The Firewall and Authorization

The Firewall and Authorization

Central to the Security component is authorization. This is handled by an instance of Symfony\Component\Security\Core\Authorization\AuthorizationCheckerInterface. When all steps in the process of authenticating the user have been taken successfully, you can ask the authorization checker if the authenticated user has access to a certain action or resource of the application:

  1. use Symfony\Component\Security\Core\Authorization\AuthorizationChecker;
  2. use Symfony\Component\Security\Core\Exception\AccessDeniedException;
  4. // instance of Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorageInterface
  5. $tokenStorage = ...;
  7. // instance of Symfony\Component\Security\Core\Authentication\AuthenticationManagerInterface
  8. $authenticationManager = ...;
  10. // instance of Symfony\Component\Security\Core\Authorization\AccessDecisionManagerInterface
  11. $accessDecisionManager = ...;
  13. $authorizationChecker = new AuthorizationChecker(
  14.     $tokenStorage,
  15.     $authenticationManager,
  16.     $accessDecisionManager
  17. );
  19. // ... authenticate the user
  21. if (!$authorizationChecker->isGranted('ROLE_ADMIN')) {
  22.     throw new AccessDeniedException();
  23. }

Note

Read the dedicated articles to learn more about Authentication and Authorization.

A Firewall for HTTP Requests

Authenticating a user is done by the firewall. An application may have multiple secured areas, so the firewall is configured using a map of these secured areas. For each of these areas, the map contains a request matcher and a collection of listeners. The request matcher gives the firewall the ability to find out if the current request points to a secured area. The listeners are then asked if the current request can be used to authenticate the user:

  1. use Symfony\Component\HttpFoundation\RequestMatcher;
  2. use Symfony\Component\Security\Http\Firewall\ExceptionListener;
  3. use Symfony\Component\Security\Http\FirewallMap;
  5. $firewallMap = new FirewallMap();
  7. $requestMatcher = new RequestMatcher('^/secured-area/');
  9. // array of callables
  10. $listeners = [...];
  12. $exceptionListener = new ExceptionListener(...);
  14. $firewallMap->add($requestMatcher, $listeners, $exceptionListener);

The firewall map will be given to the firewall as its first argument, together with the event dispatcher that is used by the Symfony\Component\HttpKernel\HttpKernel:

  1. use Symfony\Component\HttpKernel\KernelEvents;
  2. use Symfony\Component\Security\Http\Firewall;
  4. // the EventDispatcher used by the HttpKernel
  5. $dispatcher = ...;
  7. $firewall = new Firewall($firewallMap, $dispatcher);
  9. $dispatcher->addListener(
  10.     KernelEvents::REQUEST,
  11.     [$firewall, 'onKernelRequest']
  12. );

The firewall is registered to listen to the kernel.request event that will be dispatched by the HttpKernel at the beginning of each request it processes. This way, the firewall may prevent the user from going any further than allowed.

Firewall Config

The information about a given firewall, such as its name, provider, context, entry point and access denied URL, is provided by instances of the Symfony\Bundle\SecurityBundle\Security\FirewallConfig class.

This object can be accessed through the getFirewallConfig(Request $request) method of the Symfony\Bundle\SecurityBundle\Security\FirewallMap class and through the getConfig() method of the Symfony\Bundle\SecurityBundle\Security\FirewallContext class.

Firewall Listeners

When the firewall gets notified of the kernel.request event, it asks the firewall map if the request matches one of the secured areas. The first secured area that matches the request will return a set of corresponding firewall listeners (which each is a callable). These listeners will all be asked to handle the current request. This basically means: find out if the current request contains any information by which the user might be authenticated (for instance the Basic HTTP authentication listener checks if the request has a header called PHP_AUTH_USER).

Exception Listener

If any of the listeners throws an Symfony\Component\Security\Core\Exception\AuthenticationException, the exception listener that was provided when adding secured areas to the firewall map will jump in.

The exception listener determines what happens next, based on the arguments it received when it was created. It may start the authentication procedure, perhaps ask the user to supply their credentials again (when they have only been authenticated based on a “remember-me” cookie), or transform the exception into an Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException, which will eventually result in an “HTTP/1.1 403: Access Denied” response.

Entry Points

When the user is not authenticated at all (i.e. when the token storage has no token yet), the firewall’s entry point will be called to “start” the authentication process. An entry point should implement Symfony\Component\Security\Http\EntryPoint\AuthenticationEntryPointInterface, which has only one method: [start()](https://github.com/symfony/symfony/blob/4.4/src/Symfony/Component/Security/Http/EntryPoint/AuthenticationEntryPointInterface.php "Symfony\Component\Security\Http\EntryPoint\AuthenticationEntryPointInterface::start()"). This method receives the current Symfony\Component\HttpFoundation\Request object and the exception by which the exception listener was triggered. The method should return a Symfony\Component\HttpFoundation\Response object. This could be, for instance, the page containing the login form or, in the case of Basic HTTP authentication, a response with a WWW-Authenticate header, which will prompt the user to supply their username and password.

Flow: Firewall, Authentication, Authorization

Hopefully you can now see a little bit about how the “flow” of the security context works:

  1. The Firewall is registered as a listener on the kernel.request event;
  2. At the beginning of the request, the Firewall checks the firewall map to see if any firewall should be active for this URL;
  3. If a firewall is found in the map for this URL, its listeners are notified;
  4. Each listener checks to see if the current request contains any authentication information - a listener may (a) authenticate a user, (b) throw an AuthenticationException, or (c) do nothing (because there is no authentication information on the request);
  5. Once a user is authenticated, you’ll use Authorization to deny access to certain resources.

Read the next articles to find out more about Authentication and Authorization.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.