CORS configuration with Flask-CORS

Introduction

CORS lets your web app share resources safely with other websites. Flask-CORS helps you set this up easily in Flask apps.

You want your Flask API to be used by a web page hosted on a different domain.

You build a frontend app on one server and backend API on another and want them to talk.

You want to allow only specific websites to access your Flask app resources.

You want to avoid browser errors about blocked cross-site requests.

You want to control which HTTP methods and headers are allowed from other sites.

Syntax

Flask

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app, resources={r"/api/*": {"origins": "http://example.com"}})

Use CORS(app) to enable CORS for all routes and origins.

Use resources to limit CORS to certain URL patterns and origins to specify allowed domains.

Examples

This allows any website to access your Flask app resources.

Flask

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)  # Allow all origins for all routes

Only requests from http://myfrontend.com to URLs starting with /api/ are allowed.

Flask

CORS(app, resources={r"/api/*": {"origins": "http://myfrontend.com"}})

Allows cookies and credentials to be sent with cross-origin requests.

Flask

CORS(app, supports_credentials=True)

Sample Program

This Flask app allows only requests from http://frontend.example.com to access the /data route. Other origins will be blocked by the browser.

Flask

from flask import Flask, jsonify
from flask_cors import CORS

app = Flask(__name__)
# Allow CORS only for frontend.example.com on /data route
CORS(app, resources={r"/data/*": {"origins": "http://frontend.example.com"}})

@app.route('/data')
def data():
    return jsonify({'message': 'Hello from Flask!'})

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Browsers enforce CORS, so server must send correct headers for cross-origin requests to work.

Flask-CORS adds these headers automatically based on your config.

Always restrict origins in production to avoid security risks.

Summary

CORS controls which websites can access your Flask app resources.

Flask-CORS makes it easy to add CORS headers to your Flask routes.

Configure origins and routes carefully to keep your app safe and accessible.

Practice

1. What is the main purpose of using Flask-CORS in a Flask application?

easy

A. To allow or restrict cross-origin requests to the Flask app

B. To speed up the Flask app by caching responses

C. To handle database connections automatically

D. To manage user authentication and sessions

Solution

1. Step 1: Understand what CORS means

   CORS stands for Cross-Origin Resource Sharing, which controls which websites can access resources from your Flask app.

2. Step 2: Identify Flask-CORS role

   Flask-CORS is a tool to add headers that allow or restrict these cross-origin requests easily.

3. Final Answer:

   To allow or restrict cross-origin requests to the Flask app -> Option A

4. Quick Check:

   CORS controls access = A [OK]

Hint: Remember CORS controls cross-site access permissions [OK]

Common Mistakes:

• Confusing CORS with caching or authentication
• Thinking Flask-CORS speeds up the app
• Assuming Flask-CORS manages databases

2. Which of the following is the correct way to enable CORS for all routes in a Flask app using Flask-CORS?

easy

A. from flask_cors import CORS
app = Flask(__name__)
CORS()

B. from flask_cors import CORS
app = Flask(__name__)
app.enable_cors()

C. from flask_cors import CORS
app = Flask(__name__)
CORS(app)

D. from flask_cors import CORS
app = Flask(__name__)
CORS(app, origins='none')

Solution

1. Step 1: Import and create Flask app

   Import CORS from flask_cors and create the Flask app instance.

2. Step 2: Enable CORS on the app

   Call CORS(app) to enable CORS for all routes. This is the correct syntax.

3. Final Answer:

   from flask_cors import CORS\napp = Flask(__name__)\nCORS(app) -> Option C

4. Quick Check:

   Enable CORS by passing app to CORS() [OK]

Hint: Use CORS(app) to enable CORS globally [OK]

Common Mistakes:

• Calling CORS() without app argument
• Using non-existent app.enable_cors() method
• Setting origins='none' disables all CORS

3. Given this Flask app code snippet, what will be the CORS behavior for the route /data?

from flask import Flask, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app, origins=['https://example.com'])

@app.route('/data')
def data():
    return jsonify({'key': 'value'})

medium

A. Only localhost can access /data cross-origin

B. All origins can access /data without restriction

C. No origins can access /data due to missing CORS headers

D. Only requests from https://example.com will be allowed cross-origin access

Solution

1. Step 1: Check CORS configuration

   CORS is enabled with origins=['https://example.com'], so only that origin is allowed cross-origin access.

2. Step 2: Understand route behavior

   The /data route returns JSON and inherits the CORS policy set on the app.

3. Final Answer:

   Only requests from https://example.com will be allowed cross-origin access -> Option D

4. Quick Check:

   Origins list limits access = A [OK]

Hint: Origins list restricts which sites can access [OK]

Common Mistakes:

• Assuming all origins are allowed by default
• Thinking CORS disables all access without explicit allow
• Confusing localhost with example.com

4. Identify the error in this Flask app CORS setup:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(origins='*')

@app.route('/')
def home():
    return 'Hello'

medium

A. CORS is called without passing the Flask app instance

B. The origins parameter '*' is invalid and causes an error

C. The route function home() must return JSON for CORS to work

D. Flask app instance must be created after calling CORS

Solution

1. Step 1: Check how CORS is called

   CORS() is called without passing the app instance, so it does not enable CORS on the app.

2. Step 2: Understand consequences

   Without passing app, no CORS headers are added, so cross-origin requests will fail.

3. Final Answer:

   CORS is called without passing the Flask app instance -> Option A

4. Quick Check:

   Pass app to CORS() to enable it [OK]

Hint: Always pass app instance to CORS() [OK]

Common Mistakes:

• Forgetting to pass app to CORS()
• Thinking '*' is invalid (it's valid)
• Believing CORS requires JSON response

5. You want to allow CORS only on the route /api/data for requests from https://trusted.com. Which Flask-CORS setup correctly achieves this?

hard

A. CORS(app, resources={r'/api/*': {'origins': '*'}})

B. CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}})

C. CORS(app, origins='https://trusted.com')

D. CORS(app, resources={r'/api/data': {'origins': '*'}})

Solution

1. Step 1: Understand resource-specific CORS

   To restrict CORS to a specific route, use the resources parameter with a route pattern and origins.

2. Step 2: Match the route and origin

   CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}}) correctly sets CORS only on '/api/data' with origins 'https://trusted.com'.

3. Final Answer:

   CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}}) -> Option B

4. Quick Check:

   Use resources to limit routes and origins [OK]

Hint: Use resources dict to target specific routes [OK]

Common Mistakes:

• Setting origins globally instead of per route
• Using wildcard '*' when only one origin is needed
• Using wrong route pattern like '/api/*' for exact match