6fff287f As a blockchain developer, you are probably 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, this important security feature can be easily overlooked.
In this article, we will take a deep dive into the CORS bug as it relates to Esplora and its Docker container, focusing specifically on Blockstream Esplora.
What is CORS?
In other words, when a browser makes a request to a server-side application, it must authenticate the request and ensure that only authorized scripts can access sensitive data.
CORS in the Esplora Docker Container
If you run the Blockstream Esplora container using “docker run,” you have probably 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 may not be immediately apparent.
The problem: CORS error in Blockstream Esplora Docker container
If a Docker container is running with `CORS_ALLOW=*`, it is possible that the browser or the Esplora server itself has set up an exception for the current IP address. This may cause a CORS error when attempting to establish connections between client and server.
bash
docker run --name explora-container -p 50001:50001 -p 8094:80 --volume $PWD/data_bitcoin_regtest:/data -e CORS_ALLOW=* --rm -i -t blockstream/esplora bash -c &...
The problem occurs when you try to connect to the Esplora server using curl or another web application:
bash
curl
You will probably get a CORS error response that might look something like this:
json
{
"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.
If a web application makes a request to the Esplora server without proper authentication or authorization, it can trigger CORS restrictions, even if you have explicitly allowed cross-origin access with `CORS_ALLOW=*`.
In your case, there are several possible reasons why this issue might occur:
- **Incorrect CORS settings**: Double check that you have not set any incorrect CORS settings on the Esplora server or in your web application.
- **IP address exception**: The browser or the Esplora server may have set an IP address exception, which can cause CORS errors when trying to establish connections between client and server.
- **Docker container configuration**: Check that your Docker container is properly configured, including any required environment variables or settings.
Solutions
To fix the CORS error in your Blockstream Esplora container:
- **Check the browser’s CORS policy**: Make sure you Allow cross-origin requests from the web application making the request to the Esplora server.
- **Check IP address exception**: Make sure that no IP address exceptions are set up on either the client or the Esplora server.
- **Update Docker configuration**: Check your Docker container configuration and make sure it is set up properly for CORS access.
By understanding what a CORS error is in the context of Blockstream Esplora and its Docker container, you can take steps to resolve this issue and ensure that your Bitcoin node and web applications work together smoothly.