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Fetching Resources from Different Domains: The Power of `crossorigin`


Purpose

  • Allows fetching resources (images, scripts, audio/video) from a different domain (origin) than the one hosting the web page.
  • Enables Cross-Origin Resource Sharing (CORS) for specific HTML elements.

Where it's Used

  • <link> (external stylesheets)
  • <video> (video files)
  • <audio> (audio files)
  • <script> (external scripts)
  • <img> (images)

How it Works

  1. When an element with crossorigin is encountered, the browser initiates a CORS request to the target origin.
  2. The server at the target origin responds with CORS headers indicating whether the request is allowed and under what conditions.
  3. Based on the server's response, the browser either grants access to the resource or blocks it.

Attribute Values

  • use-credentials
    • Allows sending credentials with the request.
    • Use with caution, as it can introduce security risks if not handled properly on the server-side.
  • anonymous (default if crossorigin is present without a value)
    • No credentials (cookies, authorization headers) are sent with the request.
    • Suitable for scenarios where the resource doesn't require authentication or interaction with the server-side session.

Example

<img src="https://external-domain.com/image.jpg" crossorigin="anonymous" alt="Image from another domain">
<script src="https://external-domain.com/script.js" crossorigin></script>

Benefits

  • Improves security by controlling how resources from different domains interact with the main page.
  • Enables richer web experiences by incorporating resources from various sources.
  • The use-credentials value should only be used when necessary due to potential security implications.
  • Requires server-side configuration to enable CORS for the specific domain requesting the resource.

Fetching an Image with Anonymous Credentials (Default)

<img src="https://external-domain.com/image.jpg" alt="Image with anonymous credentials">

This code fetches an image from https://external-domain.com/image.jpg. Since crossorigin is present without a value, it defaults to anonymous, meaning no credentials are sent. This is suitable if the image doesn't require authentication or interaction with the server's session.

Loading a Script with Explicit Anonymous Credentials

<script src="https://external-domain.com/script.js" crossorigin="anonymous"></script>

This code loads a script from https://external-domain.com/script.js. The crossorigin="anonymous" attribute explicitly specifies that no credentials should be sent with the request. This reinforces the default behavior and ensures clarity.

Fetching an Audio File with Use Credentials (for Authentication)

<audio src="https://protected-audio.com/music.mp3" controls crossorigin="use-credentials"></audio>

This code attempts to fetch an audio file from https://protected-audio.com/music.mp3. Here, crossorigin="use-credentials" is used because the audio file might require authentication (e.g., cookies) to access. However, this requires careful server-side configuration to allow access from the specific domain of your web page while preventing unauthorized usage.

Including a Stylesheet with Anonymous Credentials (for Security)

<link rel="stylesheet" href="https://third-party-styles.com/styles.css" crossorigin="anonymous">

This code includes a stylesheet from https://third-party-styles.com/styles.css. Even though stylesheets typically don't need credentials, using crossorigin="anonymous" adds an extra layer of security by preventing potential information leaks if the stylesheet contains malicious code.


Server-Side Configuration (CORS Headers)

  • This approach offers fine-grained control over resource access and is generally the recommended way to handle CORS.
  • You need to configure your server to send appropriate CORS headers in the response. These headers specify which domains are allowed to access the resource, under what conditions (methods, headers allowed), and whether credentials can be sent.
  • This is the most common approach for allowing access to your resources from other domains.

Same-Origin Policy

  • This approach is simpler but might not be feasible in all cases, especially if you're integrating resources from external providers.
  • If possible, try to host the resources (images, scripts) on the same domain as your main web page. This avoids the need for CORS altogether.

Resource Proxying

  • However, it adds an additional layer of complexity and requires server infrastructure.
  • This approach allows you to manage CORS on the proxy server and potentially modify the resources before serving them to your page.
  • If you have control over a server, you can set up a proxy server on your domain that fetches the resources from the external domain and then serves them to your web page.

Third-Party Libraries (Limited Use Cases)

  • It's important to carefully evaluate the security implications and limitations of such libraries before using them.
  • In some specific cases (like fetching JSON data), you might consider using JavaScript libraries that handle CORS requests behind the scenes. However, this approach can be less secure and is not universally applicable.
  • Third-Party Libraries
    Use cautiously due to security concerns and limited use cases.
  • Resource Proxying
    Offers control and potential modifications, but adds complexity.
  • Same-Origin Policy
    Simplest approach, but only works if resources can be hosted on the same domain.
  • Server-Side Configuration
    Generally the most secure and flexible option, especially if you have control over the server hosting the resources.