nicks-cors-test

Simple HTML & JS Tool to quickly test CORS locally.

Blog

https://medium.com/pareture/simple-local-cors-test-tool-544f108311c5

CORS

Cross Origin Resource Sharing (CORS) is a simple and powerful mechanism which uses HTTP headers so that a server knows where a request is coming from and can choose whether or not to accept the request based on this.

Better information here: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

The header which is provided as the argument is the ‘Origin’. This is set by the User-Agent (the thing that makes the request) and can not be overridden (security enforced).

The server with the resource uses the Access-Control-Allow-Origin header to allowlist particular domains or allow requests from all origins using the wildcard:

Access-Control-Allow-Origin: *

Tool

You can use this simple tool to test making CORS requests and examine the outcome.

• The HTML file is simply a shell to call the Javascript function.
• The JS file executes an AJAX request based on the values you adjust.

Guide

• Download the files and open index.html in a browser. You will be faced with a blank screen and nothing else.
• Open the browser tools: e.g Right-click > Inspect > Console.
• Open main.js file in a text editor.
• Adjust the url value depending on the resource you want to check, save main.js and refresh the browser.
  • Try https://google.com and you will see it has been blocked due to CORS.
  • Try https://api.github.com and you will see it is not blocked and the content will be output.