What is an illegal Cross-Origin request

Table of Contents: Understanding Cross-Origin Requests The Role of CORS What Makes a Cross-Origin Request Illegal? Technical Mechanisms Behind Illegal Requests FAQ Ever wondered why that cool feature on your website isn't working as expected? It the might be because of an illegal cross-origin request. This occurs when a web application from one source attempts to access resources from another source without proper permission. This is a security measure designed to keep you and your users safe.

Understanding Cross-Origin Requests

A cross-origin request takes place when a webpage, loaded from one location (defined by the protocol, hostname, as well as port), tries to get resources from a different location. For instance, consider a situation. A webpage at `https://example.com` attempts to make an HTTP request to `https://api.example.org`. This is a cross-origin request.

• Modern web browsers have a built-in security policy called the Same-Origin Policy (SOP).
• It blocks this sort of request by default unless the target server allows it using specific CORS headers.

The Role of CORS

What is CORS? Cross-Origin Resource Sharing (CORS) is a security system. It allows servers to specify which origins are allowed to access their resources using HTTP headers. When a browser makes a cross-origin request, it adds an `Origin` header. This header states its own domain. After this, the server checks the `Origin` header against its allowed origins list. It sends back CORS headers, like `Access-Control-Allow-Origin`, if it grants access. In the case of no access, the browser stops the response from reaching the script.

What Makes a Cross-Origin Request Illegal?

A cross-origin request becomes "illegal" or unauthorized when it breaks SOP or CORS policies, that are enforced by both browsers also servers. These are a few instances when this might happen.

• Violation of Same-Origin Policy - Web browsers block scripts. They run on one origin from accessing data returned from another. This happens unless explicit permission is given via CORS headers.
• Absence of Proper CORS Headers - If the server of a requested resource does not contain needed CORS headers like `Access-Control-Allow-Origin`, a script from a different origin to read or use it will be stopped as illegal.
• Unauthorized Preflight Requests - Browsers do a 'preflight' request using the OPTIONS method. This happens for certain requests using custom HTTP methods or non-standard headers before doing the real call. A failed preflight, caused by missing or wrong CORS response headers on the server, causes following requests to be rejected as illegal.

In summary, an illegal cross-origin request is an attempt made across domains. It lacks express authorization with the proper security configuration, like SOP and/or correct CORS implementation.

Technical Mechanisms Behind Illegal Requests

Preflight Requests

For complex requests, browsers start preflight requests. These use the OPTIONS method. This pre-check validates whether operations proceed, based on permissions in response metadata. If required permissions are missing, meaning no valid `Access-Control-Allow-Methods` or `Access-Control-Allow-Credentials` exist, attempts will be deemed illegal per browser enforcement until fixed at the backend.

FAQ

Why do I see a CORS error?

A CORS error shows up when a web page tries to get resources from a different domain. This happens without the other domain allowing it through CORS headers. It is a security feature to stop malicious websites from accessing sensitive data.

How do I fix CORS issues?

Fixing CORS needs changes on the server hosting the resources you want to access. You should add the `Access-Control-Allow-Origin` header to the server's responses. This header lists which origins are allowed to access the resource. Use "*" to allow all origins (not suggested for production) or list specific domains.

Is CORS a browser feature?

Yes, CORS is primarily enforced by web browsers. Browsers block JavaScript code from accessing responses to cross-origin requests if the server has not included the correct CORS headers. Servers can still receive the request, but the browser prevents the client-side script from accessing the response. Resources & References:

1. https://www.moesif.com/blog/technical/api-development/What-is-CORS-Essential-Guide-to-Cross-Origin-Resource-Sharing/
2. https://konghq.com/blog/learning-center/what-is-cors-cross-origin-resource-sharing
3. https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Cross-Origin_Resource_Policy
4. https://docs.aws.amazon.com/AmazonS3/latest/userguide/cors.html
5. https://requestly.com/blog/what-is-cors-and-how-to-bypass-it/