What security issue is associated with an open TCP port 23?

An open TCP port 23 is associated with Telnet, which is an insecure protocol used for remote communication. The primary issue with Telnet is that it transmits data, including usernames and passwords, in plaintext. This lack of encryption exposes sensitive information to potential attackers who can easily intercept and read the data as it travels across the network.

Organizations should disable Telnet in favor of more secure alternatives such as SSH (Secure Shell), which provides encrypted communications and enhances security by protecting the data being transmitted. By disabling Telnet, the risk of unauthorized access and data breaches can be significantly reduced. Other options listed in the question refer to different protocols, each with their own security considerations, but they are not directly related to TCP port 23. This makes the identification of Telnet as the security issue pertinent to an open port 23 the correct choice.