config-application.rb

# frozen_string_literal: true

# ...
require_relative "../lib/middleware/marcelo"
# ...

module CoolName
  class Application < Rails::Application
    #...
    # Before anything you get me
    # config.middleware.insert_before 0, Marcelo

    # Appends it to the end of the middleware chain
    config.middleware.use Marcelo
    #...
  end
end

lib-middleware-marcelo.rb

# frozen_string_literal: true

# I am the man in the middle(ware)
#
# Reading material:
#   - https://younes.codes/posts/what-is-rack
#   - https://guides.rubyonrails.org/rails_on_rack.html
class Marcelo
  def initialize(app)
    @app = app
  end

  def call(env)
    return @app.call(env) unless "money".in?(env["REQUEST_PATH"].to_s.downcase)

    [
      Rack::Utils::SYMBOL_TO_STATUS_CODE[:misdirected_request], # Status code
      { denied: "you'll see no money here" },                   # Headers
      ["Go back where you came from, mercenary."]               # Body
    ]
  end
end

By doing the following:

curl -i  http://0.0.0.0:3000/show/me/the/money

The result should be:

HTTP/1.1 421 Misdirected Request
denied: you'll see no money here
cache-control: no-cache
content-security-policy:
x-request-id: 985326a1-d9bb-46ec-b423-bb47fb527cef
x-runtime: 0.001031
Server-Timing: total;dur=1.3340000296011567
set-cookie: __profilin=p%3Dt; path=/; httponly; samesite=lax
Content-Length: 39

Go back where you came from, mercenary.%