How to Configure CORS Headers for an S3 Bucket

When you are serving assets from an Amazon S3 bucket, making sure your CORS (Cross-Origin Resource Sharing) configurations are set correctly is crucial. This ensures that your resources can be accessed by web applications hosted on different domains without any hassle. Here’s how you can add the Access-Control-Allow-Origin header to your S3 bucket responses.

Understanding CORS in S3

S3 uses CORS to tell browsers which external domains are allowed to access your bucket’s resources. This step is essential when a web application needs to fetch resources, like images or fonts, stored in S3 and display them in an end-user’s browser.

Configuring CORS in AWS S3

The current way to define CORS rules for an S3 bucket might seem different from older configurations. Instead of XML, you now use JSON format to specify your rules. This adjustment makes it easier to manage and modify configurations given JSON’s prevalence and ease of understanding. Follow these steps:

1. Navigate to Your S3 Bucket: Open the AWS Management Console and go to the S3 service. Choose the bucket you want to configure.

2. Go to Permissions: Under your bucket, select the “Permissions” tab. Scroll down to find the “Cross-origin resource sharing (CORS)” configuration section.

3. JSON Configuration: You need to use JSON to specify your CORS settings. Below is an example configuration:

   [
       {
           "AllowedHeaders": [
               "*"
           ],
           "AllowedMethods": [
               "GET",
               "HEAD"
           ],
           "AllowedOrigins": [
               "*"
           ],
           "ExposeHeaders": [],
           "MaxAgeSeconds": 3000
       }
   ]
   

   • AllowedHeaders: ["*"] specifies that any headers may be included in requests.
   • AllowedMethods: You can specify which HTTP methods can be used. In this case, ["GET", "HEAD"] ensures read access.
   • AllowedOrigins: By using ["*"], any origin can access the resources. Be cautious with this setting in production environments for security reasons.
   • MaxAgeSeconds: The amount of seconds the browser is allowed to cache the preflight response.
   • ExposeHeaders: This can be used to specify which headers should be exposed to the web application.

4. Save Changes: Once you have edited the CORS configuration, save your changes.

Security Tip: Allowing all origins (*) is generally not recommended for production environments. It can expose your S3 content to potential abuse. Always limit the origins and methods to only what is necessary for your application.

Testing Your CORS Configuration

To ensure your CORS settings are correct, you can test it using browser tools or tools like Postman. Make sure the Access-Control-Allow-Origin header is included in the response when a web application tries to access your S3-hosted resources from a different domain.

By following these steps, you should be able to successfully configure your S3 bucket to support CORS and allow cross-origin requests correctly. This will enable your web applications to function seamlessly with resources hosted on Amazon S3.