Table of Contents:

• How CORS Works
• The `crossorigin` Attribute
• Supported Attribute Values
• Importance of the `crossorigin` Attribute
• Access-Control-Allow-Origin Header
• Purpose of the Origin Header
• Conclusion
• FAQ

Introduction to Cross-Origin Resource Sharing (CORS) and the `crossorigin` Attribute

Have you ever wondered how websites pull data from different places without causing a security nightmare? The `crossorigin` attribute combined with Cross-Origin Resource Sharing (CORS) are the unsung heroes, quietly making sure everything plays nicely together.

How CORS Works

When a web application sends a cross-origin request, the browser asks the server if such requests are permitted. This involves a “preflight” request. The browser sends an OPTIONS request to verify if the actual request is allowed. The server responds with headers indicating which methods are acceptable but also which headers are permitted.

For example, a JavaScript application runs on https://domain-a.com. It attempts to fetch data from https://domain-b.com/data.json. Then, the browser will send a preflight request to https://domain-b.com to check if this cross-origin request is acceptable. If the server responds with proper CORS headers, the browser sends the actual request.

The `crossorigin` Attribute

The crossorigin attribute appears in HTML. It dictates how a web element must handle cross-origin requests for its resources, especially scripts, stylesheets, as well as images from third-party servers. This attribute determines if user credentials, like cookies, including HTTP authentication data, are sent during a CORS request.

Supported Attribute Values

The crossorigin attribute accepts two specific values:

• anonymous – Signals that the request should be made without sending user credentials. Typically used for retrieving resources from third-party servers, where authentication is not needed.
• use-credentials – Indicates that user credentials should accompany the request. The server must include the Access-Control-Allow-Credentials header in its response for this to be effective.

Example:

<script src=”https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js” crossorigin=”anonymous”></script>

Importance of the `crossorigin` Attribute

The crossorigin attribute is quite important for the safe, but also effective, management of cross-origin requests. By specifying how resources are loaded, developers improve the performance in addition to the security of their web applications. For instance, when retrieving fonts from another domain, enabling CORS makes sure the fonts render properly on the page.

Access-Control-Allow-Origin Header

A crucial part of CORS involves the Access-Control-Allow-Origin header. This header states which domains can access resources from a server. The header functions as a gatekeeper, controlling which origins send cross-origin requests.

Purpose of the Origin Header

A browser sends an Origin header alongside any cross-origin request. The header identifies the requesting domain. The server examines this header against its CORS policy to decide whether it should grant access to the requested resources. This process helps guarantee that only approved domains access sensitive data, maintaining web security.

Conclusion

The crossorigin attribute or CORS are really important for managing cross-origin requests in web development. By controlling how resources are accessed across different domains, security is increased and performance is improved in modern web applications. So, for developers who want to build secure, but also robust, web applications that interact with resources from various origins, understanding and properly implementing CORS together with the crossorigin attribute is absolutely essential.

FAQ

What is the same-origin policy?

It’s a security feature in web browsers that restricts web pages from making requests to a different domain than the one which served the web page. This prevents malicious scripts from accessing sensitive data from other sites.

When do I need to use the `crossorigin` attribute?

You should use it whenever you’re fetching resources (like scripts, images, or fonts) from a different domain. This helps control whether user credentials are sent with the request.

What happens if CORS is not configured correctly?

If CORS is misconfigured, the browser blocks cross-origin requests, as well as you’ll see errors in the browser’s console. This can prevent your application from functioning correctly.

What’s the difference between `anonymous` and `use-credentials` for the `crossorigin` attribute?

anonymous means no user credentials (like cookies) are sent with the request. use-credentials means they are sent, but the server must also explicitly allow this by setting the Access-Control-Allow-Credentials header.

Resources & References:

1. https://www.dhiwise.com/post/using-the-crossorigin-attribute-for-better-performance
2. https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS
3. https://learn.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-9.0
4. https://www.moesif.com/blog/technical/api-development/Mastering-Access-Control-Allow-Origin-Your-Guide-to-Secure-Cross-Domain-Requests/
5. https://developers.cloudflare.com/cloudflare-one/identity/authorization-cookie/cors/