CSRF protection in Ruby on Rails - Mini Project: Build & Apply

CSRF Protection in Rails

📖 Scenario: You are building a simple Rails web application that allows users to submit a contact form. To keep your app safe from malicious attacks, you need to add protection against Cross-Site Request Forgery (CSRF).

🎯 Goal: Learn how to enable and verify CSRF protection in a Rails controller and form, so your app only accepts form submissions from trusted sources.

📋 What You'll Learn

Create a Rails controller named ContactsController with a new and create action

Add CSRF protection by enabling protect_from_forgery in the controller

Create a simple HTML form in the new.html.erb view that includes the CSRF token

Verify that the create action only accepts requests with a valid CSRF token

💡 Why This Matters

🌍 Real World

CSRF protection is essential for any web app that accepts form submissions to prevent attackers from tricking users into submitting unwanted requests.

💼 Career

Understanding and implementing CSRF protection is a key skill for Rails developers to build secure web applications.

1

Create ContactsController with new and create actions

Create a Rails controller named ContactsController with two empty actions: new and create.

Ruby on Rails

class ContactsController < ApplicationController
  # Your code here
end

Hint

Use def new and def create inside the controller class.

2

Enable CSRF protection in ContactsController

Add the line protect_from_forgery with: :exception inside the ContactsController class to enable CSRF protection.

Ruby on Rails

class ContactsController < ApplicationController
  # Add CSRF protection here
  def new
  end

  def create
  end
end

Hint

Place protect_from_forgery with: :exception inside the controller class but outside any method.

3

Create a form with CSRF token in new.html.erb

In the new.html.erb view file, create a form using form_with helper for the create action. Make sure the form includes the CSRF token automatically.

Ruby on Rails

Hint

Use form_with with url: contacts_path and method: :post. Rails adds the CSRF token automatically.

4

Verify CSRF token in create action

In the create action of ContactsController, add code to safely handle the form submission. The CSRF token will be verified automatically by Rails because of protect_from_forgery. Add a simple redirect to new after submission.

Ruby on Rails

class ContactsController < ApplicationController
  protect_from_forgery with: :exception

  def new
  end

  def create
    # Your code here
  end
end

Hint

You don't need to manually check the CSRF token. Just redirect after handling the form.

Practice

1. What is the main purpose of CSRF protection in a Rails application?

easy

A. To prevent unauthorized commands from being executed by malicious websites

B. To speed up database queries

C. To improve the user interface design

D. To encrypt user passwords

Solution

1. Step 1: Understand CSRF meaning

   CSRF stands for Cross-Site Request Forgery, which is an attack where unauthorized commands are sent from a user that the web application trusts.

2. Step 2: Identify CSRF protection goal

   The goal of CSRF protection is to stop these unauthorized commands from malicious websites.

3. Final Answer:

   To prevent unauthorized commands from being executed by malicious websites -> Option A

4. Quick Check:

   CSRF protection = prevent unauthorized commands [OK]

Hint: CSRF stops bad actions from other sites [OK]

Common Mistakes:

• Confusing CSRF with password encryption
• Thinking it speeds up queries
• Assuming it improves UI design

2. Which line of code correctly enables CSRF protection in a Rails controller?

easy

A. use_csrf_token()

B. enable_csrf_protection true

C. csrf_protect = true

D. protect_from_forgery with: :exception

Solution

1. Step 1: Recall Rails CSRF syntax

   Rails uses the method protect_from_forgery with options like :exception to enable CSRF protection.

2. Step 2: Match correct syntax

   Only protect_from_forgery with: :exception is valid Rails syntax to enable CSRF protection.

3. Final Answer:

   protect_from_forgery with: :exception -> Option D

4. Quick Check:

   Enable CSRF = protect_from_forgery [OK]

Hint: Use protect_from_forgery with: :exception [OK]

Common Mistakes:

• Using non-existent methods like enable_csrf_protection
• Assigning variables instead of calling method
• Calling undefined functions like use_csrf_token()

3. Given this Rails form code snippet, what happens if the CSRF token is missing or invalid?

<%= form_with url: '/posts' do |form| %>
  <%= form.text_field :title %>
  <%= form.submit 'Create' %>
<% end %>

medium

A. The form submits successfully without any error

B. Rails raises an ActionController::InvalidAuthenticityToken error

C. The form data is saved but with a warning in logs

D. The form redirects to the home page silently

Solution

1. Step 1: Understand form_with and CSRF token

   Rails automatically includes a CSRF token in forms created with helpers like form_with.

2. Step 2: Effect of missing or invalid token

   If the token is missing or invalid, Rails raises an ActionController::InvalidAuthenticityToken error to block the request.

3. Final Answer:

   Rails raises an ActionController::InvalidAuthenticityToken error -> Option B

4. Quick Check:

   Missing CSRF token = InvalidAuthenticityToken error [OK]

Hint: Missing token causes InvalidAuthenticityToken error [OK]

Common Mistakes:

• Assuming form submits without error
• Thinking data saves with warning
• Believing it redirects silently

4. You see this error in your Rails app: ActionController::InvalidAuthenticityToken. Which fix will most likely solve the problem?

medium

A. Ensure your forms include the CSRF token by using Rails form helpers

B. Remove the CSRF token from your forms

C. Add protect_from_forgery with: :exception in ApplicationController

D. Disable CSRF protection in config

Solution

1. Step 1: Identify cause of InvalidAuthenticityToken

   This error happens when the CSRF token is missing or invalid in the form submission.

2. Step 2: Choose fix to include CSRF token

   Using Rails form helpers like form_with automatically includes the CSRF token, fixing the error.

3. Final Answer:

   Ensure your forms include the CSRF token by using Rails form helpers -> Option A

4. Quick Check:

   Fix InvalidAuthenticityToken = include CSRF token [OK]

Hint: Use Rails form helpers to add CSRF token automatically [OK]

Common Mistakes:

• Adding protect_from_forgery again when already present
• Removing CSRF token which causes error
• Disabling protection instead of fixing token

5. You want to allow JSON API requests in your Rails app without CSRF errors but keep protection for HTML forms. Which is the best way to do this?

hard

A. Manually add CSRF tokens to JSON requests

B. Disable CSRF protection globally

C. Use protect_from_forgery with: :null_session and handle API authentication separately

D. Use protect_from_forgery with: :exception and skip CSRF check for JSON requests

Solution

1. Step 1: Understand CSRF and API requests

   CSRF tokens are mainly for browser forms; API JSON requests often don't use them and need different handling.

2. Step 2: Choose best Rails approach

   Using protect_from_forgery with: :null_session disables CSRF exceptions for APIs but keeps protection for forms, requiring separate API authentication.

3. Final Answer:

   Use protect_from_forgery with: :null_session and handle API authentication separately -> Option C

4. Quick Check:

   API JSON + CSRF = null_session + separate auth [OK]

Hint: Use null_session for APIs, keep exception for forms [OK]

Common Mistakes:

• Disabling CSRF protection globally
• Trying to add CSRF tokens manually to JSON
• Skipping CSRF only without alternative auth