Understanding CORS Error in Esplora Docker Container

As a blockchain developer, you’re likely familiar with the importance of cross-origin resource sharing (CORS) when building applications that interact with external services. However, when working on projects like Bitcoin, it’s easy to overlook this critical security feature.

In this article, we’ll delve into what CORS error is in the context of Esplora and its Docker container, specifically focusing on Blockstream Esplora.

What is CORS?

CORS is a security feature implemented in web browsers to prevent malicious scripts from making unauthorized requests on behalf of the user. It allows websites to communicate with their backend services without the need for a direct request.

In other words, when a browser makes a request to a server-side application, it needs to authenticate the request and ensure that only authorized scripts can access sensitive data.

CORS in Esplora Docker Container

As you run the Blockstream Esplora container using docker run, you’ve likely set up CORS to allow web applications like your Bitcoin node to connect to the Esplora server. However, if everything is working as expected, the error might not be immediately apparent.

The Issue: CORS Error in Blockstream Esplora Docker Container

When running a Docker container with CORS_ALLOW=*, it’s possible that the browser or the Esplora server itself has set up an exception for the current IP address. This can cause a CORS error when trying to establish connections between the client and server.

In your case, you’re running the following command:

docker run --name esplora-container -p 50001:50001 -p 8094:80 --volume $PWD/data_bitcoin_regtest:/data -e CORS_ALLOW=* --rm -i -t blockstream/esplora bash -c &...

The issue arises when you try to establish a connection to the Esplora server using curl or another web application:

curl 

You’ll likely receive a CORS error response, which might look something like this:

{

  "message": "Cross-Origin Resource Sharing (CORS) Warning",

  "status": 0,

  "url": "/

}

What’s Behind the CORS Error?

The CORS Allow parameter is set to *, which means that all web applications can make requests to the Esplora server. However, this can lead to unexpected behavior and security issues.

When a web application makes a request to the Esplora server without proper authentication or authorization, it may trigger CORS restrictions, even if you’ve explicitly allowed cross-origin access with CORS_ALLOW=*.

In your case, there are several possible reasons why you might be experiencing this issue:

• Incorrect CORS settings: Double-check that you haven’t set any incorrect CORS settings on the Esplora server or in your web application.

• IP address exception: The browser or the Esplora server might have an IP address exception set up, which can cause CORS errors when trying to establish connections between the client and server.

• Docker container configuration

  : Verify that your Docker container is configured correctly, including any necessary environment variables or settings.

Solutions

To resolve the CORS error in your Blockstream Esplora container:

• Check the browser’s CORS policy: Make sure you’re allowing cross-origin requests from the web application making the request to the Esplora server.

• Verify the IP address exception: Ensure that there are no IP address exceptions set up on either the client or the Esplora server.

• Update Docker configuration: Review your Docker container configuration and make sure it’s correctly set up for CORS access.