What is it?
Telnet allows a user to open a command terminal on a remote computer or device and execute commands. Telnet runs on port 23. Telnet is primarily used to log in to a remote server, but it can also be used to log in to routers and other network devices.
Why is it a risk?
When telnet is used over an unencrypted channel (like the Internet), things like usernames and passwords are transferred in clear text. This allows an attacker to eavesdrop on connections and discover confidential information.
How can you mitigate the risk?
Telnet is obsolete and should be disabled.
Secure SHell (SSH) (more accurately SSH-2) serves the same purpose as telnet but encrypts all traffic so that eavesdropping doesn’t work. There are no documented vulnerabilities in SSH-2 but the 2013 National Security Agency (NSA) information leak by Edward Snowden suggests that such a vulnerability may exist.
Nearly every device available today supports SSH.
Resources:
SolarWinds SSH Overview
https://www.solarwindsmsp.com/blog/ssh-network-protocol-overview
