This is just a short notice for you in case you are not aware of it. Cisco announced a vulnerability on Cisco IOS and IOS XE operating system. For short, you only have to disable incoming TELNET service onto the router itself to avoid this vulnerability. You can use Secure Shell (SSH) instead for remote management. SSH is not vulnerable in this problem.

The Jin-Dai Bridge (錦帶橋) in Dahu Park (大湖公園).
Taipei City, Taiwan.

You can read the original announcement for technical details.

Cisco IOS and IOS XE Software Cluster Management Protocol Remote Code Execution Vulnerability

You can disable TELNET service and enable SSH at the same time by this command:

transport input ssh

You can list listening ports by these commands:

show control-plane host open-ports

show tcp brief