Cross-Origin Resource Sharing (CORS) is a security mechanism implemented by web browsers that restricts web pages from making requests to a different domain than the one which served the web page. This policy, known as the "same-origin policy," is a fundamental security feature that prevents malicious websites from accessing sensitive data from other websites. CORS provides a controlled way to relax this restriction, allowing legitimate cross-origin requests while still protecting users from potential security threats.
The "origin" is defined by the scheme (protocol), host (domain), and port of a URL. Two URLs have the same origin only if all three of these components are identical. For example:
- http://example.com/page1.html and http://example.com/page2.html have the same origin.
- http://example.com/page.html and https://example.com/page.html have different origins (different scheme).
- http://example.com/page.html and http://subdomain.example.com/page.html have different origins (different host).
- http://example.com:8080/page.html and http://example.com/page.html have different origins (different port; the default port 80 is implied in the second URL).
CORS affects web applications when they make requests to APIs or resources hosted on different domains. Without proper CORS configuration, the browser will block these cross-origin requests, resulting in an error and preventing the application from accessing the necessary data.
Here’s how CORS works:
1. Browser Initiates a Request: When a web page makes a request to a different origin, the browser first checks if it's a "simple request." A simple request meets the following criteria:
- Method: GET, HEAD, or POST
- Headers: Only the following headers are allowed: Accept, Accept-Language, Content-Language, Content-Type (with a value of application/x-www-form-urlencoded, multipart/form-data, or text/plain).
2. Simple Request: If ....
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