Telnet
Telnet (RFC 854) is a protocol for opening a remote terminal session over a network, typically on port 23, letting a user type commands on one machine and run them on another. It predates modern transport security by decades and carries no encryption at all: a username, password, and every subsequent keystroke are sent in plain text, readable by anyone who can observe the traffic. This made Telnet the standard remote-access method for routers, switches, and embedded devices for years, and many of those devices, along with industrial and IoT equipment, still ship with Telnet enabled by default. SSH replaced Telnet for interactive remote access decades ago precisely because it adds encryption and authentication Telnet never had, so a reachable Telnet port today is usually a sign of outdated or neglected configuration rather than an intentional choice.
By AdminKit team, Netbase JSC · Updated