Information
Telnet is a clear-text administrative protocol. As such, both the credentials used to establish the session and all commands and data within the session are readable in clear-text, so can be intercepted or modified by an attacker
Telnet doesn't natively support encryption or message integrity checks. Hence, any transmitted information (including credentials) is exposed to an attacker that gains access to the communication. Furthermore communication could be intercepted and modified by a man-in-the-middle attacker.
Solution
It's recommended to restrict VTYs to SSH for management access and not allow this device to SSH to other devices:
vty-pool default 0 4 line-template default
line default
transport input ssh
transport output none
Impact:
Ensure you have already configured an alternative management access to the device before disabling telnet access. Otherwise you might be locked out of the device with no management access.