CORS configuration in Ruby on Rails

Introduction

CORS helps your web app share resources safely with other websites. It controls who can access your app's data from different places.

When your Rails API serves data to a frontend app on a different domain.

When you want to allow specific websites to use your API but block others.

When building a public API that many websites can access safely.

When your frontend and backend are on different servers or ports during development.

When you want to prevent security issues from unwanted cross-site requests.

Syntax

Ruby on Rails

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'example.com'
    resource '*', headers: :any, methods: [:get, :post, :options]
  end
end

Use origins to specify which websites can access your app.

The resource line controls which paths and HTTP methods are allowed.

Examples

This allows any website to access all resources with any HTTP method. Use carefully for public APIs.

Ruby on Rails

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins '*'
    resource '*', headers: :any, methods: :any
  end
end

This allows only 'https://myfrontend.com' to access API routes with GET and POST methods.

Ruby on Rails

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'https://myfrontend.com'
    resource '/api/*', headers: :any, methods: [:get, :post]
  end
end

This is useful during development when your frontend runs on localhost port 3000.

Ruby on Rails

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'localhost:3000'
    resource '*', headers: :any, methods: [:get, :post, :patch, :delete, :options]
  end
end

Sample Program

This config allows a frontend running on http://localhost:3000 to make GET, POST, and OPTIONS requests to your Rails backend.

Ruby on Rails

# config/initializers/cors.rb
Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'http://localhost:3000'
    resource '*', headers: :any, methods: [:get, :post, :options]
  end
end

Important Notes

Always restrict origins to trusted domains in production for security.

Use methods to limit allowed HTTP actions to only what your app needs.

Remember to restart your Rails server after changing CORS settings.

Summary

CORS controls which websites can access your Rails app's resources.

Configure CORS in config/initializers/cors.rb using Rack::Cors.

Be careful to allow only trusted origins and needed HTTP methods.

Practice

1. What is the main purpose of configuring CORS in a Rails application?

easy

A. To speed up database queries

B. To control which external websites can access your app's resources

C. To manage user sessions securely

D. To style the frontend pages

Solution

1. Step 1: Understand CORS role

   CORS (Cross-Origin Resource Sharing) controls access from other websites to your app's resources.

2. Step 2: Identify main purpose

   It restricts or allows external origins to make requests, enhancing security.

3. Final Answer:

   To control which external websites can access your app's resources -> Option B

4. Quick Check:

   CORS controls access = B [OK]

Hint: Remember CORS is about cross-site access control [OK]

Common Mistakes:

• Confusing CORS with database or session management
• Thinking CORS affects frontend styling
• Assuming CORS speeds up app performance

2. Which of the following is the correct way to add CORS middleware in a Rails app initializer?

easy

A. Rails.application.config.middleware.insert_before 0, Rack::Cors do
allow do
origins '*'
resource '*', headers: :any, methods: [:get, :post]
end
end

B. Rails.middleware.use Rack::Cors do
allow do
origins '*'
resource '*', headers: :any, methods: [:get, :post]
end
end

C. config.middleware.add Rack::Cors do
allow do
origins '*'
resource '*', headers: :any, methods: [:get, :post]
end
end

D. Rails.application.middleware.insert_after 0, Rack::Cors do
allow do
origins '*'
resource '*', headers: :any, methods: [:get, :post]
end
end

Solution

1. Step 1: Recall correct middleware insertion

   In Rails, middleware is added using Rails.application.config.middleware.insert_before or insert_after.

2. Step 2: Identify correct syntax

   The common pattern is insert_before 0, Rack::Cors with a block for configuration.

3. Final Answer:

   Rails.application.config.middleware.insert_before 0, Rack::Cors do allow do origins '*' resource '*', headers: :any, methods: [:get, :post] end end -> Option A

4. Quick Check:

   Use insert_before 0 with config.middleware = C [OK]

Hint: Use Rails.application.config.middleware.insert_before 0 for CORS [OK]

Common Mistakes:

• Using Rails.middleware instead of Rails.application.config.middleware
• Using insert_after instead of insert_before 0
• Missing config in middleware insertion

3. Given this CORS config in Rails initializer:

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'https://example.com'
    resource '*', headers: :any, methods: [:get, :post]
  end
end

What happens if a request comes from 'https://evil.com'?

medium

A. The request is blocked due to origin mismatch

B. The request causes a server error

C. The request is allowed but only GET method works

D. The request is allowed with all headers and methods

Solution

1. Step 1: Analyze allowed origins

   The config allows only 'https://example.com' origin.

2. Step 2: Check request origin

   Request from 'https://evil.com' is not in allowed origins, so it is blocked by CORS policy.

3. Final Answer:

   The request is blocked due to origin mismatch -> Option A

4. Quick Check:

   Origin not allowed = request blocked = D [OK]

Hint: Only origins listed in config are allowed [OK]

Common Mistakes:

• Assuming wildcard '*' allows all origins when specific origins set
• Thinking request causes server error instead of CORS block
• Confusing allowed methods with allowed origins

4. You wrote this CORS config:

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins '*'
    resource '*', headers: :any, methods: [:get, :post, :put]
  end
end

But PUT requests from frontend are failing with CORS errors. What is the likely problem?

medium

A. The methods list is missing :put

B. The frontend is not sending the correct Origin header

C. The server needs to allow OPTIONS method for preflight

D. The origins '*' is too restrictive

Solution

1. Step 1: Check methods allowed

   PUT is included in methods, so it's not missing.

2. Step 2: Understand CORS preflight

   PUT requests trigger a preflight OPTIONS request which must be allowed by server.

3. Step 3: Identify missing OPTIONS method

   OPTIONS method is not listed, so preflight fails causing CORS error.

4. Final Answer:

   The server needs to allow OPTIONS method for preflight -> Option C

5. Quick Check:

   Preflight OPTIONS missing = CORS fail = A [OK]

Hint: Always allow OPTIONS method for CORS preflight [OK]

Common Mistakes:

• Forgetting to allow OPTIONS method
• Assuming '*' origin is restrictive
• Blaming frontend Origin header without checking server

5. You want to allow CORS only for two trusted domains: 'https://app1.com' and 'https://app2.com'. Which config correctly implements this securely in Rails?

hard

A. Rails.application.config.middleware.insert_before 0, Rack::Cors do
allow do
origins 'app1.com'
resource '*', headers: :any, methods: [:get, :post, :options]
end
allow do
origins 'app2.com'
resource '*', headers: :any, methods: [:get, :post, :options]
end
end

B. Rails.application.config.middleware.insert_before 0, Rack::Cors do
allow do
origins '*'
resource '*', headers: :any, methods: :any
end
end

C. Rails.application.config.middleware.insert_before 0, Rack::Cors do
allow do
origins 'app1.com', 'app2.com'
resource '*', headers: :any, methods: [:get, :post]
end
end

D. Rails.application.config.middleware.insert_before 0, Rack::Cors do
allow do
origins 'https://app1.com', 'https://app2.com'
resource '*', headers: :any, methods: [:get, :post, :options]
end
end

Solution

1. Step 1: Identify correct origins format

   Origins must include full URLs with protocol for security, so 'https://app1.com' and 'https://app2.com' are correct.

2. Step 2: Check methods and resource

   Allowing common methods including OPTIONS for preflight is necessary.

3. Step 3: Compare options

   Rails.application.config.middleware.insert_before 0, Rack::Cors do allow do origins 'https://app1.com', 'https://app2.com' resource '*', headers: :any, methods: [:get, :post, :options] end end correctly lists both origins in one allow block with proper methods and full URLs.

4. Final Answer:

   Rails.application.config.middleware.insert_before 0, Rack::Cors do allow do origins 'https://app1.com', 'https://app2.com' resource '*', headers: :any, methods: [:get, :post, :options] end end -> Option D

5. Quick Check:

   Trusted full origins with methods including OPTIONS = A [OK]

Hint: List full trusted origins with needed methods including OPTIONS [OK]

Common Mistakes:

• Using '*' origin which is insecure
• Omitting protocol in origins
• Not allowing OPTIONS method for preflight
• Splitting origins into multiple allow blocks unnecessarily