Saturday, April 24, 2010

SecurityContextPersistenceFilter

This bean is configured as shown below:
<bean id="securityContextPersistenceFilter" class="org.springframework.security.web.context.SecurityContextPersistenceFilter"/>
This bean has two main tasks and they are very important
  • In a typical web application like Skyphoto the user will login once and then subsequently do several operations or click several links and buttons generating several authenticated requests to the server. Since the user was authenticated once it is important to store the security context somewhere and by some means. Otherwise he/she has to authenticate for each request and will sooner or later never use Skyphoto. In a typical web application you will store an user object in HttpSession and each time a request comes in, the server identifies this session with a session id and you get the user object. It is the responsibility of the server to cache/store and manage these session objects during the lifetime of the session.
“In Spring Security, the responsibility for storing the SecurityContext between requests falls to the SecurityContextPersistenceFilter, which by default stores the context as an HttpSession attribute between HTTP requests. It restores the context to the SecurityContextHolder for each request….”
  • The SecurityContextPersistenceFilter stores the security context in ThreadLocal for the current execution. Hence it is essential to clear this thread local object in a web application as the server uses a pool of thread and chances are that this thread will be reused. If this security context is not cleared there is a high possibility that the set information will be used for request from a different user leading to unpredictable results and possible violation of security.
The SecurityContextPersistenceFilter takes care of clearing the security context object from thread local once the request completes.
This filter MUST be executed BEFORE any authentication processing filter. Authentication processing mechanisms used in those filters expect the SecurityContextHolder to contain a valid SecurityContext by the time they execute.
The task of saving and retrieving the security context is delegated to a separate strategy interface SecurityContextRepository. The interface is shown below:
public interface SecurityContextRepository {
  SecurityContext loadContext(HttpRequestResponseHolder requestResponseHolder);
  void saveContext(SecurityContext context, HttpServletRequest request,
         HttpServletResponse response);
}
“The HttpRequestResponseHolder is simply a container for the incoming request and response objects, allowing the implementation to replace these with wrapper classes. The returned contents will be passed to the filter chain.”
The default implementation of SecurityContextRepository is the HttpSessionSecurityContextRepository class. As the name suggests it stores the security context as an HttpSession attribute.This class has an important attribute - allowSessionCreation  which by default is set to true. Hence this class can create a new session if it needs to for storing security context of an authenticated user. Note that session is only created after authentication is done and contents of security context now has been changed with authenticated user information. If you want to prevent session creation, then set this parameter to false. A sample configuration is shown below.
<bean id="securityContextPersistenceFilter" class="org.springframework.security.web.context.SecurityContextPersistenceFilter"> <property name='securityContextRepository'>
<bean class='org.springframework.security.web.context.HttpSessionSecurityContextRepository'> <property name='allowSessionCreation' value='false' />
</bean>
</property>
</bean>

How the filter chain works?

Going back to the filter chain configuration in my last post, you can see a chain filters configured as shown below:
<bean id="springSecurityFilterChain" class="org.springframework.security.web.FilterChainProxy">
<sec:filter-chain-map path-type="ant">
<sec:filter-chain pattern="/SkyPhotoWeb/**" filters=" securityContextPersistenceFilterWithASCFalse, basicAuthenticationFilter, exceptionTranslationFilter, filterSecurityInterceptor" />
<sec:filter-chain pattern="/**" filters=" securityContextPersistenceFilterWithASCTrue, formLoginFilter, exceptionTranslationFilter, filterSecurityInterceptor" />
</sec:filter-chain-map>
</bean>
Let us assume that a request is send as http://www.skyphoto.com/SkyPhotoWeb/dosomething.html the request runs through the filters as shown in picture below:
blog1
In my next post I will explain all these and few other important filters in greater detail.