Adversaries may send unauthorized command messages to instruct control system assets to perform actions outside of their intended functionality, or without the logical preconditions to trigger their expected function. Command messages are used in ICS networks to give direct instructions to control systems devices. If an adversary can send an unauthorized command message to a control system, then it can instruct the control systems device to perform an action outside the normal bounds of the device's actions. An adversary could potentially instruct a control systems device to perform an action that will cause an [Impact](https://attack.mitre.org/tactics/TA0105).(Citation: Bonnie Zhu, Anthony Joseph, Shankar Sastry 2011) In the Dallas Siren incident, adversaries were able to send command messages to activate tornado alarm systems across the city without an impending tornado or other disaster.(Citation: Zack Whittaker April 2017)(Citation: Benjamin Freed March 2019)
When communicating over an untrusted network, utilize secure network protocols that both authenticate the message sender and can verify its integrity. This can be done either through message authentication codes (MACs) or digital signatures, to detect spoofed network messages and unauthorized connections.
M0818Validate Program InputsDevices and programs designed to interact with control system parameters should validate the format and content of all user inputs and actions to ensure the values are within intended operational ranges. These values should be evaluated and further enforced through the program logic running on the field controller. If a problematic or invalid input is identified, the programs should either utilize a predetermined safe value or enter a known safe state, while also logging or alerting on the event.(Citation: PLCTop20 Mar 2023)
Use network appliances to filter ingress or egress traffic and perform protocol-based filtering. Configure software on endpoints to filter network traffic. Perform inline allow/denylisting of network messages based on the application layer (OSI Layer 7) protocol, especially for automation protocols. Application allowlists are beneficial when there are well-defined communication sequences, types, rates, or patterns needed during expected system operations. Application denylists may be needed if all acceptable communication sequences cannot be defined, but instead a set of known malicious uses can be denied (e.g., excessive communication attempts, shutdown messages, invalid commands). Devices performing these functions are often referred to as deep-packet inspection (DPI) firewalls, context-aware firewalls, or firewalls blocking specific automation/SCADA protocol aware firewalls. (Citation: Centre for the Protection of National Infrastructure February 2005)
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