Securing Your Rails App with Pundit: A Comprehensive Guide

Authorization is a critical component of any web application. It ensures that users can only access the resources and perform the actions they are permitted to. While authentication verifies a user‘s identity, authorization controls what they can see and do once logged in.

In the Ruby on Rails ecosystem, Pundit is a popular and powerful gem for handling authorization. Its simple, expressive syntax and object-oriented design make it well-suited for implementing granular access control policies in a maintainable way.

In this in-depth guide, we‘ll explore Pundit‘s key features and walk through integrating it into a Rails application for robust security. We‘ll cover defining authorization policies, enforcing them in controllers and views, testing at various levels, and advanced techniques for more complex use cases.

Whether you‘re a Rails beginner looking to shore up your app‘s security or an experienced developer seeking to refine your authorization approach, this guide will equip you with the knowledge and best practices to effectively leverage Pundit. Let‘s dive in!

Understanding Pundit‘s Approach

At its core, Pundit is all about policy classes. Policies are plain old Ruby objects that encapsulate authorization logic for a particular model or resource. By convention, Pundit looks for policies in the app/policies directory with names like UserPolicy or ArticlePolicy.

A basic policy class looks like this:

class ArticlePolicy
  attr_reader :user, :article

  def initialize(user, article)
    @user = user
    @article = article 
  end

  def show?
    true
  end

  def update?
    user.admin? || article.author == user
  end

  def destroy?
    user.admin?  
  end
end

Here the policy is initialized with the current user and the article record being authorized. Pundit makes these available within the policy automatically.

Then for each controller action – show, update, destroy, etc. – you define a corresponding policy predicate method. These methods simply return true or false to indicate if the user is authorized for that action on that record.

This approach keeps authorization logic neatly encapsulated within policies, separately from models and controllers. As your application grows, you can easily define more granular policies for different models.

Policies in Practice

Let‘s look at some more realistic policy examples to see Pundit‘s expressiveness.

Consider a multi-tenant SaaS app where each account has users with different roles:

class DocumentPolicy
  attr_reader :user, :document

  def show?
    user.account == document.account
  end

  def update?
    user.account == document.account && (
      user.admin? || user == document.creator
    )
  end

  def share?
    user.account == document.account && user.manager?  
  end
end

This DocumentPolicy ensures users can only view and share documents within their own account. Only admins or the document creator can make updates.

Here‘s a different scenario with more complex rules:

class PaymentPolicy  
  def create?
    user.account.billing_plan != ‘free‘ && (
      user.account.balance > payment.amount ||      
      user.account.credit_limit >= payment.amount
    )
  end

  def refund?
    user.admin? && payment.completed? && payment.refundable?
  end
end

The PaymentPolicy allows creating payments only for non-free billing plans and when the account has sufficient balance or credit. Refunds can only be issued by admins for payments in a completed, refundable state.

As you can see, Pundit makes it easy to clearly express even multi-faceted authorization criteria in an organized, object-oriented way.

Controllers and Scopes

So where do policies actually get applied? The primary place is in your controllers via the authorize method:

class ArticlesController < ApplicationController
  def show
    @article = Article.find(params[:id])
    authorize @article
  end

  def create
    @article = Article.new(article_params)
    authorize @article
    if @article.save
      # ...
    end
  end
end

Pundit will infer which policy to use based on the model name and call the appropriate action-specific predicate method. If authorization fails, a Pundit::NotAuthorizedError is raised.

To handle authorization errors globally, you can define an user_not_authorized method in your ApplicationController:

class ApplicationController < ActionController::Base
  protect_from_forgery
  include Pundit

  rescue_from Pundit::NotAuthorizedError, with: :user_not_authorized

  private

  def user_not_authorized
    flash[:alert] = "You are not authorized to perform this action."
    redirect_to(request.referrer || root_path)
  end
end

This will flash an alert message and redirect the user when authorization fails.

Beyond simple allow/deny for individual records, Pundit also provides scopes for filtering collections based on authorization rules. Define a scope class within your policy:

class ArticlePolicy < ApplicationPolicy
  class Scope
    def initialize(user, scope)
      @user  = user
      @scope = scope
    end

    def resolve
      if user.admin?
        scope.all
      else
        scope.published.where(account_id: user.account_id)
      end
    end
  end
end

The resolve method takes the initial scope (usually an Active Record relation) and returns the filtered scope. You can then apply the scope with:

@articles = policy_scope(Article)
# or 
@articles = Article.all
@articles = Pundit.policy_scope!(current_user, @articles)

This ensures users only see records they‘re authorized for when fetching collections.

Auditability and Monitoring

Security isn‘t a one-time effort. To maintain a secure application over time, it‘s important to audit authorization activity and monitor for potential issues.

Pundit uses a consistent pattern for performing authorization via the authorize method. This makes it straightforward to wrap, log, and monitor authorization successes and failures.

For example, you could define a custom logged_authorize method to record authorization attempts:

class ApplicationController < ActionController::Base
  # ...

  def logged_authorize(*args)
    record = args.first
    begin
      authorize(*args)
    rescue Pundit::NotAuthorizedError => e      
      # Log unauthorized attempt
      Rails.logger.warn "Authorization failure for #{record.class} #{record.id} - #{e.query}"
      raise
    else
      # Log successful authorization
      Rails.logger.info "Authorized #{record.class}  #{record.id} for #{args.last}"
    end
  end
end

Then use logged_authorize instead of authorize in your controllers:

def show
  @article = Article.find(params[:id])
  logged_authorize @article
end

This will log both failed and successful authorizations for later auditing and analysis.

You can take monitoring a step further by integrating with an error tracking or security monitoring service. Many services provide Rails integrations and can alert on spikes in authorization failures or other anomalous patterns.

Testing

Authorization rules are critical to get right. A mistake can expose sensitive data or allow unauthorized actions. As such, it‘s important to thoroughly test your policies and authorized controller actions.

At the policy level, you can unit test the behavior of individual rules:

class ArticlePolicyTest < ActiveSupport::TestCase
  def test_create
    user = users(:normal_user)  
    admin = users(:admin)

    assert ArticlePolicy.new(user, Article).create?
    assert ArticlePolicy.new(admin, Article).create?
  end

  def test_approve
    user = users(:normal_user)
    admin = users(:admin)
    article = articles(:pending)

    refute ArticlePolicy.new(user, article).approve?
    assert ArticlePolicy.new(admin, article).approve?    
  end
end

At a higher level, controller and request specs can verify that authorization is properly enforced:

RSpec.describe ArticlesController do
  let(:user) { users(:normal_user) }
  let(:admin) { users(:admin) }
  let(:article) { articles(:pending) }

  describe "PUT #approve" do
    context "as a normal user" do
      it "does not allow approving articles" do
        sign_in user
        put :approve, params: { id: article.id }
        expect(response).to redirect_to(root_path) 
        expect(flash[:alert]).to match(/not authorized/)
      end
    end

    context "as an admin" do  
      it "allows approving articles" do
        sign_in admin
        put :approve, params: { id: article.id }
        expect(article.reload).to be_approved
      end
    end
  end  
end

Between policy unit tests and integration tests for authorized controller actions, you can effectively validate the correctness of your authorization rules.

Performance Considerations

Authorization checks add overhead to web requests, so it‘s important to keep performance in mind as your application scales.

One optimization is to use memoization in your policies to avoid redundant calls to the underlying models:

class ArticlePolicy
  def initialize(user, article)
    @user = user
    @article = article
  end

  def show?  
    article_published? && user_subscribed?
  end

  private

  def article_published?
    @article_published ||= @article.published?  
  end

  def user_subscribed?
    @user_subscribed ||= @user.subscribed?
  end
end

Here the show? rule depends on the article‘s published status and the user‘s subscription status. By memoizing these calls, we avoid multiple queries if show? is called more than once per request.

For more advanced cases, Pundit provides hooks to customize caching of policy results. You can set a cache_key and cache_value in your policies:

class ArticlePolicy
  def cache_key
    "#{@article.id}-#{@article.updated_at}"
  end

  def cache_value  
    "#{user_subscribed?}-#{user.admin?}"  
  end
end

Pundit will then cache the value of each rule for a given cache key, so subsequent calls can return the cached result for improved performance.

Evolving with Pundit

As your application‘s authorization needs evolve, Pundit provides escape hatches and extension points for handling more advanced use cases.

For example, if you have a model that doesn‘t follow typical naming conventions, you can override the policy_class method to specify the policy to use:

class AdminDashboard
  def self.policy_class 
    DashboardPolicy
  end
end

If your policies need access to more information than just the user and record, you can create a custom pundit_user method to provide additional context:

class ApplicationController
  # ...
  def pundit_user
    UserContext.new(current_user, current_account)  
  end
end

And if you need to authorize actions outside the scope of a particular model, you can pass a symbol to authorize:

def export
  authorize :report, :export?
  # ...
end

Pundit will look for a ReportPolicy and call its export? method.

These hooks, along with Pundit‘s clear conventions and modular design, make it adaptable to a wide range of authorization scenarios. As your needs change, you can refactor and extend your policies without major upheaval.

Conclusion

Pundit is a powerful tool for keeping your Rails applications secure and maintainable as they grow. By encapsulating authorization logic in policy objects, it ensures access rules are clearly defined and easily testable, separate from other application concerns.

Over the course of this guide, we‘ve looked in depth at:

• Defining fine-grained authorization rules in policy classes
• Enforcing policies in controllers and scopes
• Handling authorization failures
• Logging and monitoring authorization attempts
• Testing at the policy and integration level
• Optimizing performance with memoization and caching
• Extending Pundit for more advanced use cases

With these techniques in your toolbelt, you‘re well-equipped to implement robust, scalable authorization in your Rails applications. But remember, Pundit is just one piece of the security puzzle.

To keep your application truly secure, it‘s important to apply the principles of defense-in-depth: encrypting sensitive data, validating and sanitizing user input, following the principle of least privilege, promptly applying security patches, and continuously monitoring for potential vulnerabilities.

When combined with these other security best practices, Pundit provides a strong foundation for keeping your users‘ data and your application safe. Now go forth and authorize with confidence!