Hi team,

Thanks for making this repo, it must have been a lot of work.

Summary: Remote Code Execution

Unsafe handling of request parameters in the RPC HTTP server results in command injection

Details

In lib/bitcoin/rpc/http_server.rb line 30-39, the JSON body of a POST request is parsed into command and args variables. These values are then passed to send, which is used to call an arbitrary class method. However, there is no validation that the provided command value is one of the expected RPC methods.
This means that an attacker could supply a command value such as system, and then pass arbitrary system commands into the args parameter and achieve remote code execution.

PoC

1. Start the RPC server
2. Send a request to the RPC server as so:

curl -X POST http://127.0.0.1:18443 -H 'Content-Type: application/json' \
   -d '{"method":"eval","params":["File.write(\"/tmp/pwned\",\"owned\")"]}'

3. Check the /tmp folder on the machine where the RPC server is being run. If a folder /pwned now exists, the vulnerability is confirmed.

Impact

This vulnerability would impact anyone running the RPC server. The impact is higher for those who are running it publicly exposed to the internet.

Remediation

Mitigating Factors:

• The RPC server is part of the experimental SPV node feature, which is not documented and has very few users.
• The SPV-related features may be removed in future releases.

Resolution:

• Added whitelist validation to allow only RPC methods defined in RequestHandler.
• Fixed in version 1.12.0.

Thanks all!