How can I implement an Access Control List in my Web MVC application?

Question

First question

Please, could you explain me how simpliest ACL could be implemented in MVC.

Here is the first approach of using Acl in Controller...

<?php
class MyController extends Controller {

  public function myMethod() {        
    //It is just abstract code
    $acl = new Acl();
    $acl->setController('MyController');
    $acl->setMethod('myMethod');
    $acl->getRole();
    if (!$acl->allowed()) die("You're not allowed to do it!");
    ...    
  }

}
?>

It is very bad approach, and it's minus is that we have to add Acl piece of code into each controller's method, but we don't need any additional dependencies!

Next approach is to make all controller's methods private and add ACL code into controller's __call method.

<?php
class MyController extends Controller {

  private function myMethod() {
    ...
  }

  public function __call($name, $params) {
    //It is just abstract code
    $acl = new Acl();
    $acl->setController(__CLASS__);
    $acl->setMethod($name);
    $acl->getRole();
    if (!$acl->allowed()) die("You're not allowed to do it!");
    ...   
  }

}
?>

It is better than previous code, but main minuses are...

• All controller's methods should be private
• We have to add ACL code into each controller's __call method.

The next approach is to put Acl code into parent Controller, but we still need to keep all child controller's methods private.

What is the solution? And what is the best practice? Where should I call Acl functions to decide allow or disallow method to be executed.

Second question

Second question is about getting role using Acl. Let's imagine that we have guests, users and user's friends. User have restricted access to viewing his profile that only friends can view it. All guests can't view this user's profile. So, here is the logic..

• we have to ensure that method being called is profile
• we have to detect owner of this profile
• we have to detect is viewer is owner of this profile or no
• we have to read restriction rules about this profile
• we have to decide execute or not execute profile method

The main question is about detecting owner of profile. We can detect who is owner of profile only executing model's method $model->getOwner(), but Acl do not have access to model. How can we implement this?

Answer 1

First part/answer (ACL implementation)

In my humble opinion, the best way to approach this would be to use decorator pattern, Basically, this means that you take your object, and place it inside another object, which will act like a protective shell. This would NOT require you to extend the original class. Here is an example:

class SecureContainer
{

    protected $target = null;
    protected $acl = null;

    public function __construct( $target, $acl )
    {
        $this->target = $target;
        $this->acl = $acl;
    }

    public function __call( $method, $arguments )
    {
        if ( 
             method_exists( $this->target, $method )
          && $this->acl->isAllowed( get_class($this->target), $method )
        ){
            return call_user_func_array( 
                array( $this->target, $method ),
                $arguments
            );
        }
    }

}

And this would be how you use this sort of structure:

// assuming that you have two objects already: $currentUser and $controller
$acl = new AccessControlList( $currentUser );

$controller = new SecureContainer( $controller, $acl );
// you can execute all the methods you had in previous controller 
// only now they will be checked against ACL
$controller->actionIndex();

As you might notice, this solution has several advantages:

1. containment can be used on any object, not just instances of Controller
2. check for authorization happens outside the target object, which means that:
   • original object is not responsible for access control, adheres to SRP
   • when you get "permission denied", you are not locked inside a controller, more options
3. you can inject this secured instance in any other object, it will retain the protection
4. wrap it & forget it .. you can pretend that it is the original object, it will react the same

But, there are one major issue with this method too - you cannot natively check if secured object implements and interface ( which also applies for looking up existing methods ) or is part of some inheritance chain.

Second part/answer (RBAC for objects)

In this case the main difference you should recognize is that you Domain Objects (in example: Profile) itself contains details about owner. This means, that for you to check, if (and at which level) user has access to it, it will require you to change this line:

$this->acl->isAllowed( get_class($this->target), $method )

Essentially you have two options:

• Provide the ACL with the object in question. But you have to be careful not to violate Law of Demeter:

  $this->acl->isAllowed( get_class($this->target), $method )
  

• Request all the relevant details and provide the ACL only with what it needs, which will also make it a bit more unit-testing friendly:

  $command = array( get_class($this->target), $method );
  /* -- snip -- */
  $this->acl->isAllowed( $this->target->getPermissions(), $command )
  

Couple videos that might help you to come up with your own implementation:

• Inheritance, Polymorphism, & Testing
• Don't Look For Things!

Side notes

You seem to have the quite common ( and completely wrong ) understanding of what Model in MVC is. Model is not a class. If you have class named FooBarModel or something that inherits AbstractModel then you are doing it wrong.

In proper MVC the Model is a layer, which contains a lot of classes. Large part of the classes can be separated in two groups , based on the responsibility:

- Domain Business Logic

(read more: here and here):

Instances from this group of classes deal with computation of values, check for different conditions, implement sales rules and do all the rest what you would call "business logic". They have no clue how data is stored, where it is stored or even if storage exists in first place.

Domain Business object do not depend on database. When you are creating an invoice, it does not matter where data comes from. It can be either from SQL or from a remote REST API, or even screenshot of a MSWord document. The business logic does no change.

- Data Access and Storage

Instances made from this group of classes are sometimes called Data Access Objects. Usually structures that implement Data Mapper pattern ( do not confuse with ORMs of same name .. no relation ). This is where your SQL statements would be (or maybe your DomDocument, because you store it in XML).

Beside the two major parts, there is one more group of instances/classes, that should be mentioned:

- Services

This is where your and 3rd party components come in play. For example, you can think of "authentication" as service, which can be provided by your own, or some external code. Also "mail sender" would be a service, which might knit together some domain object with a PHPMailer or SwiftMailer, or your own mail-sender component.

Another source of services are abstraction on to on domain and data access layers. They are created to simplify the code used by controllers. For example: creating new user account might require to work with several domain objects and mappers. But, by using a service, it will need only one or two lines in the controller.

What you have to remember when making services, is that the whole layer is supposed to be thin. There is no business logic in services. They are only there to juggle domain object, components and mappers.

One of things they all have in common would be that services do not affect the View layer in any direct way, and are autonomous to such an extent, that they can be ( and quit often - are ) used outside the MVC structure itself. Also such self-sustained structures make the migration to a different framework/architecture much easier, because of extremely low coupling between service and the rest of application.

Answer 2

ACL and Controllers

First of all: These are different things / layers most often. As you criticize the exemplary controller code, it puts both together - most obviously too tight.

tereško already outlined a way how you could decouple this more with the decorator pattern.

I'd go one step back first to look for the original problem you're facing and discuss that a bit then.

On the one hand you want to have controllers that just do the work they're commanded to (command or action, let's call it command).

On the other hand you want to be able to put ACL in your application. The field of work of these ACLs should be - if I understood your question right - to control access to certain commands of your applications.

This kind of access control therefore needs something else that brings these two together. Based on the context in which a command is executed in, ACL kicks in and decisions need to be done whether or not a specific command can be executed by a specific subject (e.g. the user).

Let's summarize to this point what we have:

• Command
• ACL
• User

The ACL component is central here: It needs to know at least something about the command (to identify the command to be precise) and it needs to be able to identify the user. Users are normally easily identified by a unique ID. But often in webapplications there are users that are not identified at all, often called guest, anonymous, everybody etc.. For this example we assume that the ACL can consume a user object and encapsulate these details away. The user object is bound to the application request object and the ACL can consume it.

What about identifying a command? Your interpretation of the MVC pattern suggests that a command is compound of a classname and a method name. If we look more closely there are even arguments (parameters) for a command. So it's valid to ask what exactly identifies a command? The classname, the methodname, the number or names of arguments, even the data inside any of the arguments or a mixture of all this?

Depending on which level of detail you need to identify a command in your ACL'ing, this can vary a lot. For the example let's keep it simply and specify that a command is identified by the classname and method name.

So the context of how these three parts (ACL, Command and User) are belonging to each other is now more clear.

We could say, with an imaginary ACL compontent we can already do the following:

$acl->commandAllowedForUser($command, $user);

Just see what is happeninig here: By making both the command and the user identifiable, the ACL can do it's work. The job of the ACL is unrelated to the work of both the user object and the concrete command.

There is only one part missing, this can't live in the air. And it doesn't. So you need to locate the place where the access control needs to kick in. Let's take a look what happens in a standard webapplication:

User -> Browser -> Request (HTTP)
   -> Request (Command) -> Action (Command) -> Response (Command) 
   -> Response(HTTP) -> Browser -> User

To locate that place, we know it must be before the concrete command is executed, so we can reduce that list and only need to look into the following (potential) places:

User -> Browser -> Request (HTTP)
   -> Request (Command)

At some point in your application you know that a specific user has requested to perform a concrete command. You already do some sort of ACL'ing here: If a user requests a command which does not exists, you don't allow that command to execute. So where-ever that happens in your application might be a good place to add the "real" ACL checks:

The command has been located and we can create the identification of it so the ACL can deal with it. In case the command is not allowed for a user, the command will not be executed (action). Maybe a CommandNotAllowedResponse instead of the CommandNotFoundResponse for the case a request could not be resolved onto a concrete command.

The place where the mapping of a concrete HTTPRequest is mapped onto a command is often called Routing. As the Routing already has the job to locate a command, why not extend it to check if the command is actually allowed per ACL? E.g. by extending the Router to a ACL aware router: RouterACL. If your router does not yet know the User, then the Router is not the right place, because for the ACL'ing to work not only the command but also the user must be identified. So this place can vary, but I'm sure you can easily locate the place you need to extend, because it's the place that fullfills the user and command requirement:

User -> Browser -> Request (HTTP)
   -> Request (Command)

User is available since the beginning, Command first with Request(Command).

So instead of putting your ACL checks inside each command's concrete implementation, you place it before it. You don't need any heavy patterns, magic or whatever, the ACL does it's job, the user does it's job and especially the command does it's job: Just the command, nothing else. The command has no interest to know whether or not roles apply to it, if it's guarded somewhere or not.

So just keep things apart that don't belong to each other. Use a slightly rewording of the Single Responsibility Principle (SRP): There should be only one reason to change a command - because the command has changed. Not because you now introduce ACL'ing in your application. Not because you switch the User object. Not because you migrate from an HTTP/HTML interface to a SOAP or command-line interface.

The ACL in your case controls the access to a command, not the command itself.