Modbus/TCP

Modbus/TCP is the TCP/IP version of Modbus, a decades-old industrial communication protocol used to read and write values — sensor readings, relay states, setpoints — on programmable logic controllers (PLCs) and other industrial equipment, typically over port 502. IANA’s service name and port number registry lists port 502 for the Modbus Application Protocol (mbap) and a separate port, 802, for its secure variant (mbap-s). It was designed in an era when industrial networks were assumed to be physically isolated, so the protocol itself has no authentication, authorization, or encryption built in: whoever can reach a Modbus/TCP device on the network can read its registers and, in many configurations, write to them too. Modern industrial networks are increasingly connected to the broader internet, directly or via misconfigured firewalls, which exposes this decades-old assumption as a real risk rather than a theoretical one. OPC UA, a newer industrial protocol, instead makes authentication and encryption built-in, configurable options rather than leaving them out entirely. The protocol itself is maintained by the Modbus Organization, and NIST SP 800-82, the federal guide to operational technology security, covers internet-exposed industrial protocols like Modbus/TCP as a defense-in-depth problem. AdminKit’s Modbus exposure check tests a device’s own reachability directly, live, against the public IP making the request.

See the full AdminKit glossary for other related terms.