I'm currently trying to track down what I think is a false positive from Nessus, my attempts to netcat into the device are failing, but I believe the service is shut off anyway and Nessus is being Nessus.

This brought up a question I've had for a while and any time I look it up on Google, I get 9001 answers to different things... How do vuln scanners authenticate to web services using either bash or cmd. I know it can be done with netcat or ncat, but I can't seem to find proper results. How is it done?

edit: forgot to mention this is http.