When developing web applications, you might encounter issues when making requests to a different domain than your own. This is where Cross-Origin Resource Sharing (CORS) comes into play. CORS is a security feature implemented by browsers to prevent malicious websites from making unauthorized requests to your servers. In this post, we’ll dive into what CORS is, why it’s important, and how to configure it properly.

What is CORS?

Cross-Origin Resource Sharing (CORS) is a mechanism that allows web servers to specify who can access their resources and how the requests should be handled. It uses HTTP headers to tell browsers whether different web applications are permitted to share resources.

Why is CORS Important?

CORS is crucial for security. By default, web browsers block cross-origin requests to protect users from potential security threats like cross-site scripting (XSS) and cross-site request forgery (CSRF). However, legitimate use cases, such as APIs and web services, often need to be accessed from different origins, which makes configuring CORS essential for these scenarios.

How CORS Works

CORS relies on a set of HTTP headers that define how resources should be shared between different origins. Here are the key headers involved:

• Access-Control-Allow-Origin - Specifies which origins are allowed to access the resource. Can be set to a specific origin or * to allow all origins.
• Access-Control-Allow-Methods - Specifies the HTTP methods that are allowed when accessing the resource (e.g., GET, POST, PUT).
• Access-Control-Allow-Headers - Lists the headers that are permitted in the request.
• Access-Control-Allow-Credentials - Indicates whether credentials (cookies, HTTP authentication) are allowed to be included in the request.
• Access-Control-Expose-Headers - Lists the headers that clients can access.
• Access-Control-Max-Age - Specifies how long the results of a preflight request can be cached.

Preflight Requests

When making certain types of cross-origin requests, browsers send a preflight request to the server to check if the actual request is safe to send. This preflight request is an OPTIONS request that checks the CORS headers before making the actual request.