How to Use Access Control Lists (ACLs)

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In complex applications, you will often face the problem that access decisions cannot only be based on the person (Token) who is requesting access, but also involve a domain object that access is being requested for. This is where the ACL system comes in.

Alternatives to ACLs

Using ACL's isn't trivial, and for simpler use cases, it may be overkill. If your permission logic could be described by just writing some code (e.g. to check if a Blog is owned by the current User), then consider using voters. A voter is passed the object being voted on, which you can use to make complex decisions and effectively implement your own ACL. Enforcing authorization (e.g. the isGranted() part) will look similar to what you see in this article, but your voter class will handle the logic behind the scenes, instead of the ACL system.

Imagine you are designing a blog system where your users can comment on your posts. Now, you want a user to be able to edit their own comments, but not those of other users; besides, you want to be able to edit all comments. In this scenario, Comment would be the domain object that you want to restrict access to. You could take several approaches to accomplish this using Symfony, two basic approaches are (non-exhaustive):

  • Enforce security in your business methods: Basically, that means keeping a reference inside each Comment to all users who have access, and then compare these users to the provided Token.
  • Enforce security with roles: In this approach, you would add a role for each Comment object, i.e. ROLE_COMMENT_1, ROLE_COMMENT_2, etc.

Both approaches are perfectly valid. However, they couple your authorization logic to your business code which makes it less reusable elsewhere, and also increases the difficulty of unit testing. Besides, you could run into performance issues if many users would have access to a single domain object.

Fortunately, there is a better way, which you will find out about now.

Bootstrapping

Now, before you can finally get into action, you need to do some bootstrapping. First, you need to configure the connection the ACL system is supposed to use:

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# app/config/security.yml
security:
    # ...

    acl:
        connection: default

Note

The ACL system requires a connection from either Doctrine DBAL (usable by default) or Doctrine MongoDB (usable with MongoDBAclBundle). However, that does not mean that you have to use Doctrine ORM or ODM for mapping your domain objects. You can use whatever mapper you like for your objects, be it Doctrine ORM, MongoDB ODM, Propel, raw SQL, etc. The choice is yours.

After the connection is configured, you have to import the database structure running the following command:

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$ php app/console init:acl

Getting Started

Coming back to the small example from the beginning, you can now implement ACL for it.

Once the ACL is created, you can grant access to objects by creating an Access Control Entry (ACE) to solidify the relationship between the entity and your user.

Creating an ACL and Adding an ACE

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// src/AppBundle/Controller/BlogController.php
namespace AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\Security\Core\Exception\AccessDeniedException;
use Symfony\Component\Security\Acl\Domain\ObjectIdentity;
use Symfony\Component\Security\Acl\Domain\UserSecurityIdentity;
use Symfony\Component\Security\Acl\Permission\MaskBuilder;

class BlogController extends Controller
{
    // ...

    public function addCommentAction(Post $post)
    {
        $comment = new Comment();

        // ... setup $form, and submit data

        if ($form->isSubmitted() && $form->isValid()) {
            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($comment);
            $entityManager->flush();

            // creating the ACL
            $aclProvider = $this->get('security.acl.provider');
            $objectIdentity = ObjectIdentity::fromDomainObject($comment);
            $acl = $aclProvider->createAcl($objectIdentity);

            // retrieving the security identity of the currently logged-in user
            $tokenStorage = $this->get('security.token_storage');
            $user = $tokenStorage->getToken()->getUser();
            $securityIdentity = UserSecurityIdentity::fromAccount($user);

            // grant owner access
            $acl->insertObjectAce($securityIdentity, MaskBuilder::MASK_OWNER);
            $aclProvider->updateAcl($acl);
        }
    }
}

There are a couple of important implementation decisions in this code snippet. For now, I only want to highlight two:

First, you may have noticed that ->createAcl() does not accept domain objects directly, but only implementations of the ObjectIdentityInterface. This additional step of indirection allows you to work with ACLs even when you have no actual domain object instance at hand. This will be extremely helpful if you want to check permissions for a large number of objects without actually hydrating these objects.

The other interesting part is the ->insertObjectAce() call. In the example, you are granting the user who is currently logged in owner access to the Comment. The MaskBuilder::MASK_OWNER is a pre-defined integer bitmask; don't worry the mask builder will abstract away most of the technical details, but using this technique you can store many different permissions in one database row which gives a considerable boost in performance.

Tip

The order in which ACEs are checked is significant. As a general rule, you should place more specific entries at the beginning.

Checking Access

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// src/AppBundle/Controller/BlogController.php

// ...

class BlogController
{
    // ...

    public function editCommentAction(Comment $comment)
    {
        $authorizationChecker = $this->get('security.authorization_checker');

        // check for edit access
        if (false === $authorizationChecker->isGranted('EDIT', $comment)) {
            throw new AccessDeniedException();
        }

        // ... retrieve actual comment object, and do your editing here
    }
}

In this example, you check whether the user has the EDIT permission. Internally, Symfony maps the permission to several integer bitmasks, and checks whether the user has any of them.

Note

You can define up to 32 base permissions (depending on your OS PHP might vary between 30 to 32). In addition, you can also define cumulative permissions.

Cumulative Permissions

In the first example above, you only granted the user the OWNER base permission. While this effectively also allows the user to perform any operation such as view, edit, etc. on the domain object, there are cases where you may want to grant these permissions explicitly.

The MaskBuilder can be used for creating bit masks easily by combining several base permissions:

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$builder = new MaskBuilder();
$builder
    ->add('view')
    ->add('edit')
    ->add('delete')
    ->add('undelete')
;
$mask = $builder->get(); // int(29)

This integer bitmask can then be used to grant a user the base permissions you added above:

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$identity = new UserSecurityIdentity('johannes', 'AppBundle\Entity\User');
$acl->insertObjectAce($identity, $mask);

The user is now allowed to view, edit, delete, and un-delete objects.

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