Configure Cross-Origin Resource Sharing (CORS)

Cross-Origin Resource Sharing (CORS) Open external link is a standardized method that prevents domain X from accessing the resources of domain Y. It does so by using special headers in HTTP responses from domain Y, that allow your browser to verify that domain Y permits domain X to access these resources.

While CORS can help protect your data from malicious websites, CORS is also used to interact with objects in your bucket and configure policies on your bucket.

CORS is used when you interact with a bucket from a web browser, and you have two options:

Set a bucket to public: This option makes your bucket accessible on the Internet as read-only, which means anyone can request and load objects from your bucket in their browser or anywhere else. This option is ideal if your bucket contains images used in a public blog.

Presigned URLs: Allows anyone with access to the unique URL to perform specific actions on your bucket.

​​ Prerequisites

Before you configure CORS, you must have:

• An R2 bucket with at least one object. If you need to create a bucket, refer to Create a public bucket.
• A domain you can use to access the object. This can also be a localhost.
• (Optional) Access keys. An access key is only required when creating a presigned URL.

​​ Use CORS with a public bucket

To use CORS with a public bucket, ensure your bucket is set to allow public access.

Next, add a CORS policy to your bucket to allow the file to be shared.

​​ Use CORS with a presigned URL

Presigned URLs are an S3 concept that contain a special signature that encodes details of an S3 action, such as GetObject or PutObject. Presigned URLs are only used for authentication, which means they are generally safe to distribute publicly without revealing any secrets.

​​ Create a presigned URL

You will need a pair of S3-compatible credentials to use when you generate the presigned URL.

The example below shows how to generate a presigned PutObject URL using the @aws-sdk/client-s3 Open external link package for JavaScript.

import { PutObjectCommand, S3Client } from "@aws-sdk/client-s3";
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";
const S3 = new S3Client({
	endpoint: "https://4893d737c0b9e484dfc37ec392b5fa8a.r2.cloudflarestorage.com",
	credentials: {
		accessKeyId: "7dc27c125a22ad808cd01df8ec309d41",
		secretAccessKey:
			"1aa5c5b0c43defdb88f567487c071d17e234126133444770a706ae09336c57a4",
	},
	region: "auto",
});
const url = await getSignedUrl(
	S3,
	new PutObjectCommand({
		Bucket: bucket,
		Key: object,
	}),
	{
		expiresIn: 60 * 60 * 24 * 7, // 7d
	}
);
console.log(url);

​​ Test the presigned URL

Test the presigned URL by uploading an object using cURL. The example below would upload the 123 text to R2 with a Content-Type of text/plain.

$ curl --request PUT <URL> --header "Content-Type: text/plain" --data "123"

​​ Add CORS policies from the dashboard

1. From the Cloudflare dashboard, select R2.
2. Locate and select your bucket from the list.
3. From your bucket’s page, select Settings.
4. Under CORS Policy, select Add CORS policy.
5. From the JSON tab, manually enter or copy and paste your policy into the text box.
6. When you are done, select Save.

Your policy displays on the Settings page for your bucket.

​​ Response headers

The following fields in an R2 CORS policy map to HTTP response headers. These response headers are only returned when the incoming HTTP request is a valid CORS request.

​​ Example

This example shows a CORS policy added for a bucket that contains the Roboto-Light.ttf object, which is a font file.

The AllowedOrigins specify the web server being used, and localhost:3000 is the hostname where the web server is running. The AllowedMethods specify that only GET requests are allowed and can read objects in your bucket.

[
  {
    "AllowedOrigins": [
       "http://localhost:3000"
    ],
    "AllowedMethods": [
       "GET"
    ]
  }
]

In general, a good strategy for making sure you have set the correct CORS rules is to look at the network request that is being blocked by your browser.

• Make sure the rule’s AllowedOrigins includes the origin where the request is being made from. (like http://localhost:3000 or https://yourdomain.com)
• Make sure the rule’s AllowedMethods includes the blocked request’s method.
• Make sure the rule’s AllowedHeaders includes the blocked request’s headers.

Also note that CORS rule propagation can, in rare cases, take up to 30 seconds.

​​ Common Issues

• Only a cross-origin request will include CORS response headers.
  • A cross-origin request is identified by the presence of an Origin HTTP request header, with the value of the Origin representing a valid, allowed origin as defined by the AllowedOrigins field of your CORS policy.
  • A request without an Origin HTTP request header will not return any CORS response headers. Origin values must match exactly.
• The value(s) for AllowedOrigins in your CORS policy must be a valid HTTP Origin header value Open external link . A valid Origin header does not include a path component and must only be comprised of a scheme://host[:port] (where port is optional).
  • Valid AllowedOrigins value: https://static.example.com - includes the scheme and host. A port is optional and implied by the scheme.
  • Invalid AllowedOrigins value: https://static.example.com/ or https://static.example.com/fonts/Calibri.woff2 - incorrectly includes the path component.
• If you need to access specific header values via JavaScript on the origin page, such as when using a video player, ensure you set Access-Control-Expose-Headers correctly and include the headers your JavaScript needs access to, such as Content-Length.