Modbus

Modbus: Simple industrial protocol for reading and writing registers and discrete I/O. It is the most widespread protocol in operational technologyOperational TechnologyHardware and software used to monitor or control physical equipment, machinery, and industrial processes. and to this day contains no security mechanisms.

How it works and where it fits

Modbus dates from 1979 and is deliberately minimal. A request consists of a unit identifier, a function code, and an address; the data models are coils, discrete inputs, input registers, and holding registers. A handful of function codes read and write single or multiple registers. In IP networks the protocol runs as Modbus TCP on port 502, serially as Modbus RTU. A register has no semantics in the protocol — its meaning comes solely from the plant documentation.

Practical security relevance

There is no authentication, no integrity protection, and no encryption: whoever reaches the network can read and write. Any Modbus endpoint reachable from IT should therefore be treated as a critical finding. Protection happens outside the protocol through a zone model, firewalls with function-code filtering, unidirectional crossings for read-only analytics, and the Modbus/TCP Security extension with TLS where devices support it.

Related concepts

• Operational TechnologyOperational TechnologyHardware and software used to monitor or control physical equipment, machinery, and industrial processes.: Hardware and software used to monitor or control physical equipment, machinery, and industrial processes.
• Programmable Logic ControllerProgrammable Logic ControllerIndustrial computer that cyclically reads sensor values and derives actuator commands for a process.: Industrial computer that cyclically reads sensor values and derives actuator commands for a process.
• Operational Technology SecurityOperational Technology SecurityProtects industrial control, production, and process control systems.: Protects industrial control, production, and process control systems.
• SCADASCADAControl system for monitoring and operating geographically distributed industrial processes.: Control system for monitoring and operating geographically distributed industrial processes.