Need

Prevent execution of remote files to maintain application integrity and confidentiality of data.

Context

• Usage of Elixir for building scalable and fault-tolerant applications
• Usage of Plug and Cowboy for HTTP request and response handling
• Usage of server to accept URLs or file paths from user inputs

Description

Non compliant code

        defmodule VulnerableApp do
  use Plug.Router

  plug :match
  plug :dispatch

  get '/' do
    filename = get_param(conn, 'filename')
    file_content = File.read!(filename)
    send_resp(conn, 200, file_content)
  end
end
        
        

This code includes a file specified by user input in the server's execution context, which could lead to Remote File Inclusion.

Steps

• Don't allow file paths or URLs to be specified directly by user inputs.
• Sanitize all user inputs to ensure they don't contain malicious code.
• Use a safe method for handling files, such as storing file references in a database and retrieving them by ID.

Compliant code

        defmodule SecureApp do
  use Plug.Router

  plug :match
  plug :dispatch

  get '/' do
    file_id = get_param(conn, 'file_id')
    filename = lookup_filename(file_id)
    file_content = File.read!(filename)
    send_resp(conn, 200, file_content)
  end

  defp lookup_filename(file_id) do
    # Query database to retrieve file path by ID
  end
end
        
        

This code retrieves files in a safe manner by using a database of file references rather than directly including files from user inputs.

References

• 061. Remote File Inclusion