The Firewall and Security Context

The Firewall and Security Context

Central to the Security component is the security context, which is an instance of SecurityContextInterface. When all steps in the process of authenticating the user have been taken successfully, you can ask the security context if the authenticated user has access to a certain action or resource of the application:

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use Symfony\Component\Security\Core\SecurityContext;
use Symfony\Component\Security\Core\Exception\AccessDeniedException;

// instance of Symfony\Component\Security\Core\Authentication\AuthenticationManagerInterface
$authenticationManager = ...;

// instance of Symfony\Component\Security\Core\Authorization\AccessDecisionManagerInterface
$accessDecisionManager = ...;

$securityContext = new SecurityContext(
    $authenticationManager,
    $accessDecisionManager
);

// ... authenticate the user

if (!$securityContext->isGranted('ROLE_ADMIN')) {
    throw new AccessDeniedException();
}

2.6As of Symfony 2.6, the SecurityContext class was split in the AuthorizationChecker and TokenStorage classes.

Note

Read the dedicated sections 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:

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use Symfony\Component\Security\Http\FirewallMap;
use Symfony\Component\HttpFoundation\RequestMatcher;
use Symfony\Component\Security\Http\Firewall\ExceptionListener;

$map = new FirewallMap();

$requestMatcher = new RequestMatcher('^/secured-area/');

// instances of Symfony\Component\Security\Http\Firewall\ListenerInterface
$listeners = array(...);

$exceptionListener = new ExceptionListener(...);

$map->add($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 HttpKernel:

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use Symfony\Component\Security\Http\Firewall;
use Symfony\Component\HttpKernel\KernelEvents;

// the EventDispatcher used by the HttpKernel
$dispatcher = ...;

$firewall = new Firewall($map, $dispatcher);

$dispatcher->addListener(
    KernelEvents::REQUEST,
    array($firewall, 'onKernelRequest')
);

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 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 implement ListenerInterface). 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 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 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 AuthenticationEntryPointInterface, which has only one method: start(). This method receives the current Request object and the exception by which the exception listener was triggered. The method should return a 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 sections to find out more about Authentication and Authorization.

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