Setting CORS in Firebase

Cross-Origin Resource Sharing (CORS) is a mechanism that allows restricted resources (e.g., fonts, scripts, images, stylesheets, XHRs) on a web page to be requested from another domain outside the domain from which the first script was served.

Firebase provides a number of ways to set CORS, depending on the specific service you are using.

If CORS is not configured properly, you will encounter some issues in console like this.

Access to fetch at ‘____' from origin ‘___' has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.

Make sure to read the additional tips at the end of this article.

Create cors.json File

The cors.json file should contain a JSON object with the following properties:

• origins: A list of allowed origins. You can use the wildcard character (*) to allow all origins.
• methods: A list of allowed HTTP methods.
• headers: A list of allowed HTTP headers.

Here is an example of a cors.json file:

[
  {
    "origin": ["*"],
    "method": ["GET"],
    "maxAgeSeconds": 3600
  }
]

Firebase Storage

To set CORS for Firebase Storage, you can use the gsutil command line tool.

1. Install gsutil:

pip install gsutil

2. Authenticate with gsutil:

gsutil config

3. Set CORS for your bucket:

Now, open command terminal in the directory were cors.json file is stored and run this command.

gsutil cors set cors.json gs://<your-bucket-name>

Additional tips

• If you are using a custom domain for your Firebase app, you will need to add the domain to the list of allowed origins in your CORS configuration.
• If you are using a third-party library to make requests to Firebase, make sure that the library supports CORS.
• You can test your CORS configuration using a tool like curl or Postman.

The need for setting CORS arises from the same-origin policy, which is a security feature implemented in all major web browsers. The same-origin policy prevents scripts running on a web page from accessing resources from another domain, unless the server hosting the resources explicitly allows it.

CORS provides a mechanism for servers to relax the same-origin policy and allow cross-origin requests. This is useful for a variety of applications, such as:

• Single sign-on (SSO): SSO allows users to log in once and access multiple websites and applications without having to log in again. This is often implemented using cross-origin requests to allow the user’s credentials to be passed from one website to another.
• APIs: APIs often allow developers to build applications that interact with data and resources on a remote server. CORS allows these applications to be made available to users on any website, regardless of the domain.
• Content delivery networks (CDNs): CDNs are used to deliver static content, such as images, CSS, and JavaScript files, from servers that are located close to the user’s location. CORS allows CDNs to be used to deliver content to websites on any domain.

Benefits of setting CORS

There are a number of benefits to setting CORS, including:

• Improved security: CORS can help to improve the security of your application by preventing unauthorized access to your resources.
• Enhanced user experience: CORS can allow your users to use your application with other websites and services, without having to switch accounts or log in again.
• Increased reach: CORS allows your application to be used by users on any website, regardless of the domain.

Best practices for setting CORS

When setting CORS, it is important to follow best practices to ensure the security of your application. Here are a few tips:

• Limit the allowed origins: Only allow requests from origins that you trust.
• Limit the allowed methods: Only allow the HTTP methods that your application needs.
• Limit the allowed headers: Only allow the HTTP headers that your application needs.
• Use a preflight request: CORS uses a preflight request to check if the server will allow the actual request. You should implement a preflight request endpoint on your server.

Conclusion

Setting CORS in Firebase is a relatively straightforward process. By following the steps above, you can ensure that your Firebase app is accessible to users from all domains.