Reframing PLC Logic as a Foundational Layer of OT Defense
Cybersecurity in Operational Technology (OT) has emerged within a short period of time. Firewalls, network separation, intrusion identification frameworks, and observing platforms have been seen as the take of the day in industrial security plans. However, even with this advancement, a very thin line of defense is not fully analyzed, and that is the internal logic of the programmable logic controller (PLC) itself.
This gap was the focal point of a recent webinar conducted by Mubarak Mustafa, who is a consultant with over three decades’ experience in industrial automation and control systems. Mustafa opinion is straightforward and unconventional: the outcomes of cybersecurity in the OT settings are directly influenced, in which the control systems are programmed. In the absence of security consciousness in the design of PLC logic, even well secured networks can be exposed to low effort high impact attacks.
To understand how seemingly routine PLC programming decisions can create or reduce cyber risk, watch the full webinar: How Control System Programming Methodology Impacts Cybersecurity – Episode 7 – Webinar Series – YouTube.
In this article we will discusses the systematic analysis of the issues that control system programming brings to the problem of cybersecurity risk, the inefficiency of traditional defenses, and the benefit of engineering-sensitive logic design in terms of mitigating cyber-physical harm.
Why Traditional PLC Mindsets No Longer Align with Modern Threats
PLC programming has historically been framed as an operational and safety-focused discipline. Security considerations were assumed to belong elsewhere, typically at the network perimeter or within IT-managed infrastructure. As a result, PLC logic often reflects decades-old assumptions: trusted inputs, benign operators, and predictable environments.
Modern threat realities have invalidated those assumptions.
Core Components of Industrial Control Architecture
To understand how programming choices affect security, it is essential to examine the roles and limitations of the three foundational components of industrial control systems.
Programmable Logic Controller (PLC)
The PLC sits at the heart of the process, interfacing directly with sensors, actuators, valves, motors, and safety interlocks. Unlike IT systems, PLCs operate on proprietary operating systems designed for deterministic control rather than defensive computing.
While this proprietary nature reduces exposure to common malware, it introduces other risks. Most legacy PLCs still constitute most deployed controllers worldwide lack meaningful authentication mechanisms. In many cases, any system with the appropriate vendor software and network access can download logic, modify parameters, or manipulate runtime data.
Engineering Workstation (EWS)
EWS is responsible for creating, compiling, and deploying PLC logic. Every aspect of this process, file formats, protocols, compilers, and communications, is vendor-specific. Because the EWS has full control authority, compromising enables deep system manipulation. Attacks such as Stuxnet demonstrated the destructive potential of this vector, although such attacks require advanced capability and planning.
Human Machine Interface (HMI)
The HMI displays process data and allows operators to issue commands. Unlike EWS, the HMI does not require proprietary development tools. Any application capable of speaking about the correct protocol can function as an HMI, including general-purpose software.
Although HMIs cannot typically modify PLC logic, they can freely read and write process variables and tags. This capability, combined with widespread trust in operator inputs, makes the HMI the most accessible and frequently exploited attack surface.
Operational Efficiency and the Hidden Cost of Flexibility
To illustrate how standard programming practices introduce cyber risk, lets explore a powerful example involving a water tank.
Hardcoded Logic: Secure but Rigid
In early automation designs, engineers often embedded operational limits directly into PLC logic. For example, a pump would start when the tank level dropped below 20 percent and stop when it exceeded 80 percent. Changing these thresholds requires downloading a new program, typically during a planned shutdown.
From a cybersecurity perspective, this approach is robust. Operational limits cannot be altered remotely through data manipulation. From an operational perspective, however, it is inflexible and costly.
Variable-Based Logic: Efficient but Exposed
To improve efficiency, engineers began externalizing these limits into configurable variables such as low set points and high set points. Operators could adjust values through the HMI without stopping production.
This design choice improved availability and responsiveness, but it quietly transferred authority from trusted logic to mutable data. An attacker who gains access to the HMI can inject malicious values without altering the program itself. For example, setting a high set point above the physical maximum can prevent a pump from ever stopping, leading to overflow or hazardous conditions.
The vulnerability arises not from malicious code, but from unvalidated trust in data.
Security-Aware Programming: Engineering Controls Inside the PLC
The solution proposed is neither theoretical nor disruptive. It involves enhancing PLC logic with intentional defensive constraints.
Input Validation and Hard Limits
PLC logic should enforce absolute operational boundaries regardless of external input. If a process requires operation between defined limits, the PLC should cap values accordingly. Any input exceeding those bounds should be ignored or constrained to safe values.
State Stability Logic
Logic can be designed to preserve stable states when inputs fall within acceptable ranges. This prevents rapid cycling or oscillation caused by malicious or erratic input changes.
Impact Reduction Rather Than Traffic Prevention
These techniques do not block communication or prevent data transmission. Instead, they ensure that unsafe commands never translate into unsafe physical actions. The PLC remains responsive, but no longer obedient to hazardous instructions.
Why External Defenses Alone Cannot Solve the Problem
Firewalls and network controls remain essential, but they are not sufficient.
Deploying firewalls for every PLC is often impractical in large facilities. Firewalls also introduce latency and operational risk. More importantly, firewalls cannot distinguish between legitimate and malicious commands if those commands originate from an authorized system such as an HMI.
When trust is misplaced, perimeter defenses fail silently.
Applying Secure Programming Across System Lifecycles
For new projects, security-aware programming should be embedded during design. For legacy environments, wholesale reprogramming is neither feasible nor necessary. Instead, organizations should prioritize critical assets and high-consequence parameters. Targeted logic updates can dramatically reduce risk without widespread disruption.
This approach mirrors long-established safety engineering practices. Safety programming evolved to prevent accidents. Secure programming evolves to prevent intentional harm. Both disciplines protect availability and integrity, and neither can succeed in isolation.
A Unifying Analogy: Internal Judgment Over Blind Obedience
A PLC can be compared to the pilot of an aircraft. The HMI functions as the control tower. In traditional designs, the pilot follows instructions without question. Secure programming gives the pilot judgment. The tower may issue commands, but the pilot refuses those that would cause disaster.
Conclusion
OT cybersecurity cannot mature if it remains confined to network boundaries. Control system logic is not merely an operational artifact; it is an enforcement mechanism with direct influence over physical outcomes. When PLCs are programmed without security awareness, they amplify risk. When programmed with intent, they absorb it.
The future of industrial cyber resilience depends on recognizing PLC programming as a cybersecurity control layer. Not by replacing firewalls or monitoring platforms, but by complementing them with logic that understands safety, context, and consequences.
Secure systems are not built by trusting inputs. They are built by engineering judgment into the machines that act on them.
To explore how security-aware control system design can be applied in real-world environments and to access more expert insights on OT cybersecurity and industrial automation, visit our website. Discover practical guidance, in-depth analysis, and proven strategies to help you engineer resilience directly into your control systems.
Related Articles
