproxy]] == DelegatingFilterProxy Spring provides a `Filter` implementation named {spring-framework-api-url}org/springframework/web/filter/DelegatingFilterProxy.html[`DelegatingFilterProxy`] that allows bridging between the Servlet container's lifecycle and Spring's `ApplicationContext`. The Servlet container allows registering ``Filter``s 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 <>. .DelegatingFilterProxy [[servlet-delegatingfilterproxy-figure]] image::{figures}/delegatingfilterproxy.png[] `DelegatingFilterProxy` looks up __Bean Filter~0~__ from the `ApplicationContext` and then invokes __Bean Filter~0~__. The pseudo code of `DelegatingFilterProxy` can be seen below. .`DelegatingFilterProxy` Pseudo Code ==== .Java [source,java,role="primary",subs="+quotes,+macros"] ---- public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) { // Lazily get Filter that was registered as a Spring Bean // For the example in <> `delegate` is an instance of __Bean Filter~0~__ Filter delegate = getFilterBean(someBeanName); // delegate work to the Spring Bean delegate.doFilter(request, response); } ---- .Kotlin [source,kotlin,role="secondary",subs="+quotes,+macros"] ---- fun doFilter(request: ServletRequest, response: ServletResponse, chain: FilterChain) { // Lazily get Filter that was registered as a Spring Bean // For the example in <> `delegate` is an instance of __Bean Filter~0~__ 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. [[servlet-filterchainproxy]] == 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 <>. Since `FilterChainProxy` is a Bean, it is typically wrapped in a <>. .FilterChainProxy [[servlet-filterchainproxy-figure]] image::{figures}/filterchainproxy.png[] [[servlet-securityfilterchain]] == SecurityFilterChain {security-api-url}org/springframework/security/web/SecurityFilterChain.html[`SecurityFilterChain`] is used by <> to determine which Spring Security ``Filter``s should be invoked for this request. .SecurityFilterChain [[servlet-securityfilterchain-figure]] image::{figures}/securityfilterchain.png[] The <> in `SecurityFilterChain` are typically Beans, but they are registered with `FilterChainProxy` instead of <>. `FilterChainProxy` provides a number of advantages to registering directly with the Servlet container or <>. 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. // FIXME: Add a link to SecurityContext For example, it clears out the `SecurityContext` to avoid memory leaks. It also applies Spring Security's xref:servlet/exploits/firewall.adoc#servlet-httpfirewall[`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, ``Filter``s are invoked based upon the URL alone. // FIXME: Link to RequestMatcher 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. .Multiple SecurityFilterChain [[servlet-multi-securityfilterchain-figure]] image::{figures}/multi-securityfilterchain.png[] In the <> 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~0~``'s pattern of `+/api/**+`, so only `SecurityFilterChain~0~` will be invoked even though it also matches on ``SecurityFilterChain~n~``. If a URL of `/messages/` is requested, it will not match on ``SecurityFilterChain~0~``'s pattern of `+/api/**+`, so `FilterChainProxy` will continue trying each `SecurityFilterChain`. Assuming that no other, `SecurityFilterChain` instances match `SecurityFilterChain~n~` will be invoked. // FIXME add link to pattern matching Notice that `SecurityFilterChain~0~` has only three security ``Filter``s instances configured. However, `SecurityFilterChain~n~` has four security ``Filter``s configured. It is important to note that each `SecurityFilterChain` can be unique and configured in isolation. In fact, a `SecurityFilterChain` might have zero security ``Filter``s if the application wants Spring Security to ignore certain requests. // FIXME: add link to configuring multiple `SecurityFilterChain` instances [[servlet-security-filters]] == Security Filters The Security Filters are inserted into the <> with the <> API. The <>s matters. It is typically not necessary to know the ordering of Spring Security's ``Filter``s. However, there are times that it is beneficial to know the ordering Below is a comprehensive list of Spring Security Filter ordering: * xref:servlet/authentication/session-management.adoc#session-mgmt-force-session-creation[`ForceEagerSessionCreationFilter`] * ChannelProcessingFilter * WebAsyncManagerIntegrationFilter * SecurityContextPersistenceFilter * HeaderWriterFilter * CorsFilter * CsrfFilter * LogoutFilter * OAuth2AuthorizationRequestRedirectFilter * Saml2WebSsoAuthenticationRequestFilter * X509AuthenticationFilter * AbstractPreAuthenticatedProcessingFilter * CasAuthenticationFilter * OAuth2LoginAuthenticationFilter * Saml2WebSsoAuthenticationFilter * xref:servlet/authentication/passwords/form.adoc#servlet-authentication-usernamepasswordauthenticationfilter[`UsernamePasswordAuthenticationFilter`] * OpenIDAuthenticationFilter * DefaultLoginPageGeneratingFilter * DefaultLogoutPageGeneratingFilter * ConcurrentSessionFilter * xref:servlet/authentication/passwords/digest.adoc#servlet-authentication-digest[`DigestAuthenticationFilter`] * BearerTokenAuthenticationFilter * xref:servlet/authentication/passwords/basic.adoc#servlet-authentication-basic[`BasicAuthenticationFilter`] * RequestCacheAwareFilter * SecurityContextHolderAwareRequestFilter * JaasApiIntegrationFilter * RememberMeAuthenticationFilter * AnonymousAuthenticationFilter * OAuth2AuthorizationCodeGrantFilter * SessionManagementFilter * <> * xref:servlet/authorization/authorize-requests.adoc#servlet-authorization-filtersecurityinterceptor[`FilterSecurityInterceptor`] * SwitchUserFilter [[servlet-exceptiontranslationfilter]] == Handling Security Exceptions The {security-api-url}org/springframework/security/web/access/ExceptionTranslationFilter.html[`ExceptionTranslationFilter`] allows translation of {security-api-url}org/springframework/security/access/AccessDeniedException.html[`AccessDeniedException`] and {security-api-url}/org/springframework/security/core/AuthenticationException.html[`AuthenticationException`] into HTTP responses. `ExceptionTranslationFilter` is inserted into the <> as one of the <>. image::{figures}/exceptiontranslationfilter.png[] * image:{icondir}/number_1.png[] First, the `ExceptionTranslationFilter` invokes `FilterChain.doFilter(request, response)` to invoke the rest of the application. * image:{icondir}/number_2.png[] If the user is not authenticated or it is an `AuthenticationException`, then __Start Authentication__. ** The xref:servlet/authentication/architecture.adoc#servlet-authentication-securitycontextholder[SecurityContextHolder] is cleared out ** The `HttpServletRequest` is saved in the {security-api-url}org/springframework/security/web/savedrequest/RequestCache.html[`RequestCache`]. When the user successfully authenticates, the `RequestCache` is used to replay the original request. // FIXME: add link to authentication success ** 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. // FIXME: link to AuthenticationEntryPoint * image:{icondir}/number_3.png[] Otherwise if it is an `AccessDeniedException`, then __Access Denied__. The `AccessDeniedHandler` is invoked to handle access denied. // FIXME: link to AccessDeniedHandler [NOTE] ==== 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 [source,java] ---- 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 <> 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. xref:servlet/authorization/authorize-requests.adoc#servlet-authorization-filtersecurityinterceptor[`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__