What is considered a cross-origin request

Understanding Cross-Origin Requests: A Comprehensive Guide

Explore what defines a cross-origin request, why browsers restrict them, and how to manage Cross-Origin Resource Sharing (CORS) for secure web interactions.

In the world of web development, understanding how different parts of a web application interact is crucial for both functionality and security. A fundamental concept in this interaction is the 'cross-origin request'. This article will demystify what constitutes a cross-origin request, delve into the security implications, and explain how the Cross-Origin Resource Sharing (CORS) mechanism addresses these challenges.

What Defines a Cross-Origin Request?

A request is considered 'cross-origin' if the protocol, domain (host), or port of the requesting resource differs from that of the target resource. This concept is central to the browser's Same-Origin Policy (SOP), a critical security feature that prevents malicious scripts on one web page from accessing sensitive data on another web page. If any of these three components (protocol, domain, or port) do not match, the request is deemed cross-origin.

flowchart TD
    A[Requesting Origin] --> B{Protocol Match?}
    B -- No --> F[Cross-Origin]
    B -- Yes --> C{Domain Match?}
    C -- No --> F[Cross-Origin]
    C -- Yes --> D{Port Match?}
    D -- No --> F[Cross-Origin]
    D -- Yes --> E[Same-Origin]

Decision flow for determining if a request is cross-origin

Let's break down each component with examples:

Examples of Cross-Origin Scenarios

Consider a web page loaded from https://www.example.com:8080. Here's how different requests would be classified:

1. Same Protocol, Different Domain:
   - Request to: `https://api.example.com:8080`
   - Result: Cross-origin (domain `api.example.com` vs `www.example.com`)

2. Same Domain, Different Protocol:
   - Request to: `http://www.example.com:8080`
   - Result: Cross-origin (protocol `http` vs `https`)

3. Same Domain, Different Port:
   - Request to: `https://www.example.com:3000`
   - Result: Cross-origin (port `3000` vs `8080`)

4. Completely Different Origin:
   - Request to: `https://another-site.org:443`
   - Result: Cross-origin (all three components differ)

5. Same Origin:
   - Request to: `https://www.example.com:8080/data`
   - Result: Same-origin (protocol, domain, and port all match)

Illustrative examples of cross-origin and same-origin requests

The Role of Cross-Origin Resource Sharing (CORS)

While the Same-Origin Policy is vital for security, it can also be restrictive for legitimate web applications that need to access resources from different origins (e.g., an API hosted on a separate domain). This is where CORS comes in. CORS is a mechanism that uses additional HTTP headers to tell browsers to give a web application running at one origin permission to access selected resources from a different origin.

When a browser detects a cross-origin request, it often performs a 'preflight' request using the OPTIONS HTTP method. This preflight request asks the server for permission to make the actual request. The server responds with CORS headers, such as Access-Control-Allow-Origin, indicating whether the requesting origin is permitted to access the resource. If the preflight is successful, the browser proceeds with the actual request.

sequenceDiagram
    participant Browser
    participant WebServer

    Browser->>WebServer: OPTIONS /api/data (Preflight Request)
    Note over WebServer: Checks Origin header
    WebServer-->>Browser: HTTP/1.1 204 No Content
    WebServer-->>Browser: Access-Control-Allow-Origin: https://client.com
    WebServer-->>Browser: Access-Control-Allow-Methods: GET, POST
    alt Preflight Successful
        Browser->>WebServer: GET /api/data (Actual Request)
        WebServer-->>Browser: HTTP/1.1 200 OK
        WebServer-->>Browser: Access-Control-Allow-Origin: https://client.com
        WebServer-->>Browser: { "data": "sensitive info" }
    else Preflight Failed
        Browser--xBrowser: Blocked by CORS policy
    end

Sequence diagram of a CORS preflight request and subsequent actual request