Telnet

Telnet is a protocol used for remotely accessing network devices or servers over a TCP/IP network. It provides a command-line interface (CLI) that allows a user to control and manage the device remotely, as if they were sitting at the physical console. Telnet has been widely used for remote administration and maintenance of network devices such as routers, switches, and servers, but it has largely been replaced by more secure protocols such as SSH. The telnet protocol uses port 23 for communication and sends data in plain text, which means that it is vulnerable to eavesdropping and interception.

Telnet, by default, does not encrypt any data sent over the connection (including passwords), and so it is often practical to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, hub or gateway located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login, password and whatever else is typed with a packet analyzer. Most implementations of Telnet have no authentication that would ensure communication is carried out between the two desired hosts and not intercepted in the middle. Several vulnerabilities have been discovered over the years in commonly used Telnet daemons.