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Access Control Lists

class AclComponent(ComponentCollection $collection, array $settings = array())

Understanding How ACL Works

Powerful things require access control. Access control lists are a way to manage application permissions in a fine-grained, yet easily maintainable and manageable way.

Access control lists, or ACL, handle two main things: things that want stuff, and things that are wanted. In ACL lingo, things (most often users) that want to use stuff are represented by access request objects, or AROs. Things in the system that are wanted (most often actions or data) are represented by access control objects, or ACOs. The entities are called ‘objects’ because sometimes the requesting object isn’t a person. Sometimes you might want to limit the ability of certain CakePHP controllers to initiate logic in other parts of your application. ACOs could be anything you want to control, from a controller action, to a web service, to a line in your grandma’s online diary.

To review:

  • ACO - Access Control Object - Represents something that is wanted
  • ARO - Access Request Object - Represents something that wants something else

Essentially, ACLs are used to decide when an ARO can have access to an ACO.

In order to help you understand how everything works together, let’s use a semi-practical example. Imagine, for a moment, a computer system used by a familiar group of adventurers from the fantasy novel Lord of the Rings. The leader of the group, Gandalf, wants to manage the party’s assets while maintaining a healthy amount of privacy and security for the other members of the party. The first thing he needs to do is create a list of the AROs (requesters) involved:

  • Gandalf
  • Aragorn
  • Bilbo
  • Frodo
  • Gollum
  • Legolas
  • Gimli
  • Pippin
  • Merry

Note

Realize that ACL is not the same as authentication. ACL is what happens after a user has been authenticated. Although the two are usually used in concert, it’s important to realize the difference between knowing who someone is (authentication) and knowing what they can do (ACL).

The next thing Gandalf needs to do is make an initial list of ACOs (resources) the system will handle. His list might look something like:

  • Weapons
  • The One Ring
  • Salted Pork
  • Diplomacy
  • Ale

Traditionally, systems were managed using a sort of matrix that showed a basic set of users and permissions relating to objects. If this information were stored in a table, it might look like this:

x Weapons The Ring Salted Pork Diplomacy Ale
Gandalf     Allow Allow Allow
Aragorn Allow   Allow Allow Allow
Bilbo         Allow
Frodo   Allow     Allow
Gollum     Allow    
Legolas Allow   Allow Allow Allow
Gimli Allow   Allow    
Pippin       Allow Allow
Merry       Allow Allow

At first glance, it seems that this sort of system could work rather well. Assignments can be made to protect security (only Frodo can access the ring) and protect against accidents (keeping the hobbits out of the salted pork and weapons). It seems sufficiently fine-grained and easy to read, right?

For a small system like this, maybe a matrix setup would work. But for a growing system, or a system with a large number of resources (ACOs) and users (AROs), a table can quickly become unwieldy. Imagine trying to control access to the hundreds of war encampments and trying to manage them by unit. Another drawback to matrices is that you can’t create logical groups of users or make cascading permissions changes to groups of users based on those logical groupings. For example, it would sure be nice to automatically allow the hobbits access to the ale and pork once the battle is over. Doing it on an individual user basis would be tedious and error prone. Making a cascading permissions change to all members of the ‘hobbit’ group at once would be easy.

ACL is most usually implemented in a tree structure, with a tree of AROs and a tree of ACOs. By organizing your objects in trees, you can deal out permissions in a granular fashion while maintaining a good grip on the big picture. Being the wise leader he is, Gandalf elects to use ACL in his new system, and organizes his objects along the following lines:

  • Fellowship of the Ring™
    • Warriors
      • Aragorn
      • Legolas
      • Gimli
    • Wizards
      • Gandalf
    • Hobbits
      • Frodo
      • Bilbo
      • Merry
      • Pippin
    • Visitors
      • Gollum

Using a tree structure for AROs allows Gandalf to define permissions that apply to entire groups of users at once. So, using our ARO tree, Gandalf can tack on a few group-based permissions:

  • Fellowship of the Ring (Deny: all)
    • Warriors (Allow: Weapons, Ale, Elven Rations, Salted Pork)
      • Aragorn
      • Legolas
      • Gimli
    • Wizards (Allow: Salted Pork, Diplomacy, Ale)
      • Gandalf
    • Hobbits (Allow: Ale)
      • Frodo
      • Bilbo
      • Merry
      • Pippin
    • Visitors (Allow: Salted Pork)
      • Gollum

If we wanted to use ACL to see whether Pippin was allowed to access the ale, we’d first consult the tree to retrieve his path through it, which is Fellowship->Hobbits->Pippin. Then we see the different permissions that reside at each of those points, and use the most specific permission relating to Pippin and the Ale.

ARO Node Permission Info Result
Fellowship of the Ring Deny all Denying access to ale.
Hobbits Allow ‘ale’ Allowing access to ale!
Pippin Still allowing ale!

Note

Since the ‘Pippin’ node in the ACL tree doesn’t specifically deny access to the ale ACO, the final result is that we allow access to that ACO.

The tree also allows us to make finer adjustments for more granular control, while still keeping the ability to make sweeping changes to groups of AROs:

  • Fellowship of the Ring (Deny: all)
    • Warriors (Allow: Weapons, Ale, Elven Rations, Salted Pork)
      • Aragorn (Allow: Diplomacy)
      • Legolas
      • Gimli
    • Wizards (Allow: Salted Pork, Diplomacy, Ale)
      • Gandalf
    • Hobbits (Allow: Ale)
      • Frodo (Allow: Ring)
      • Bilbo
      • Merry (Deny: Ale)
      • Pippin (Allow: Diplomacy)
    • Visitors (Allow: Salted Pork)
      • Gollum

This approach allows us the ability to make both wide-reaching permissions changes and fine-grained adjustments. This allows us to say that all hobbits can have access to ale, with one exception: Merry. To see whether Merry can access the Ale, we’d find his path in the tree: Fellowship->Hobbits->Merry. Then we’d work our way down, keeping track of ale-related permissions:

ARO Node Permission Info Result
Fellowship of the Ring Deny all Denying access to ale.
Hobbits Allow ‘ale’ Allowing access to ale!
Merry Deny Ale Denying ale.

Defining Permissions: CakePHP’s INI-based ACL

CakePHP’s first ACL implementation was based on INI files stored in the CakePHP installation. While it’s useful and stable, we recommend that you use the database backed ACL solution, mostly because of its ability to create new ACOs and AROs on the fly. We meant it for usage in simple applications - and especially for those folks who for some reason might not be using a database.

By default, CakePHP’s ACL is database-driven. To enable INI-based ACL, you’ll need to tell CakePHP what system you’re using by updating the following lines in app/Config/core.php

// Change these lines:
Configure::write('Acl.classname', 'DbAcl');
Configure::write('Acl.database', 'default');

// to look like this:
Configure::write('Acl.classname', 'IniAcl');
//Configure::write('Acl.database', 'default');

ARO/ACO permissions are specified in /app/Config/acl.ini.php. The basic idea is that AROs are specified in an INI section that has three properties: groups, allow, and deny.

  • groups: names of ARO groups of which this ARO is a member
  • allow: names of ACOs to which this ARO has access
  • deny: names of ACOs to which this ARO should be denied access

ACOs are specified in INI sections that only include the allow and deny properties.

As an example, let’s see how the Fellowship ARO structure we’ve been crafting would look in INI syntax:

;-------------------------------------
; AROs
;-------------------------------------
[aragorn]
groups = warriors
allow = diplomacy

[legolas]
groups = warriors

[gimli]
groups = warriors

[gandalf]
groups = wizards

[frodo]
groups = hobbits
allow = ring

[bilbo]
groups = hobbits

[merry]
groups = hobbits
deny = ale

[pippin]
groups = hobbits

[gollum]
groups = visitors

;-------------------------------------
; ARO Groups
;-------------------------------------
[warriors]
allow = weapons, ale, salted_pork

[wizards]
allow = salted_pork, diplomacy, ale

[hobbits]
allow = ale

[visitors]
allow = salted_pork

Now that you’ve got your permissions defined via the INI mechanism, you can skip to the section on checking permissions using the ACL component. Alternatively, you can keep reading to see how you would define the same permissions using a database ACL.

Defining Permissions: CakePHP’s Database ACL

Now that we’ve covered INI-based ACL permissions, let’s move on to the (more commonly used) database ACL.

Getting Started

The default ACL permissions implementation is powered by a database. CakePHP’s database ACL consists of a set of core models and a console application that comes with your CakePHP installation. The models are used by CakePHP to interact with your database in order to store and retrieve nodes in tree format. The console application is used to initialize your database and interact with your ACO and ARO trees.

To get started, first you’ll need to make sure your /app/Config/database.php is present and correctly configured. See section 4.1 for more information on database configuration.

Once you’ve done that, use the CakePHP console to create your ACL database tables:

$ cake schema create DbAcl

Running this command will drop and re-create the tables necessary to store ACO and ARO information in tree format. The output of the console application should look something like the following:

---------------------------------------------------------------
Cake Schema Shell
---------------------------------------------------------------

The following tables will be dropped.
acos
aros
aros_acos

Are you sure you want to drop the tables? (y/n)
[n] > y
Dropping tables.
acos updated.
aros updated.
aros_acos updated.

The following tables will be created.
acos
aros
aros_acos

Are you sure you want to create the tables? (y/n)
[y] > y
Creating tables.
acos updated.
aros updated.
aros_acos updated.
End create.

Note

This replaces an older deprecated command, “initdb”.

You can also use the SQL file found in app/Config/Schema/db_acl.sql, but that’s nowhere near as fun.

When finished, you should have three new database tables in your system: acos, aros, and aros_acos (the join table to create permissions information between the two trees).

Note

If you’re curious about how CakePHP stores tree information in these tables, read up on modified database tree traversal. The ACL component uses CakePHP’s Tree to manage the trees’ inheritances. The model class files for ACL can be found in lib/Cake/Model/.

Now that we’re all set up, let’s work on creating some ARO and ACO trees.

Creating Access Request Objects (AROs) and Access Control Objects (ACOs)

When creating new ACL objects (ACOs and AROs), realize that there are two main ways to name and access nodes. The first method is to link an ACL object directly to a record in your database by specifying a model name and foreign key value. The second can be used when an object has no direct relation to a record in your database - you can provide a textual alias for the object.

Note

In general, when you’re creating a group or higher-level object, use an alias. If you’re managing access to a specific item or record in the database, use the model/foreign key method.

You create new ACL objects using the core CakePHP ACL models. In doing so, there are a number of fields you’ll want to use when saving data: model, foreign_key, alias, and parent_id.

The model and foreign_key fields for an ACL object allow you to link the object to its corresponding model record (if there is one). For example, many AROs will have corresponding User records in the database. Setting an ARO’s foreign_key to the User’s ID will allow you to link up ARO and User information with a single User model find() call if you’ve set up the correct model associations. Conversely, if you want to manage edit operation on a specific blog post or recipe listing, you may choose to link an ACO to that specific model record.

An alias is just a human-readable label you can use to identify an ACL object that has no direct model record correlation. Aliases are generally useful in naming user groups or ACO collections.

The parent_id for an ACL object allows you to fill out the tree structure. Supply the ID of the parent node in the tree to create a new child.

Before we can create new ACL objects, we’ll need to load up their respective classes. The easiest way to do this is to include CakePHP’s ACL Component in your controller’s $components array:

public $components = array('Acl');

Once we’ve got that done, let’s see some examples of creating these objects. The following code could be placed in a controller action:

Note

While the examples here focus on ARO creation, the same techniques can be used to create an ACO tree.

Remaining with our Fellowship example, let’s first create our ARO groups. Because they won’t have specific records tied to them, we’ll use aliases to create the ACL objects. Here, we create them via a controller action, but we could do it elsewhere.

Our approach shouldn’t be drastically new - we’re just using models to save data like we always do:

public function any_action() {
    $aro = $this->Acl->Aro;

    // Here's all of our group info in an array we can iterate through
    $groups = array(
        0 => array(
            'alias' => 'warriors'
        ),
        1 => array(
            'alias' => 'wizards'
        ),
        2 => array(
            'alias' => 'hobbits'
        ),
        3 => array(
            'alias' => 'visitors'
        ),
    );

    // Iterate and create ARO groups
    foreach ($groups as $data) {
        // Remember to call create() when saving in loops...
        $aro->create();

        // Save data
        $aro->save($data);
    }

    // Other action logic goes here...
}

Once we’ve got the groups, we can use the ACL console application to verify the tree structure.

$ cake acl view aro

Aro tree:
---------------------------------------------------------------
  [1]warriors

  [2]wizards

  [3]hobbits

  [4]visitors

---------------------------------------------------------------

The tree is still simple at this point, but at least we’ve got some verification that we’ve got four top-level nodes. Let’s add some children to those ARO nodes by putting our specific user AROs under these groups. Every good citizen of Middle Earth has an account in our new system, so we’ll tie these ARO records to specific model records in our database.

Note

When adding child nodes to a tree, make sure to use the ACL node ID, rather than a foreign_key value.

public function any_action() {
    $aro = new Aro();

    // Here are our user records, ready to be linked to new ARO records.
    // This data could come from a model and be modified, but we're using static
    // arrays here for demonstration purposes.

    $users = array(
        0 => array(
            'alias' => 'Aragorn',
            'parent_id' => 1,
            'model' => 'User',
            'foreign_key' => 2356,
        ),
        1 => array(
            'alias' => 'Legolas',
            'parent_id' => 1,
            'model' => 'User',
            'foreign_key' => 6342,
        ),
        2 => array(
            'alias' => 'Gimli',
            'parent_id' => 1,
            'model' => 'User',
            'foreign_key' => 1564,
        ),
        3 => array(
            'alias' => 'Gandalf',
            'parent_id' => 2,
            'model' => 'User',
            'foreign_key' => 7419,
        ),
        4 => array(
            'alias' => 'Frodo',
            'parent_id' => 3,
            'model' => 'User',
            'foreign_key' => 7451,
        ),
        5 => array(
            'alias' => 'Bilbo',
            'parent_id' => 3,
            'model' => 'User',
            'foreign_key' => 5126,
        ),
        6 => array(
            'alias' => 'Merry',
            'parent_id' => 3,
            'model' => 'User',
            'foreign_key' => 5144,
        ),
        7 => array(
            'alias' => 'Pippin',
            'parent_id' => 3,
            'model' => 'User',
            'foreign_key' => 1211,
        ),
        8 => array(
            'alias' => 'Gollum',
            'parent_id' => 4,
            'model' => 'User',
            'foreign_key' => 1337,
        ),
    );

    // Iterate and create AROs (as children)
    foreach ($users as $data) {
        // Remember to call create() when saving in loops...
        $aro->create();

        //Save data
        $aro->save($data);
    }

    // Other action logic goes here...
}

Note

Typically you won’t supply both an alias and a model/foreign_key, but we’re using both here to make the structure of the tree easier to read for demonstration purposes.

The output of that console application command should now be a little more interesting. Let’s give it a try:

$ cake acl view aro

Aro tree:
---------------------------------------------------------------
  [1]warriors

    [5]Aragorn

    [6]Legolas

    [7]Gimli

  [2]wizards

    [8]Gandalf

  [3]hobbits

    [9]Frodo

    [10]Bilbo

    [11]Merry

    [12]Pippin

  [4]visitors

    [13]Gollum

---------------------------------------------------------------

Now that we’ve got our ARO tree setup properly, let’s discuss a possible approach for structuring an ACO tree. While we can put together a more abstract representation of our ACO’s, it’s often more practical to model an ACO tree after CakePHP’s Controller/Action setup. We’ve got five main objects we’re handling in this Fellowship scenario. The natural setup for this in a CakePHP application consists of a group of models, and ultimately the controllers that manipulate them. Beyond the controllers themselves, we’ll want to control access to specific actions in those controllers.

Let’s set up an ACO tree that will mimic a CakePHP app setup. Since we have five ACOs, we’ll create an ACO tree that should end up looking something like the following:

  • Weapons
  • Rings
  • PorkChops
  • DiplomaticEfforts
  • Ales

You can create children nodes under each of these five main ACOs, but using CakePHP’s built-in action management covers basic CRUD operations on a given object. Keeping this in mind will make your ACO trees smaller and easier to maintain. We’ll see how these are used later on when we discuss how to assign permissions.

Since you’re now a pro at adding AROs, use those same techniques to create this ACO tree. Create these upper level groups using the core Aco model.

Assigning Permissions

After creating our ACOs and AROs, we can finally assign permissions between the two groups. This is done using CakePHP’s core Acl component. Let’s continue with our example.

Here we’ll work with Acl permisions in the context of a controller action. Let’s set up some basic permissions using the AclComponent in an action inside our controller.

class SomethingsController extends AppController {
    // You might want to place this in the AppController
    // instead, but here works great too.
    public $components = array('Acl');

    public function index() {
        // Allow warriors complete access to weapons
        // Both these examples use the alias syntax
        $this->Acl->allow('warriors', 'Weapons');

        // Though the King may not want to let everyone
        // have unfettered access
        $this->Acl->deny('warriors/Legolas', 'Weapons', 'delete');
        $this->Acl->deny('warriors/Gimli',   'Weapons', 'delete');
public function index() {
    // Allow warriors complete access to weapons
    // Both these examples use the alias syntax
    $this->Acl->allow('warriors', 'Weapons');

    // However, the King does not want to let everyone
    // have unfettered access
    $this->Acl->deny('warriors/Legolas', 'Weapons', 'delete');
    $this->Acl->deny('warriors/Gimli',   'Weapons', 'delete');

    die(print_r('done', 1));
}

The first call we make to the AclComponent allows any user under the ‘warriors’ ARO group full access to anything under the ‘Weapons’ ACO group. Here we’re just addressing ACOs and AROs by their aliases.

Notice the usage of the third parameter? One nice thing about the CakePHP ACL setup is that permissions contain four built-in properties related to CRUD (create, read, update, and delete) actions for convenience. The default options for that parameter are create, read, update, and delete but you can add a column in the aros_acos database table (prefixed with _ - for example _admin) and use it alongside the defaults.

The second set of calls is an attempt to make a more fine-grained permission decision. We want Aragorn to keep his full-access privileges, but we want to deny other warriors in the group the ability to delete Weapons records. We’re using the alias syntax to address the AROs above, but you might want to use the model/foreign key syntax yourself. What we have above is equivalent to this:

// 6342 = Legolas
// 1564 = Gimli

$this->Acl->deny(
  array('model' => 'User', 'foreign_key' => 6342),
  'Weapons',
  'delete'
);
$this->Acl->deny(
  array('model' => 'User', 'foreign_key' => 1564),
  'Weapons',
  'delete'
);

Note

Addressing a node using the alias syntax uses a slash-delimited string (‘/users/employees/developers’). Addressing a node using model/foreign key syntax uses an array with two parameters: array('model' => 'User', 'foreign_key' => 8282).

The next section will help us validate our setup by using the AclComponent to check the permissions we’ve just set up.

Checking Permissions: The ACL Component

Let’s use the AclComponent to make sure dwarves and elves can’t remove things from the armory. At this point, we should be able to use the AclComponent to make a check between the ACOs and AROs we’ve created. The basic syntax for making a permissions check is:

$this->Acl->check($aro, $aco, $action = '*');

Let’s give it a try inside a controller action:

public function index() {
    // These all return true:
    $this->Acl->check('warriors/Aragorn', 'Weapons');
    $this->Acl->check('warriors/Aragorn', 'Weapons', 'create');
    $this->Acl->check('warriors/Aragorn', 'Weapons', 'read');
    $this->Acl->check('warriors/Aragorn', 'Weapons', 'update');
    $this->Acl->check('warriors/Aragorn', 'Weapons', 'delete');

    // Remember, we can use the model/id syntax
    // for our user AROs
    $this->Acl->check(array('User' => array('id' => 2356)), 'Weapons');

    // These also return true:
    $result = $this->Acl->check('warriors/Legolas', 'Weapons', 'create');
    $result = $this->Acl->check('warriors/Gimli', 'Weapons', 'read');

    // But these return false:
    $result = $this->Acl->check('warriors/Legolas', 'Weapons', 'delete');
    $result = $this->Acl->check('warriors/Gimli', 'Weapons', 'delete');
}

The usage here is for demonstration, but this type of checking can be used to decide whether to allow an action, show an error message, or redirect the user to a login.