Modbus is a widely used industrial communication protocol for moving commands and process data between control systems, PLCs, sensors, and other OT devices. It was built for reliability and simplicity, not for security, so many deployments do not include native authentication, encryption, or message integrity checks. In practice, that means a device that can reach the network can often send Modbus commands unless another control blocks it.
This matters in cyber security because Modbus is common in legacy ICS environments where equipment is long-lived and hard to replace. Attackers can abuse that trust to read process values, change setpoints, or issue unauthorized control actions. Defenders usually compensate with segmentation, industrial firewalls, strict allow-lists, gateways, and protocol-aware monitoring. Those controls reduce exposure, limit lateral movement, and help detect traffic that is syntactically valid but operationally suspicious.