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Advanced Topic 40 of 51 Web Development

CORS

Browser policy controlling cross-origin requests.

How it works

Cross-Origin Resource Sharing is a browser mechanism that relaxes the same-origin policy, which blocks scripts from reading responses from other origins, defined by scheme, host and port. A server opts in by sending headers such as Access-Control-Allow-Origin. For non-simple requests, the browser first sends a preflight OPTIONS request asking which methods and headers are allowed, and credentials need an explicit Access-Control-Allow-Credentials and a specific origin.

Step-by-step walkthrough

  1. A page on site A calls the API on site B.
  2. For risky requests the browser first sends an OPTIONS preflight.
  3. The API replies with Access-Control-Allow-Origin headers.
  4. If allowed the real request is sent.
  5. If not the browser blocks the response, the policy protects users.

Where it is used

Front ends hosted on one domain calling APIs on another, third-party widgets and public APIs configure CORS.

Common pitfall

CORS is enforced by browsers, not servers, and does not protect the API from other clients. Using a wildcard origin together with credentials is disallowed, and reflecting any origin blindly creates vulnerabilities. Missing headers on error responses and preflight handling cause confusing failures. Debug with the network panel, allow only needed origins and cache preflights with max-age.

Where it fits in the course

CORS is topic 40 of 51 in Web Development, rated Advanced. It follows OAuth 2.0. The next topic is Web Security Basics.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Web Development quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is CORS?
Browser policy controlling cross-origin requests. It belongs to Web Development and is an advanced-level topic.
Where do people use CORS?
Front ends hosted on one domain calling APIs on another, third-party widgets and public APIs configure CORS.
What is the most common mistake with CORS?
CORS is enforced by browsers, not servers, and does not protect the API from other clients.
What should I learn before this topic?
Start with OAuth 2.0, the topic just before it in Web Development.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Web Development quiz.
Is CORS hard to learn?
It is an Advanced topic. Expect to read the explanation more than once and to connect it with earlier topics. Working through a small example by hand helps most.
How long does it take to learn CORS?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to CORS?
OAuth 2.0, Web Security Basics, JWT are closely connected. The full list is in the related topics section of this page.
What should I study after CORS?
Continue with Web Security Basics, the next topic in Web Development.

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