The same-origin policy is a critical security concept in web development that restricts interactions between documents or scripts from different origins. An origin is defined as a combination of the protocol, domain name, and port number of a URL. The same-origin policy restricts a web page from making requests to a different domain than the one that served the original content, which helps to prevent malicious attacks such as Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF).
In practice, the same-origin policy prevents a script running on one domain from accessing or modifying the content of a web page on another domain. For example, a script running on www.example.com cannot access the cookies or other data associated with www.anotherdomain.com, nor can it modify the DOM or make XMLHttpRequests to that domain. This policy is enforced by web browsers and is a fundamental aspect of web security.
However, there are some exceptions to the same-origin policy. For example, if a script running on a page served from www.example.com includes an image from www.anotherdomain.com, the browser will allow the image to be displayed because it is not considered an active interaction. Additionally, modern web technologies such as Cross-Origin Resource Sharing (CORS) allow for controlled access to resources from different domains.
Overall, the same-origin policy plays a critical role in preventing many types of web-based attacks and is an essential concept for web developers to understand and implement properly.