Rack middleware and a Railtie: a firewall with rate limits, shared blocks, attack patterns and a browser check; error monitoring (Rails.error, Sidekiq); server speed per route, with its database queries, outgoing calls, job queues and how close the server is to its limits; crawler counts; server-side events, leads and audit log; feature flags and maintenance mode; and the gems, settings and machine figures the Security check needs. Standard library and rack only.