CSRF protection in Ruby on Rails - Step-by-Step Execution

Concept Flow - CSRF protection

User sends form request

↓

Rails adds CSRF token to form

↓

User submits form with token

↓

Rails receives request

↓

Check token validity

↓

Process

↓

Action done

Rails adds a secret token to forms and checks it on submission to stop fake requests from other sites.

Execution Sample

Ruby on Rails

class PostsController < ApplicationController
  protect_from_forgery with: :exception

  def create
    # create post logic
  end
end

This code enables CSRF protection in a Rails controller, raising an error if the token is missing or wrong.

Execution Table

Step	Action	Token Present?	Token Valid?	Result
1	Render form	N/A	N/A	Form includes CSRF token hidden field
2	User submits form	Yes	Yes	Request accepted, post created
3	User submits form	No	N/A	Request rejected, raises exception
4	User submits form	Yes	No	Request rejected, raises exception

💡 Execution stops when request is accepted or rejected based on token check

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
csrf_token	nil	generated token string	submitted token string (valid)	nil	submitted token string (invalid)
request_status	pending	pending	accepted	rejected	rejected

Key Moments - 3 Insights

Why does Rails add a hidden token field to forms?

Rails adds the hidden token so it can verify the request came from the same site, as shown in Step 1 of the execution_table.

What happens if the token is missing in the submitted form?

The request is rejected and an exception is raised, as shown in Step 3 of the execution_table.

How does Rails know if the token is valid or not?

Rails compares the submitted token with the one stored in the user session, rejecting the request if they don't match, as shown in Step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at Step 2 when the token is present and valid?

A Form is re-rendered with a new token

B Request is rejected with an error

C Request is accepted and post is created

D User is redirected to login

💡 Hint

Check the Result column in Step 2 of the execution_table

At which step does the request get rejected because the token is missing?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for 'Token Present?' column with 'No' in the execution_table

If the token submitted does not match the stored token, what is the result?

A Request accepted

B Request rejected with exception

C Token is regenerated automatically

D Form is submitted twice

💡 Hint

See Step 4 in the execution_table where Token Valid? is 'No'

Concept Snapshot

Rails CSRF protection:
- Adds a hidden token to forms automatically
- Checks token on form submission
- Rejects requests with missing or invalid tokens
- Enabled by protect_from_forgery in controllers
- Stops attackers from forging requests
- Raises exception on failure

Full Transcript

CSRF protection in Rails works by adding a secret token to forms when they are rendered. When the user submits the form, Rails checks if the token is present and matches the one stored in the session. If the token is valid, the request is processed normally. If the token is missing or invalid, Rails rejects the request and raises an exception. This prevents attackers from tricking users into submitting unwanted requests from other sites. The key steps are rendering the form with the token, submitting the form, and verifying the token on the server side.

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