The short answer
Passive OT monitoring analyses network traffic without interacting with devices, making it the standard approach for sensitive industrial environments where unplanned network packets can cause operational disruption. Active monitoring queries devices directly to gather richer asset intelligence but carries risk if applied without OT-specific controls. Most mature OT security programmes require both, applied deliberately, in the right zones, by analysts who understand the operational consequences of each approach.
Why this debate has become more consequential
For most of the history of OT security, passive monitoring was the only acceptable option. Industrial control systems, SCADA networks, programmable logic controllers, and the legacy devices that populate most OT environments were simply too fragile to tolerate active network interrogation. A misplaced scan could overload a PLC’s limited network stack, interrupt a control loop, or trigger a safety system response. Passive monitoring, listening to traffic rather than generating it, was the safest path to visibility.
That picture has changed for two reasons.
First, the threat landscape has evolved to the point where passive monitoring alone is no longer sufficient to meet the visibility requirements that a serious OT security programme demands. Dormant devices, silent assets, and threat actors specifically designed to evade traffic-based detection create blind spots that passive approaches cannot close.
Second, the regulatory environment has made those blind spots consequential. CAF Objective C, NIS2 detection obligations, and the incoming UK Cyber Security and Resilience Bill all require demonstrable monitoring capability across the systems supporting essential functions. Organisations that cannot demonstrate coverage of their full OT asset inventory, including assets that do not regularly communicate, are increasingly exposed at audit.
Understanding passive and active monitoring, when each applies, what each cannot do, and how a managed SOC deploys both safely, has become a practical compliance question as much as a technical one.
Passive OT monitoring: how it works and where it excels
Passive monitoring captures and analyses network traffic without injecting packets or interacting with devices. In OT environments, it is typically implemented through SPAN ports or network TAPs on key network switches, which create a copy of all passing traffic and forward it to a monitoring sensor or platform for deep packet inspection.
The monitoring sensor analyses this traffic to build an asset inventory from observed communications, identify the protocols in use, establish behavioural baselines, and detect anomalies that deviate from normal patterns. Because it only listens, it imposes no additional load on the OT network and does not risk disrupting device operation.
Where passive monitoring excels:
Passive monitoring is uniquely suited to the real-time detection of behavioural anomalies in active industrial processes. When a PLC begins issuing commands outside its established communication profile, when a new device appears on the network for the first time, or when traffic crosses a zone boundary in an unexpected direction, passive monitoring detects these events as they happen, without touching the devices involved.
It is also the safest approach for legacy environments. Many OT assets in operation across UK critical infrastructure were manufactured before cybersecurity requirements existed. They operate on deterministic communication cycles and have strict thresholds for network load. Passive monitoring respects these constraints completely.
For OT threat detection, passive monitoring provides the continuous behavioural baseline that makes anomaly detection meaningful. Without a baseline built from weeks or months of observed traffic patterns, it is impossible to distinguish a lateral movement attempt from a legitimate control system message.
The limitations of passive monitoring:
Passive monitoring can only detect what communicates. This creates a category of assets that are invisible to a passive-only approach: devices that are powered on but idle, legacy equipment that only communicates during specific maintenance windows, backup systems that have no regular network activity, rogue devices connected but not yet active, and vendor laptops left on the network following a commissioning engagement.
These are not theoretical edge cases. In OT security assessments, silent and dormant assets are among the most common sources of undiscovered risk. A threat actor who establishes persistence on a device that rarely communicates can remain undetected for extended periods in a passive-only monitoring environment.
Active OT monitoring: capabilities, risks, and the case for controlled deployment
Active monitoring queries devices directly, sending requests to retrieve firmware versions, software configurations, open ports, and other asset details that are not visible in network traffic. In IT environments, this is standard practice. Network scanners, vulnerability assessment tools, and endpoint agents routinely interrogate devices to maintain an accurate asset inventory and identify exploitable conditions.
In OT environments, active monitoring has historically been treated with significant caution, and for good reason.
The risk in OT environments:
Many OT devices were not designed to handle the volume of network requests that active scanning generates. A standard IT vulnerability scanner running against an OT network can cause PLCs to reboot, interrupt control loops, trigger safety system responses, or generate alarms that pull operator attention away from genuine process anomalies. The consequences are real. In a water treatment plant, a disrupted control loop could affect chemical dosing. In a power grid, an unexpected device reboot could interrupt generation or switching operations.
This remains a current concern. Even today, incorrectly configured active monitoring tools cause stability issues and unplanned outages in OT environments. The risk is real, and it is the reason that experienced OT security teams treat active monitoring with considerably more discipline than their IT counterparts.
The capabilities that justify controlled active deployment:
Despite these risks, there are asset intelligence requirements that passive monitoring genuinely cannot meet, and that have direct security and compliance implications.
Firmware versions are not transmitted in network traffic for some OT devices. Without active querying, an asset inventory cannot confirm whether an OT device is running a firmware version with a known critical vulnerability. Open ports and running services on OT workstations and servers are not always visible in traffic analysis. Devices that do not communicate regularly are entirely absent from a passive-only asset inventory.
Active monitoring, applied with OT-specific controls, fills these gaps. The key phrase is OT-specific controls. The approach required in an OT environment is fundamentally different from an IT vulnerability scan. It involves targeted queries to specific devices using protocols those devices were designed to respond to, at rates that do not approach the device’s connection threshold, at times coordinated with operations teams, and with pre-defined rollback procedures if unexpected behaviour is observed.
When applied with this discipline, active querying provides the firmware visibility, full asset inventory coverage, and vulnerability context that passive monitoring cannot deliver. The practical question becomes how to apply active monitoring safely, selectively, and with operational awareness.
A decision framework: which approach applies where
Rather than defaulting to “use both,” the practical decision about passive and active monitoring in an OT environment should be made at the zone level, informed by four factors.
Device criticality and sensitivity. Safety-critical control systems, process controllers, and any device whose disruption would have immediate physical consequences should be treated as passive-only unless a specific, tested active query method has been validated for that device type. Workstations, historians, and network infrastructure in OT-adjacent zones can typically tolerate controlled active queries.
Protocol support. Modern industrial protocols including OPC-UA and certain implementations of DNP3 support structured information requests that are safe to use for active querying. Legacy protocols such as Modbus do not. Before applying active monitoring to any OT asset, the protocol must be confirmed and the vendor’s guidance on querying consulted.
Network capacity. Many OT networks, particularly in older critical infrastructure environments, operate at bandwidth levels that are low by IT standards. Active queries must be rate-limited to avoid saturating network segments that control systems depend on for time-critical communication.
Regulatory and compliance requirements. CAF Objective C requires monitoring across the systems supporting essential functions. For an organisation with a significant number of silent or dormant OT assets, a passive-only approach will leave identifiable gaps in the evidence base. Demonstrating complete asset visibility, which both active and passive monitoring contribute to, is increasingly important in CAF assessments and NIS2 compliance reviews.
The silent device problem: why passive-only approaches fail at audit
This deserves specific attention because it is the gap most likely to surface in a CAF assessment and least likely to be addressed in a passive-only monitoring programme.
Passive monitoring can only observe devices that generate network traffic during the monitoring period. In a typical OT environment, a significant proportion of assets do not communicate continuously. Remote terminal units at distributed sites, backup PLCs in standby mode, engineering workstations used only during maintenance windows, and test systems left connected after commissioning all fall into this category.
These assets carry real risk. A threat actor conducting low-and-slow reconnaissance in an OT environment will specifically target assets that do not generate regular traffic, because they are less likely to be observed and their behaviour is less likely to deviate from an established baseline. Establishing persistence on a dormant backup system and using it to stage lateral movement is precisely the kind of patient, process-aware attacker methodology identified in the Dragos 2026 OT Cybersecurity Year in Review.
A passive-only monitoring programme cannot detect this activity. More critically for compliance purposes, it cannot demonstrate that it has not occurred.
Active discovery methods, including periodic targeted queries and controlled subnet enumeration, are the mechanism for maintaining a current inventory of all assets including those that do not regularly communicate. In a mature OT security programme, this inventory is the foundation of every other control, including the risk assessment, segmentation validation, and incident response planning that CAF Objectives A, B, and D respectively require.
How a managed IT/OT SOC implements both approaches safely
The decision about when and how to apply passive and active monitoring in a specific OT environment requires human judgement, OT operational knowledge, and a structured deployment methodology.
At e2e-assure, our OT Security service deploys our second-generation telemetry analyser, which is available on-premises, cloud, or hybrid, and is designed specifically for the constraints of industrial environments. Before any monitoring configuration is finalised, our analysts work with the client’s operations team to understand the network architecture, identify critical zones, confirm protocol support, and establish the boundaries within which active queries can be safely applied.
The CUMULO platform then correlates telemetry from both passive and active sources, alongside EDR, cloud, and identity data, into a unified detection surface. This means that a passive detection of anomalous OT traffic and an active discovery of an unexpected firmware version on an adjacent workstation are correlated as related events, are correlated as related events and analysed together
This is the practical advantage of a fully converged IT/OT SOC: the correlation layer that turns individual monitoring signals into coherent threat narratives. Passive and active data sources derive their value from the analysis applied to them. Without OT-specialist analysts building and validating the detection logic, monitoring data produces either noise or false confidence.
Our CUMULO platform provides the single unified detection surface across both monitoring approaches, reducing alert fatigue and cutting ingestion costs while maintaining the full visibility that CAF Objective C and NIS2 detection obligations require.
UK compliance implications: what CAF and NIS2 require from your monitoring approach
CAF Objective C, covering the detection of cyber events, requires organisations to demonstrate that they have monitoring capability across the systems supporting their essential functions. The NCSC assesses this through observable evidence.
For an organisation with OT assets forming part of its essential function delivery, this means demonstrating three things. First, that OT network traffic is actively monitored and that anomalies are detected and investigated. Second, that the monitoring coverage extends to the full asset inventory, including assets that do not communicate regularly. Third, that the monitoring capability is operated by analysts with the OT expertise to distinguish genuine threats from operational noise.
A passive-only monitoring programme typically satisfies the first requirement but creates demonstrable gaps in the second. The third requirement cannot be met by a tool alone, regardless of how sophisticated it is.
NIS2 Article 21 and its equivalent obligations under the incoming UK Cyber Security and Resilience Bill require appropriate and proportionate technical and organisational measures to detect incidents. For OT environments, this requires a monitoring approach that provides visibility across the full asset surface, including dormant and legacy devices. It also requires the incident detection to be timely, meaning that detection latency created by passive-only approaches in environments with slow-communicating assets must be accounted for.
Our NIS2 and CAF compliance support includes a monitoring coverage review as a standard component, assessing whether the current combination of passive and active approaches provides the evidence base needed to satisfy both frameworks, and identifying the specific gaps that need to be addressed before a formal assessment.
Frequently asked questions
What is passive OT monitoring? Passive OT monitoring analyses network traffic by observing communications between devices without sending any additional packets or interacting with devices directly. It uses SPAN ports or network TAPs to capture a copy of all passing traffic, which is then analysed for asset inventory, anomaly detection, and threat identification. Because it does not generate network activity, it is non-disruptive and safe for use in sensitive industrial environments.
What is active OT monitoring? Active OT monitoring queries devices directly, sending requests to retrieve information such as firmware versions, software configurations, open ports, and running services. Unlike passive monitoring, it generates network traffic and interacts with devices, which creates risk in OT environments where legacy devices may not tolerate unexpected network requests. Applied with OT-specific controls and rate limiting, it provides asset intelligence that passive monitoring cannot deliver.
What are the limitations of passive-only monitoring? Passive monitoring only detects devices and activity that generate network traffic during the monitoring period. Devices that are powered on but not actively communicating, legacy systems that only communicate during maintenance windows, and dormant backup systems are all invisible to a passive-only approach. These silent assets represent real security risk and create demonstrable gaps in asset inventory that can surface in CAF assessments and NIS2 compliance reviews.
Is active monitoring safe in OT environments? Active monitoring can be deployed safely in OT environments when applied with appropriate controls. These include using protocols the target device was designed to respond to, limiting query rates to avoid exceeding device network capacity, coordinating queries with operations teams, and restricting active interrogation to zones where it has been validated as safe. Applying IT-style active scanning tools to OT networks without these controls carries significant risk of operational disruption.
What does CAF Objective C require for OT monitoring? CAF Objective C requires organisations to demonstrate active monitoring capability across the systems supporting their essential functions, including OT assets. This requires evidence that OT network traffic is monitored, that the monitoring covers the full asset inventory including non-communicating devices, and that the monitoring capability is operated by analysts with the expertise to investigate OT-specific anomalies. A passive-only monitoring programme that cannot demonstrate coverage of silent or dormant OT assets will typically result in a CAF assessment gap.
How does e2e-assure’s monitoring approach work in practice? e2e-assure deploys a second-generation telemetry analyser across on-premises, cloud, or hybrid OT environments. Before deployment, analysts work with operations teams to map network architecture, identify critical zones, and determine the boundary conditions for safe active querying. Passive monitoring provides continuous behavioural baseline detection across active assets. Controlled active discovery provides firmware visibility, full asset inventory coverage, and vulnerability context for non-communicating assets. All telemetry is correlated in the CUMULO platform alongside IT, cloud, and identity data, and is analysed by SC-cleared OT specialists operating 24/7.
Assess your OT monitoring coverage
If your current monitoring programme relies on passive detection alone, or if you cannot demonstrate asset inventory coverage for silent and dormant OT devices, you have gaps that are increasingly visible in CAF assessments and NIS2 compliance reviews. Speak with an e2e-assure OT security specialist to understand where your coverage stands and what it takes to close the gaps.