Internet-Security.com TCP port 23 details

TCP port 23 is used by the Telnet protocol for remote terminal access.
Unix/Linux systems historically run telnet servers on this port (inetutils-telnetd, netkit-telnetd, in.telnetd on Solaris, AIX telnetd), though often disabled by default today.
Microsoft Telnet Server (TlntSvr) on Windows Server 2003/2008 (and some older Windows editions) exposes port 23 for remote login.
Network gear commonly supports Telnet on 23 for CLI management, including Cisco IOS/IOS-XE, Juniper Junos, MikroTik RouterOS, HP ProCurve/Aruba switches, and firewalls like Fortinet and SonicWall (usually optional).
Many embedded/IoT devices ship with BusyBox telnetd on 23 (e.g., consumer routers, IP cameras, DVRs), and HP JetDirect print servers provide Telnet-based configuration on 23.
Mainframe/midrange environments use Telnet-based terminal services on 23, such as IBM z/OS TN3270/TN3270E (unencrypted) and IBM i (AS/400) TN5250 by default.
Some RTOS devices (e.g., VxWorks targets) expose a Telnet console on port 23 for debugging/administration.
Security: Telnet on 23 is unencrypted and heavily exploited (e.g., Mirai and similar botnets brute-force default credentials on routers/IoT devices).

TCP 23