Access Control & Permissions in Rails

Access Control is a simple idea. We want company employees to be able to delete inappropriate content; but random Users cannot. Here I propose one way to implement Access Control that has the particular advantage of being very general, very concise, and unlikely to be violated. I call it RESTful Access Control.

What is RESTful Access Control?

When we model RESTfully the Application Interface (i.e., the HTTP interface), we construe Resources as responding to a simple set of Verbs: Get, Put, Post, Delete. To the extent that our entire application is designed this way, we can Control all Access to our Application with perfect granularity: as being a set of Permissions to perform these four Verbs on a corresponding Resource. Let’s start with an example.

Suppose a User can add Members to a Group. How do we determine if this or that User can create a Membership? This logic doesn’t belong in the controller — call me a prig, but Business Logic belongs in the Model. Anything else is unbecoming. Therefore, we have a Controller like:

class MembershipsController < ApplicationController
  before_filter :load_group

  def create
    membership = @group.memberships.build(params[:membership])
    raise SecurityTransgression unless current_user.can_create?(membership)
    ...
  end

  def load_group
    @group = Group.find(params[:group_id])
  end
end

Let’s implement the can_create? method. Despite the above Formulaic code, the User Class isn’t really the proper place for the Logic: different kinds of resources are likely to have different rules. Thus, we implement the Logic in the Model being created:

class Membership < ActiveRecord::Base
  def can_be_created_by?(user)
    ...
  end
end

I call the above “the Passive Voice.” I prefer, for various reasons, to use the “Active Voice”, and therefore we implement a simple Proxy in the User Class:

class User < ActiveRecord::Base
  def can_create?(resource)
    resource.can_be_created_by?(self)
  end
end

Of course, this “Active Voice” implementation requires that we follow the Null Object Pattern for Users who are not Logged In, otherwise we’ll produce Errors when we ask if a Logged Out User has Permission to do something:

class AnonymousUser < User
  ...
end

Why we Always Create An Instance Before Checking Permission

In our Controller Action above, we first build an instance of a Model in memory, then ask for Permission.

membership = @group.memberships.build(params[:membership])
raise SecurityTransgression unless current_user.can_create?(membership)

But why even create the Model in memory it shall not be persisted? Well, for some advanced Access Control Rules, we need all of the data available to an Instance in order to proceed. Suppose, for example, that only the Owner of a Group can create Memberships. The simplest way to enforce this rule is to reference the Group Owner in the can_be_created_by? method:

class Membership
  belongs_to :group

  def can_be_created_by?(user)
    group.owner == user
  end
end

Similarly, can_be_destroyed_by?, can_be_updated_by?, and can_be_read_by? all must be instance methods. This provides the maximum flexibility and a consistent Interface.

This is a robust Pattern for Access Control. It extends brilliantly to User Roles:

def can_be_destroyed_by?(user)
  user.admin? or ...
end

Even a per-user Access Control scheme where we grant individual Users permission to manipulate individual objects can elegantly fit in. Consider something like the following:

class Permission
  belongs_to :user
  belongs_to :resource, :polymorphic => true
end

class CreatePermission < Permission
end

class Membership
  has_many :create_permissions, :as => :resource

  def can_be_created_by?(user)
    create_permissions.find_by_user_id(user)
  end
end

Handling Access Control in the Views

Access Control has View consequences as well. For example, we shouldn’t show a destroy button if the User lacks permission to perform that Action. Given the above implementation, this is simple enough:

 <%= link_to_unless current_user.can_destroy?(resource), 'destroy', ... %>

Why Exceptions?

Above, I’ve proposed using Exceptions to handle Security Transgressions. (At Pivotal, we call these Exceptions SecurityTransgressions because it has a more romantic air than the banal AccessDenied). Normally, these Transgressions cannot happen: in your Templates you will not even display links to modify an object if the User lacks Permission. So Transgressions occur only when a User is fiddling about with nefarious intent. So we can handle misbehavior consistently across the site, by (for example) rendering a HTTP 403 Forbidden Response. Exceptions are a neat way to do this, because Rails gives us a systematic way to handle exceptions with rescue_action:

 class SecurityTransgression < StandardError; end

 def create
   raise SecurityTransgression unless ...
 end

class ApplicationController < ActionController::Base
  def rescue_action(e)
    case e
    when SecurityTransgression
      head :forbidden
    end
  end
end

Ensuring Programmers Remember to do Access Control And How to Avoid Over-Engineering

The biggest danger with Access Control rules is the possibility that a careless Programmer might forget to implement them. Having a consistent, terse pattern for Access Control goes some way towards mitigating this problem. When all Access Control is as simple as adding a can_create? check to your Actions, Access Control is unlikely to be overlooked.

The Second biggest danger in Access Control is over-engineering. Suppose we want to enforce Access Control logic. Rails gives us some tools to do this. Well, Rails gives us just enough rope to hang ourselves. There are some serious disadvantages to the approach I am going to propose now, but we’ll get to that shortly.

Rails has something called an Around Filter. An Around Filter, like a Before and After Filter, is a Filter applied to every Action. But this Around filter is like the Heavenly Union of the Before and After Filters: both great tasting and less filling, it can do work before and after invoking the Action.

class ApplicationController
  around_filter :ensure_permission_to_create, :only => :create

  def ensure_permission_to_create
    class_name = ModelsController.to_s.demodulize.gsub(/Controller$/,'').singularize
param_name = class_name.downcase
    instance_variable_name = "@#{class_name.downcase}"

    instance_variable_set instance_variable_name, class_name.constantize.new(params[param_name])
    if current_user.can_create?(instance_variable_get instance_variable_name)
      yield # the create acction is invoked here.
    else
      head :forbidden
    end
  end
end

Whew. In the above implementation, control is yielded to the Action only if the User has permission. The instance of the Model is also built for us automatically; so, our Formulaic create Action becomes:

def create
  if @model.save
    ...
  end
end

Very DRY, But this is, again, just enough rope to hang ourselves with. Our Controllers need to be named exactly right, our parameters without an inconsistency, and so forth. This is not a problem in the World of Forms, but in Reality it may require some coding contortions. The tricky metaprogramming and impertinent assumptions in ensure_permision... only get nastier when we want to make a Nested Resource, and build our Model using a proxy (as in the previous example: @group.memberships.build(params([:membership])). Finally, where we have Controllers that do not map directly to ActiveRecords, we would need to introduce a Model layer that responded to the Interface defined in ensure_permission... We often want to do this, but not always. Being forced to might suck. So I offer this design strategy with much caution.

The Exception based approach, while a bit more verbose, is battle-tested and works in Practice. In any case, pushing the logic to the Model with can_be_created_by? will certainly work, regardless of whether you take the Around Filter approach or the Exception-Based one.

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