1What is an Industrial Control System (ICS)?

An ICS is a collection of hardware and software that monitors, automates and controls a physical process such as generating power, treating water or running a production line.

Traditional IT protects information. In an industrial environment you also protect machinery, the environment and human lives. A cyberattack on an ICS can therefore end in a physical outcome: an overflowing tank, a damaged turbine or a power cut.

The three technologies you must know

2How a control loop works

Take a water treatment plant that must keep a storage tank at the right level. The control loop has four parts: a sensor, a controller, an actuator and a human interface.

1. The sensor measures the water level and reports it to the PLC.
2. The PLC compares the reading with the configured target (the setpoint).
3. If the level is too low, the PLC commands a pump to start or a valve to open.
4. When the target is reached, the PLC stops the pump or closes the valve.
5. The HMI shows the operator the level, alarms and equipment status.

This is closed-loop control because feedback is used continuously to correct the process.

3PLC – Programmable Logic Controller

A PLC is a ruggedised industrial computer that automates a machine or process. It tolerates heat, vibration and electrical noise and runs continuously with predictable (deterministic) timing.

The scan cycle

A PLC does not wait for events like a normal program. It runs the same short loop over and over.

Example: a factory conveyor

• A sensor detects a box and the PLC receives the signal.
• The PLC switches on the motor and the belt moves the box to the next station.
• A second sensor detects arrival and the PLC stops the motor, with no operator involved.

Good to know

• Programming: done in IEC 61131-3 languages such as ladder logic, structured text and function block diagram, from an engineering workstation. You do not need to learn the languages for CISSP, but you must know the workstation is a crown-jewel asset.
• Mode switch: many PLCs have a physical key position (for example RUN, PROGRAM, REMOTE). Locking it in RUN is a simple physical control against unauthorised program changes.
• Fragile networking: small embedded network stacks can crash under unexpected traffic, which is why aggressive scanning is dangerous.

PLC threats

Key controls

Network segmentation, strict access control, controlled program changes, secure remote maintenance, physical protection (cabinets, mode keys, port locks) and verified offline backups of PLC programs. Safety functions such as emergency stops must not depend only on ordinary PLC logic; they use dedicated safety-rated devices.

4DCS – Distributed Control System

A DCS controls a complex process by spreading control across many controllers in one facility, all managed from a central control room.

It is typical of continuous and batch process industries: refineries, chemical plants, power stations and pharmaceutical manufacturing. Because control is distributed, a single controller failure does not stop the whole plant, and DCS designs commonly include redundancy.

Example: an oil refinery

If cooling fails, the system may need to reduce heating or shut down affected equipment safely. That coordination is what a DCS is built for.

Why a DCS instead of one PLC?

A PLC controls one machine or cell. A DCS gives a coordinated environment: shared operator interfaces, alarm management, engineering configuration, historian and process management across many controllers.

DCS security considerations

• A compromised operator station can send harmful process commands.
• Changing setpoints can destabilise the process.
• A compromised engineering system can alter control logic.
• Weak segmentation lets an incident spread between process areas.
• Shared services (time, directory, patch servers) become common points of failure.

5SCADA – Supervisory Control and Data Acquisition

SCADA is a system architecture to supervise, monitor and sometimes remotely control industrial assets, especially when those assets are geographically spread out.

Example: an electricity distribution network

A utility runs 100 substations across a country. Without SCADA, technicians would drive to each one to read values and change settings. With SCADA:

• Each substation has sensors, protection devices and an RTU or PLC.
• Measurements and status flow to the control center, where operators see power flows, breaker status and faults.
• Authorised operators send supervisory commands, for example opening or closing a breaker.
• Local controllers and protective relays keep doing their job even if the WAN link fails, where designed to do so.

PLC vs RTU

Modern PLCs and RTUs overlap a lot, so treat this as a typical distinction rather than a hard line.

SCADA security risks

Because SCADA links separated assets, it has the largest exposure of the three: unauthorised remote access, insecure legacy protocols, compromised HMI or SCADA servers, malicious supervisory commands, spoofed or replayed telemetry and communication outages.

6PLC vs DCS vs SCADA

These are memory aids, not absolute rules. Real systems overlap and are often combined.

Supporting components you must recognise

7ICS security vs traditional IT security

This is one of the highest-value areas for the exam. Both worlds care about confidentiality, integrity and availability, but they rank them differently, and OT adds safety on top.

Why standard IT actions can be dangerous in OT

A security engineer finds a vulnerability in a production PLC. In IT you might patch immediately or run a scanner. In OT, doing that without assessment could crash a legacy controller, stop production or affect safety.

The CISSP approach:

1. Assess the operational and safety risk.
2. Coordinate with OT engineers and the system owner.
3. Test the change in a representative environment.
4. Schedule an approved maintenance window.
5. Have recovery and rollback ready.
6. Use compensating controls when patching is not safe yet.

8Why ICS is hard to secure

Most ICS weaknesses come from one root cause: these systems were designed for reliability and safety in isolated networks, where everything on the wire was trusted.

9Purdue model, Industrial DMZ and zones

The Purdue Enterprise Reference Architecture (the Purdue model) organises enterprise and industrial systems into functional levels. It tells you where a system lives and therefore where its controls belong. It is a reference model, not a literal network diagram.

The Industrial DMZ

Staff need production reports, but letting enterprise PCs talk directly to PLCs would let malware walk into the control network. The DMZ solves this: IT talks to the DMZ, OT talks to the DMZ, and IT never talks directly to OT.

The DMZ does not make you secure by itself. It needs correctly configured firewalls, identity controls, monitoring and secure administration.

ISA/IEC 62443: zones, conduits and security levels

ISA/IEC 62443 is the main industrial automation and control systems (IACS) security standard family, developed by the ISA99 committee and the IEC. Two concepts matter most:

• Zone: a group of assets with the same security requirements (for example, the PLC network).
• Conduit: a controlled communication path between zones (for example, a firewall rule set between the PLC network and the engineering workstation network).

10Industrial communication protocols

Industrial devices exchange sensor values, commands, alarms and status. Most protocols date from an era of isolated networks, so trust was assumed and security was never part of the design.

Example: why Modbus TCP is risky

11Safety Instrumented Systems (SIS)

Process control keeps the plant running. Process safety keeps it from hurting anyone. They are separate jobs and must be built as separate systems.

Take a reactor held at a set pressure. The DCS regulates the pressure in normal operation. If pressure rises to a dangerous level, an independent safety system must shut the reactor down, even if the DCS has failed or has been hacked.

12Real-world ICS attacks

Real incidents turn abstract concepts into exam scenarios. Know the year, the target and the lesson.

13How to secure an ICS environment

The exam wants a security manager’s answer: layered, risk-based, and coordinated with operations. Think defence in depth, adapted for OT.

14Standards and regulation to recognise

15The CISSP exam angle

Where it appears: mainly Domain 3 (Security Architecture and Engineering), under assessing and mitigating vulnerabilities of security architectures, which lists ICS. It also touches Domain 4 (network segmentation and protocols), Domain 7 (change management and incident response) and Domain 1 (risk and safety-aware governance).

Six decision rules

Tempting wrong answers

16Practice questions

Ten exam-style questions with BEST, MOST and FIRST wording. Pick an answer to see feedback.

17Final revision sheet

Further reading

• NIST SP 800-82 Rev. 3, Guide to Operational Technology (OT) Security
• ISA/IEC 62443 series of standards
• NIS2 Directive (EU) 2022/2555
• CISA/FBI advisory AA21-042A (Oldsmar water treatment facility) and CISA alert IR-ALERT-H-16-056-01 (Ukraine power grid)

Independent study notes. CISSP is a registered mark of ISC2, which is not affiliated with this article. Figures are simplified teaching diagrams. Last reviewed October 2026.