Telnet is an outdated network protocol for text-oriented command-line access to remote hosts.

Problem

With Telnet, all communication including username and password is transmitted unencrypted in clear text and is therefore susceptible to eavesdropping.

Many IoT devices (routers, network cameras, etc.) are running Telnet servers by default. If the devices are openly accessible from the Internet and standard login credentials have not been changed, an attacker can easily gain full control of the devices. Malware like Mirai automatically exploits insecure Telnet servers openly accessible from the Internet using to compromise devices and connect them to a botnet.

Solution

If the Telnet server is not required, disable or deinstall it. Otherwise, restrict access to trusted local networks. In particular for IoT devices: Check if default login credentials have been changed.

CERT-Bund recommends using (Open)SSH with key-based authentication for secure access to remote hosts.

Further information

• Wikipedia: Telnet