[[servlet-architecture]]
= Servlet Security: The Big Picture
: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 <<servlet-filters-review,``Filter``s and the `FilterChain`>>.

.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 <<servlet-delegatingfilterproxy-figure>> `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 <<servlet-delegatingfilterproxy-figure>> `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 up.
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 <<servlet-securityfilterchain,`SecurityFilterChain`>>.
Since `FilterChainProxy` is a Bean, it is typically wrapped in a <<servlet-delegatingfilterproxy>>.

.FilterChainProxy
[[servlet-filterchainproxy-figure]]
image::{figures}/filterchainproxy.png[]

[[servlet-securityfilterchain]]
== SecurityFilterChain

{security-api-url}org/springframework/security/web/SecurityFilterChain.html[`SecurityFilterChain`] is used by <<servlet-filterchainproxy>> to determine which Spring Security ``Filter``s should be invoked for this request.

.SecurityFilterChain
[[servlet-securityfilterchain-figure]]
image::{figures}/securityfilterchain.png[]

The <<servlet-security-filters,Security Filters>> in `SecurityFilterChain` are typically Beans, but they are registered with `FilterChainProxy` instead of <<servlet-delegatingfilterproxy>>.
`FilterChainProxy` provides a number of advantages to registering directly with the Servlet container or <<servlet-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.
// 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 <<servlet-multi-securityfilterchain-figure>> 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 <<servlet-filterchainproxy>> with the <<servlet-securityfilterchain>> API.
The <<servlet-filters-review,order of ``Filter``>>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:

* 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
* <<servlet-exceptiontranslationfilter,`ExceptionTranslationFilter`>>
* 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 <<servlet-filterchainproxy>> as one of the <<servlet-security-filters>>.

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/index.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 <<servlet-filters-review>> 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__
