"Allow same origin" refers to a fundamental web browser security mechanism where resources from the same origin are permitted to interact with each other, while interaction between resources from different origins is restricted by default. This concept is governed by the Same-Origin Policy.
Understanding the Same-Origin Policy
The Same-Origin Policy is a critical security model for web browsers. Its primary purpose is to prevent malicious scripts on one web page from accessing sensitive data on another web page.
Based on the reference provided: Under the policy, a web browser permits scripts contained in a first web page to access data in a second web page, but only if both web pages have the same origin.
This means if a script is loaded from https://site-a.com, it can generally interact with data loaded from https://site-a.com/another-page. However, it cannot freely access data from https://site-b.com without specific permissions.
What Defines an "Origin"?
Two URLs are considered to have the same origin if they share the exact same:
- Protocol: (e.g.,
http,https) - Domain: (e.g.,
example.com,sub.example.com) - Port: (e.g.,
80,443,8080) - often implicitly defined by the protocol (80 for http, 443 for https)
Here are some examples illustrating the concept of same vs. different origins relative to https://www.example.com/dir/page.html:
| URL | Outcome | Reason (if different) |
|---|---|---|
https://www.example.com/dir2/other.html |
Same Origin | Protocol, domain, and port are the same. |
https://www.example.com:8080/page.html |
Different | Different port (443 vs 8080). |
https://blog.example.com/page.html |
Different | Different subdomain (www vs blog). |
http://www.example.com/page.html |
Different | Different protocol (https vs http). |
Why is the Same-Origin Policy Important?
The policy is a fundamental defense against common web security threats, primarily Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) attacks.
Without this policy, a malicious website could:
- Load a legitimate bank website in an iframe.
- Run a script to read information (like account balances) from the bank website's page content.
- Perform actions (like transferring money) on the bank website using the user's active session, without the user's knowledge or consent.
The Same-Origin Policy prevents the malicious script (from the different origin) from accessing or manipulating the content of the bank website (the same origin).
Where Does the Policy Apply?
The Same-Origin Policy primarily affects interactions involving:
- XMLHttpRequest (XHR) and Fetch API requests: Prevents scripts from making HTTP requests to retrieve data from a different origin unless explicitly allowed.
- DOM Manipulation: Prevents scripts from one origin from accessing or manipulating the content of documents loaded from another origin (e.g., inside an
iframe). - Web Storage (localStorage, sessionStorage): Data stored by one origin is not accessible by another.
Overcoming Same-Origin Restrictions (When Necessary)
While the policy restricts cross-origin interactions for security, legitimate scenarios often require communication between different origins (e.g., a frontend on one domain needing data from an API on another domain). Mechanisms exist to allow this in a controlled manner, including:
- Cross-Origin Resource Sharing (CORS): An HTTP-header based mechanism that allows servers to explicitly grant permission for resources (like data from an API) to be requested by web pages from different origins.
- JSONP (JSON with Padding): An older technique that leverages
<script>tags, which are not subject to the Same-Origin Policy for fetching scripts. (Less secure and less flexible than CORS). postMessage(): Allows secure communication between scripts from different origins, typically between a window and an iframe.
Understanding "allow same origin" is key to grasping web security fundamentals and how browsers protect users by isolating content based on its source.
