CORS configuration in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CORS stand for and why is it important in web applications?

CORS stands for Cross-Origin Resource Sharing. It is important because it controls how resources on a web server can be requested from another domain, helping to keep web applications secure by preventing unauthorized cross-origin requests.

beginner

In Rails, where do you typically configure CORS settings?

In Rails, CORS settings are usually configured in the file config/initializers/cors.rb using the rack-cors gem.

intermediate

What is the purpose of the 'origins' option in a Rails CORS configuration?

The 'origins' option specifies which domains are allowed to make cross-origin requests to your Rails server. You can set it to '*' to allow all domains or list specific domains for tighter security.

intermediate

Explain what the 'resource' block does in the Rails CORS configuration.

The 'resource' block defines which paths on the server accept cross-origin requests and what HTTP methods and headers are allowed for those requests.

intermediate

Why should you avoid setting 'origins' to '*' in production environments?

Setting 'origins' to '*' allows any website to access your API, which can lead to security risks like data leaks or unauthorized actions. It's safer to specify only trusted domains in production.

Where is the CORS configuration typically placed in a Rails app?

A config/routes.rb

B app/controllers/application_controller.rb

C app/views/layouts/application.html.erb

D config/initializers/cors.rb

CORS settings are usually configured in config/initializers/cors.rb using the rack-cors gem.

What does setting 'origins' to '*' do in CORS configuration?

A Allows only localhost to access resources

B Blocks all cross-origin requests

C Allows all domains to access resources

D Enables HTTPS only

Setting 'origins' to '*' means any domain can make cross-origin requests to your server.

Which HTTP methods can you specify in the 'resource' block for CORS?

A GET, POST, PUT, DELETE, OPTIONS

B CONNECT, TRACE, PATCH only

C HEAD only

D None, methods are not configurable

You can specify common HTTP methods like GET, POST, PUT, DELETE, OPTIONS in the 'resource' block.

Why is CORS configuration important for APIs?

A It manages user authentication

B It controls which external sites can access your API

C It styles the API responses

D It speeds up database queries

CORS controls which external domains can access your API, protecting it from unauthorized cross-origin requests.

What gem is commonly used in Rails to handle CORS?

A rack-cors

B devise

C puma

D sidekiq

The rack-cors gem is commonly used in Rails apps to configure and handle CORS.

Describe how to set up CORS in a Rails application using rack-cors.

Think about the gem, the initializer file, and the configuration block.

Explain why careful CORS configuration is important for web app security.

Consider what could happen if any website could access your server.

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