Understanding CORS (Cross-Origin Resource Sharing) in Amazon S3

In this lesson, we explored CORS (Cross-Origin Resource Sharing) and its importance in web security and Amazon S3. Understanding CORS is crucial for the AWS exam, as it frequently appears as a question.

1. What is CORS?

CORS (Cross-Origin Resource Sharing) is a security feature implemented in web browsers that controls how resources (such as images, fonts, or APIs) can be requested from different origins.

What is an Origin?

An origin consists of three main components:

• Scheme/Protocol (e.g., https, http)
• Host/Domain (e.g., www.example.com)
• Port (e.g., 443 for HTTPS)

For example, the origin of:

📌 https://www.example.com

• Scheme: https
• Host: www.example.com
• Port: 443 (default for HTTPS)

Two URLs have the same origin if all three components match. If any of these differ, they are considered different origins, triggering CORS restrictions.

2. How CORS Works?

When a web page requests a resource from a different origin, the browser first sends a pre-flight request to check if the cross-origin request is allowed.

Step-by-Step CORS Process: