Telnet
$ telnet 10.10.10.3 23

uses telnet protocol to access ip at port 23

Enumerating Telnet
$ nmap -sT --open -v -p-

runs nmap

-sT = connect scan

--open = only show open/possibly open ports

-v = verbosity

-p- = all ports

$ nmap -sV -p 8012

runs nmap

-sV = service version

-p = port

Exploiting Telnet
https://www.cvedetails.com/
https://cve.mitre.org/