TCP Port 502 – Modbus TCP

Port 502 carries Modbus TCP, one of the oldest and most widely deployed industrial control protocols. PLCs, sensors, meters, and building systems use it to read values and write control registers. The protocol dates to an era of fully trusted serial networks.

About TCP Port 502

The Internet Assigned Numbers Authority (IANA) registers port 502 for mbap. The IANA description reads: ‘Modbus Application Protocol’.

Security Considerations

Modbus has no authentication and no encryption anywhere in the protocol. Any device that can reach port 502 can read values and, on writable registers, change the physical behavior of whatever the controller runs. Internet facing Modbus endpoints turn up in every public scan of industrial systems, and they are a standing target for researchers and attackers alike. Industrial gear belongs on isolated networks reached through VPN access, with port 502 firewalled from anything that does not strictly need it.

Data Sources

This information is compiled from: IANA Service Name and Transport Protocol Port Number Registry.