After operating Nmap against a system on a network, Patrick sees that TCP port 23 is open and a service is running on it. What problem should he detect?

TCP port 23 is typically associated with Telnet, an unencrypted remote shell protocol. Since Telnet sends its authentication and other traffic in the clear (clear/plain text), it should not be used, and Patrick should identify this as a configuration issue involving an insecure protocol.