Science Knowings: JavaScript Course For Social Media

Cross-Origin Resource Sharing (CORS)

Introduction to CORS

Cross-Origin Resource Sharing (CORS)

Cross-Origin Resource Sharing (CORS) is a mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served.

What is CORS?

CORS enables you to make requests from one website to another website to access and use resources like images, fonts, scripts, or data from the other website.

HTTP Request Methods and CORS

CORS applies to specific HTTP request methods:

  • GET
  • HEAD
  • POST
  • PUT

CORS Headers

CORS involves adding specific headers to HTTP requests and responses:

Request Headers:
  • Origin
  • Access-Control-Request-Method
  • Access-Control-Request-Headers
Response Headers:
  • Access-Control-Allow-Origin
  • Access-Control-Allow-Methods
  • Access-Control-Allow-Headers
  • Access-Control-Max-Age

Simple and Preflighted Requests

CORS requests can be either simple or preflighted:

Simple Requests:
  • Use safe methods (GET, HEAD, POST)
  • Set simple headers (Content-Type, Accept)
  • No preflight request is needed
Preflighted Requests:
  • Use non-simple methods (PUT, DELETE, etc.)
  • Set non-simple headers (Authorization, X-Custom-Header)
  • Preflight request is sent first to check if the actual request is allowed

Preflight Request Example

OPTIONS /api/data HTTP/1.1
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: Content-Type, Authorization

CORS and Security

CORS is a security mechanism that prevents unauthorized access to resources across different origins.

  • It protects against cross-site request forgery (CSRF) attacks.
  • It ensures that only authorized domains can access resources.

CORS and Browser Security

Browsers enforce CORS restrictions to protect users from malicious cross-origin requests.

  • Same-origin policy: Browsers restrict access to resources from different origins without proper CORS headers.
  • Preflight requests: Browsers send preflight requests for non-simple CORS requests to check for authorization.

CORS and Server Security

Servers must implement CORS to allow cross-origin requests.

  • Configure CORS headers on the server-side to specify allowed origins, methods, and headers.
  • Use CORS middleware or frameworks to handle CORS requests automatically.

CORS and Cross-Site Request Forgery (CSRF)

CSRF is a type of attack where a malicious website tricks a victim's browser into making unauthorized requests to a legitimate website.

CORS helps prevent CSRF by restricting cross-origin requests from untrusted websites.

Configuring CORS on the Server

To configure CORS on the server:

  • Add CORS headers to the HTTP response.
  • Use server-side code to set the appropriate CORS headers.
  • Use a CORS middleware or framework to handle CORS requests automatically.

Preflight Request Configuration

For preflighted requests:

  • The server must respond with the appropriate CORS headers in the preflight response.
  • The browser will then send the actual request if the preflight response is successful.

CORS Troubleshooting

If CORS requests are not working:

  • Check the CORS headers on both the client and server sides.
  • Check the browser console for any CORS-related errors.
  • Use CORS debugging tools or online resources.

Common CORS Errors

  • 403 Forbidden: CORS headers are not set or are incorrect.
  • 405 Method Not Allowed: The request method is not allowed by CORS.
  • Preflight request failed: The preflight request was not successful.
  • Origin null is not allowed by Access-Control-Allow-Origin: The origin of the request is not allowed by the server.

Debugging CORS Issues

To debug CORS issues:

  • Use browser developer tools to inspect HTTP requests and responses.
  • Log CORS-related information on the server-side.
  • Use CORS debugging tools or online resources.

Next Up: Web Application Security Best Practices

In the next session, we'll explore best practices for securing web applications. Follow us to learn more and protect your applications from vulnerabilities!