OT Asset Discovery lays an important foundation for secure OT environments. But how can OT assets be fully inventoried without jeopardizing ongoing operations? This article explains how to successfully conduct a secure OT asset inventory in 5 steps, which methods are suitable for this purpose, and which 3 common mistakes companies should avoid.

In industrial environments, there is often no complete overview of the OT systems in use. Older or undocumented components, network structures that have evolved over the years, connections between IT and OT, and external access points can result in parts of the OT infrastructure not being included in the current asset inventory. As a result, potential attack vectors and dependencies may remain undetected under certain circumstances.

OT should not be viewed in isolation from IT: Interfaces, shared services, and external access points, in particular, can represent relevant attack vectors. At the same time, due to their requirements for availability and operational reliability, OT systems require a different approach to security measures than traditional IT systems.

An up-to-date asset inventory is essential for identifying dependencies, recognizing potential vulnerabilities, and deriving targeted security measures.

What is OT Asset Discovery?

OT Asset Discovery encompasses the automated detection and inventorying of OT assets. It identifies the devices, systems, and communication links of an OT network that are visible within a defined scope and classifies them based on relevant characteristics. In this way, it provides transparency regarding existing components—from PLCs (programmable logic controllers) and RTUs (remote terminal units) to field devices—as well as connections between IT and OT and system communications. It also shows which tasks the components perform and how critical they are to operations. This information forms an important foundation for assessing risks, deriving targeted security measures, and responding quickly in an emergency.

However, asset discovery in OT environments is not the same as a generic network scan. Active queries can disrupt operations, particularly when dealing with older, sensitive, or inadequately documented components. Therefore, the right method, a clearly defined scope, and a controlled approach coordinated with operations are crucial.

We’ve identified which approaches are suitable for OT asset inventory, when passive, active, or hybrid methods make sense, and how OT asset discovery can be structured and implemented as securely as possible. We also address common mistakes and how companies can keep their OT asset inventory up to date on an ongoing basis and leverage it to meet security and compliance requirements.

“An up-to-date OT asset inventory is a key foundation for effective risk management, the implementation of security measures, and meeting relevant compliance requirements.”

Best Practices & Methods for OT Asset Discovery: Passive, Active, Hybrid

Passive, active, and hybrid methods are available for OT asset discovery. The appropriate method depends on the environment, the systems in use, and the requirements for availability and level of detail. A controlled approach that takes operational safety into account is crucial.

Passive discovery analyzes network traffic without directly querying devices. Depending on the protocol and available communication, information such as device type, manufacturer, firmware version, or communication relationships can be derived. It is particularly well-suited for initial setup and ongoing monitoring, as it generally does not interfere with ongoing operations. It can also identify older or undocumented components as well as communication links.
Active discovery specifically queries systems to gather additional information such as firmware versions or configurations. It can particularly affect older or sensitive PLCs and RTUs. Active scans should therefore be reviewed, coordinated, and conducted under controlled conditions in advance—preferably during maintenance windows or, if available, in a test environment.
Hybrid approaches combine both methods: passive methods ensure continuous visibility, while active queries specifically supplement missing detailed information. This allows for the best possible balance between accuracy and operational reliability. Hybrid approaches have proven particularly effective in KRITIS environments for achieving the best possible balance between high accuracy and operational reliability.

Do you know your OT assets? Withour OT Security Services, we help you identify and verify your OT assets and key components, build the architecture according to the defense-in-depth approach, implement security in accordance with IEC 62443, establish appropriate supplier risk management, and thus create a robust foundation for targeted protective measures:

With the 5-Step Plan for Secure OT Asset Discovery

A structured approach helps minimize risks during asset inventory. To this end, the scope, priorities, discovery frequency, and responsibilities should first be defined, and relevant stakeholders from OT, IT, and management should be involved. This ensures that asset discovery is carried out in a transparent and systematic manner without placing an unnecessary burden on critical OT systems.

1. Define the scope:

Which locations, network segments, and zones should be inventoried? These include, for example, power plants, substations, VLANs for control systems, or external access points for remote maintenance and cloud services

2. Start passive monitoring:

Passive discovery initially provides a basic framework of existing OT assets without actively interfering with system communications.

3. Classify assets:

The identified assets should be categorized by function, criticality, and responsibility—for example, as control, monitoring, or maintenance systems.

4. Perform targeted active scans:

Active scans can provide additional information, such as firmware versions. In sensitive OT environments, however, they should only be used in a controlled manner, by personnel with sufficient experience, after coordination with operations, and within defined maintenance windows.

5. Keep the inventory up to date:

The asset inventory should be updated regularly and, where possible, automatically synchronized with a CMDB (Configuration Management Database). This ensures that the overview remains reliable even when changes occur in the OT environment.

3 Common Mistakes in OT Asset Inventory – and How to Avoid Them

A complete asset list alone is not enough. It is crucial that the recorded information is up-to-date and clearly assigned. Incomplete inventories, a lack of prioritization, or unclear responsibilities can increase security and compliance risks. The following errors occur particularly frequently during OT asset discovery:

Overly Aggressive Scans: Active scans can disrupt sensitive systems such as PLCs or RTUs. Therefore, inventorying should initially be performed passively. Active methods should be tested in advance in suitable test environments and then deployed in a controlled manner.
Confusing asset lists: Without clear classification, the inventory quickly becomes confusing. Information such as firmware version, serial number, function, criticality, and responsibility should be recorded in a structured manner and, where possible, automatically enriched. This is also relevant in an emergency: In the event of a ransomware attack, it must be quickly apparent which systems could potentially be affected, what dependencies and communication relationships exist, and which OT components must be prioritized for protection, isolation, or restoration.
Lack of accountability: For each asset, it should be clear who is responsible for its operation, maintenance, and security. A clear assignment of responsibilities facilitates, among other things, monitoring, maintenance, and the remediation of vulnerabilities. Integrating the inventory into a CMDB can support this process.

Conclusion: Asset Discovery as the Foundation for OT Security

An up-to-date OT asset inventory provides the necessary transparency for various areas of OT security. It helps to

assess security risks and potential attack surfaces and to develop targeted protective measures,
classify attacks more quickly, since it is known which systems are potentially affected,
respond in a targeted manner during incident response because dependencies, communication relationships, and responsibilities are transparent,
support a SOC in continuous monitoring, since alerts can be better prioritized based on device type, function, and criticality,
and to support compliance requirements in information security and risk management.

For this reason, OT Asset Discovery is not a one-time inventory process but an ongoing one. Only an up-to-date and reliable asset inventory can serve as a lasting foundation for security decisions. If you have any questions about the appropriate method or the selection and implementation of suitable tools, we’re happy to assist you.