If systems are connecting to remote systems on TCP port 6667, what is the most likely cause?

When systems are connecting to remote systems on TCP port 6667, the most likely cause is botnet command-and-control via IRC (Internet Relay Chat). This port is commonly associated with IRC servers, which are often utilized for communication among bots in a botnet. Botnets are networks of compromised computers that can be controlled remotely by an attacker, and IRC is a popular protocol for facilitating this kind of communication.

In many cases, attackers will use IRC to send commands to the bots in their network, directing them to perform malicious activities such as launching DDoS attacks, spreading malware, or stealing data. The use of TCP port 6667 for this type of communication makes it a significant indicator of potential botnet activity.

Other options do not align with the typical use of port 6667. File sharing generally involves different protocols or ports, such as FTP or SMB. Remote desktop protocol typically uses port 3389, and web service communication usually occurs over port 80 for HTTP or port 443 for HTTPS. Thus, the association of TCP port 6667 with IRC makes it clear that the correct answer involves botnet command-and-control activities.