Reframing Risk, Engineering Safety, and Advancing Cyber Resilience
Operational Technology (OT) environments are entering a period of accelerated transformation. Industrial facilities through energy, chemicals, water, utilities, transportation, and advanced manufacturing are integrating IT systems, deploying industrial IoT, and digitalizing operations at a rapid pace. With every new interface, modernization initiative, or network expansion, one fundamental requirement intensifies: accurate, verified, and continuously updated knowledge of the assets that exist in the environment and associated vulnerabilities. Yet, one topic ‘active network scanning’ continues to evoke resistance and worry among OT asset owners.
For years, the concept of scanning in OT has been surrounded by fears: PLCs freezing during production cycles, communication loops collapsing unexpectedly, historian data gaps emerging, or entire units shutting down due to an inadequately planned scan. These incidents were real, and they shaped a culture of fear that persists today. But the reason these incidents occurred was never because “active scanning is unsafe” rather it was because the active scanning was performed without regard for the architectural and deterministic characteristics that make OT fundamentally different from IT.
To dive deeper into the engineering realities behind safe active scanning, you can watch the full webinar: Active Network Scanning in Operational Technology (OT).
This article discusses the technical realities that make OT sensitive to uncontrolled scans, demonstrates why passive monitoring alone is insufficient for modern cyber readiness, provide details on the architectural constraints that must be respected, and provides a structured engineering framework for executing scans without operational disruption.
Architectural Realities Behind OT Sensitivity to Scanning
Unlike IT networks where devices were designed with substantial memory, powerful CPUs, and defensive networking stacks OT environments contain devices engineered primarily for deterministic control logic, predictable communication cycles, and decades-long operational lifespans. Many industrial controllers still in service were manufactured in the 1990s or early 2000s, long before cybersecurity requirements or modern vulnerability management practices existed. As a result, these devices often operate with extremely small network buffers, minimal RAM, limited error-handling routines, and firmware that was never designed to receive bursts of unexpected packets.
The risk of active scanning is therefore not a matter of opinion or policy; it is rooted in architectural limitations. When an unmanaged or aggressive scan sends packets at a rate beyond what a legacy controller can process, the device may freeze, drop communication, enter STOP mode, reboot unexpectedly, or trigger internal watchdog timers (WDT). Similarly, networks that include unmanaged switches, flat VLANs, low-bandwidth links, serial-to-Ethernet converters, or industrial ring topologies may experience congestion, RSTP) recalculation events, or broadcast storms if scans introduce excessive traffic. These conditions can break HMI update cycles, delay PLC heartbeat confirmations, disrupt historian polling, and even trigger automatic shutdown mechanisms.
Understanding these constraints is essential to not avoid scanning but to perform scanning in a manner engineered around the realities of OT operations.
Defining the OT Environment: Architecture, Constraints, and Legacy Exposure
Industrial networks span a complex set of technologies: distributed control systems (DCS), supervisory control and data acquisition (SCADA), programmable logic controllers (PLCs), remote terminal units (RTUs), safety instrumented systems (SIS/ESD), human–machine interfaces (HMIs), engineering workstations (EWS), and a range of vendor-embedded devices. What unites them is a reliance on deterministic communication rather than opportunistic or best-effort networking.
Control cycles such as HMI polling, historian timestamping, safety system heartbeats, or PLC-to-PLC synchronization operate on precise timing windows. Even minor disruptions can create cascading operational consequences. For example, a 1–2 second delay in a safety system’s challenge-response sequence can cause it to interpret lost communication as a safety risk and enter a fail-safe state. In high-integrity applications, PLC heartbeat values may update every few milliseconds; any interruption or jitter is treated as a potential hazard.
Legacy devices intensify this challenge. Older PLCs using 10 Mbps Ethernet cards, serial-to-Ethernet gateways, proprietary protocols, or unsupported firmware versions lack the defensive robustness of modern IT systems. Many are particularly intolerant of malformed packets, rapid ARP requests, concurrent TCP handshakes, or multi-threaded UDP sweeps. These devices expect narrowly defined traffic patterns; any deviation, even if benign from a cybersecurity perspective, can cause faults.
In such environments, the network often exhibits sensitivity. Industrial rings using RSTP, for instance, can enter recalculation cycles when faced with unexpected broadcast traffic, momentarily interrupting communication paths between controllers and HMIs. Low-bandwidth segments, common in remote or older facilities, can choke under bursts of scanning traffic that would be trivial in IT networks.
The OT environment is not fragile; it is deterministic and demands predictable, controlled behavior.
Passive Monitoring vs. Active Scanning
A mature OT security program requires both passive and active techniques, but each serves a distinct purpose.
Passive monitoring is invaluable for understanding live communication patterns, protocol behaviors, vendor-specific traffic normalization, and real-time operations. Observing live traffic through A SPAN port is a special configuration on a network switch that stands for switched port analyzer (SPAN) ports or industrial IDS platforms. Passive monitoring provides deep insights without sending a single packet. However, passive monitoring cannot detect silent devices, unused ports, dormant RTUs, disconnected vendor laptops, or backup assets that do not regularly communicate.
Active scanning, in contrast, has the ability to discover all devices on a subnet, enumerate firmware versions, identify listening services, validate network boundaries, and uncover misconfigurations. It is indispensable for building an accurate asset inventory, something no passive tool can accomplish completely. The risks emerge only when scans are executed without technical control.
The choice is not passive or active. OT requires passive and active, applied with engineering discipline.
Why Active Scanning Historically Caused Disruptions
Incidents associated with active scanning in OT did not occur because scanning is inherently dangerous. They occurred because scanning was performed using IT-oriented assumptions. A full-speed, multi-threaded Nmap or Nessus scan is designed for gigabit-class IT networks running modern operating systems and will overwhelm many industrial assets.
An aggressive scan may generate thousands of packets per second, performing port sweeps across every TCP and UDP port, issuing duplicate connection attempts, probing protocols in rapid succession, and fingerprinting services. Legacy controllers, unable to interpret or reject unexpected packets gracefully, may freeze or drop traffic. Networks built with older switching hardware may saturate buffers or trigger topology recalculations.
In every scanning-related OT incident, the root cause is the same: the scanning activity exceeded the processing or buffering capacity of the receiving asset or network segment.
Why Active Scanning is Now Operationally Necessary
Despite historical fears, active scanning has become an unavoidable requirement for modern OT cybersecurity maturity. As regulations strengthen, threat actors become more sophisticated, and IT/OT convergence deepens; organizations can no longer rely on outdated network diagrams or manual asset lists. Visibility is the foundation of every security control, from zero trust architectures to segmentation validation, risk assessments, and patch management programs.
Active scanning uniquely enables organizations to enumerate firmware versions, detect unexpected services, validate IP-MAC mappings, and uncover unauthorized or rogue devices insights that passive monitoring alone cannot provide. In many environments, scanning reveals decades-old devices, unpatched systems, vendor access points left behind; engineering laptops forgotten on the network, or test systems still connected long after commissioning work concluded.
Without safe active scanning, security teams operate blindly. Modern OT resilience depends on asset intelligence, and asset intelligence requires controlled, engineered scanning.
Engineering Framework for Safe Active Scanning
Safe scanning in OT does not rely on guesswork; it is a methodical engineering discipline. The first principle is understanding the environment at a deep technical level: mapping device generations, identifying which assets operate on legacy firmware, determining which communication cycles are time-sensitive, evaluating whether RSTP is deployed, and reviewing bandwidth constraints. This situational awareness ensures that scanning actions never intersect with fragile devices or critical process paths unexpectedly.
Next, assets must be classified by operational criticality. Safety instrumented systems, emergency shutdown PLCs, and interlock controllers demand extreme caution and are often best scanned offline or during controlled maintenance windows. Core process controllers require serialized, ultra-slow interrogation with strict rate limiting. HMIs, historians, and servers can tolerate moderate scanning. Boundary and DMZ systems can generally be scanned using standard IT techniques. Criticality dictates timing, scoping, and packet frequency.
Tool selection and configuration are equally important. The difference between a safe scan and a dangerous one lies primarily in timing. A slow, serialized Nmap scan using timing templates like -T0 or -T1, with multi-second delays between packets and no parallelization, behaves fundamentally differently from an aggressive IT-style sweep. These controlled scans may take longer, sometimes hours, but they ensure that no device or network segment receives traffic faster than it can safely process.
Safe scanning also requires controlled scoping. Unnecessary all-port scans, UDP floods, OS fingerprinting sweeps, and curiosity-driven probing have no place in OT environments. Scanning must follow documented procedures, approval of workflows, and engineering change control. In mature organizations, scanning results feed immediately back into the asset inventory, improving configuration accuracy, and informing remediation strategies.
Steps By Step Methodology for Performing Active Scanning
1. Understand the Environment
It is mandatory to know your environment, including whether you have any redundant tree protocol (RSTP) running, ring topologies, or legacy PLCs. Without this knowledge, security personnel risk causing a plant shutdown.
2. Classify Assets by Criticality
This step determines the risk for your environment. Security professionals must assess the risk of a particular network (e.g., is it a safety system, or is it merely HMI communication?) to determine appropriate security measures. For instance, communication happening between PLCs is highly critical and should not be messed with, while communication only between a PLC and an HMI may tolerate a one-second delay.
3. Select the Right Tools and Modes
It is important to use the right tools and techniques of fine-tuned software to avoid certain high-risk traffic, such as broadcast traffic on sensitive ring networks.
4. Control the Timing (The Key to Safety)
The most important principle for safe active scanning is controlling the timing. It involves slowing down the scanning significantly. An intense scan might send 1,000 packets per second, which can crash legacy PLCs. The alteration is to send one command or request every 5 seconds, which allows the PLC to respond without malfunctioning. This slowing down removes the high-risk variable from the equation, ensuring the control system’s stability.
5. Only Scan What You Need
The last step to consider is scanning only for the required information and data. Security activities should be conducted based on actual necessity rather than running an intense scan just because “it’s fun.”
Correcting Common Misconceptions About OT Scanning
The belief that active scanning is prohibited or universally unsafe continues only because scanning has been misused. When controlled, scanning is entirely viable. Passive monitoring alone cannot meet visibility requirements; it cannot identify dormant assets or validate segmentation boundaries. Vendor documentation rarely prescribes scanning methods because their guidance focuses on product functionality, not network-level resilience; thus, operators must assume responsibility for safe scanning practice.
Another misconception is that switches or network hardware are at risk from scanning. The challenges arise from misconfigured redundancy protocols, bandwidth constraints, or outdated topologies, not from the switches themselves.
Institutionalizing Safe Scanning
As organizations mature, safe scanning must move from an ad-hoc activity to a formalized engineering process. This requires developing scanning standards that define approved tools, acceptable timing profiles, asset categories, and operational risk thresholds. Technical safeguards such as rate limiting, VLAN isolation, controlled access lists, and bandwidth shaping can further reduce risk at the network layer. Cross-training between IT and OT personnel ensures that practitioners understand both scanning techniques and industrial process behavior, minimizing the risk of misinterpretation or misuse.
Finally, every industrial organization should validate scanning behavior in testbeds, Factory Acceptance Test (FAT) environments, or simulation platforms before deploying scanning methodologies into live production. This approach builds operator confidence and verifies device tolerance under realistic conditions.
Conclusion
Active network scanning has long been stigmatized within OT environments due to historical incidents where scanning was performed without understanding deterministic communication patterns and device limitations. But the reality is clear: active scanning is not inherently unsafe; uncontrolled scanning is. With the right engineering principles-controlled timing, informed scoping, knowledge of device criticality, awareness of network architecture, and adherence to operational discipline, active scanning becomes predictable, safe, and deeply valuable.
In a world where cyber threats are intensifying and industrial digitalization continues to accelerate; organizations cannot achieve cyber resilience without accurate asset intelligence. And accurate asset intelligence depends on safe, deliberate, technically informed active scanning.
The future of OT cybersecurity will be defined not by avoiding active scanning but by elevating it into a disciplined engineering practice one that respects the operational sensitivity of industrial systems while empowering defenders to secure them effectively.
From asset discovery and protocol-aware VA to safely scoped PT and architectural hardening, ACET Solution designs programs that respect operational realities while elevating cyber-resilience.
Visit our website to learn how our OT cybersecurity services can support your operational continuity, safety objectives, and long-term resilience.
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