OT Asset Inventory: CISA's 2025 Guide to Modern Defensible Architecture

The cybersecurity landscape for operational technology (OT) environments has reached a critical inflection point. CISA's newly released "Foundations for OT Cybersecurity: Asset Inventory Guidance for Owners and Operators" represents more than technical recommendations—it's a roadmap for building the modern defensible architecture that critical infrastructure organizations desperately need.

Released in collaboration with the NSA, FBI, EPA, and international partners, this comprehensive guidance addresses a fundamental challenge: organizations cannot protect what they cannot see. For CISOs, Security Architects, and IT leaders managing manufacturing, industrial, and healthcare environments with thousands of devices, the path forward requires both methodical asset discovery and advanced microsegmentation strategies.

CISA: Foundations for OT Cybersecurity: Asset Inventory Guidance for Owners and Operators

Understanding CISA's New OT Asset Inventory Guidance

What Makes OT Asset Inventory Critical for Critical Infrastructure

CISA's guidance identifies a stark reality: creating an asset inventory is necessary for building a modern defensible architecture and represents one of CISA's Cybersecurity Performance Goals (CPGs). The authoring agencies emphasize that without an inventory, organizations simply do not know what they have and what should be secured and protected.

The challenge extends beyond simple device counting. OT environments contain diverse assets—legacy systems, specialized devices, sensors, and instrumentation—that use various proprietary protocols for communication. These systems often lack the ability to accommodate security agents, making traditional endpoint protection or legacy microsegmentation approaches ineffective.

The guidance specifically addresses how cyber actors exploit vulnerabilities in flawed or outdated software/firmware, weak authentication mechanisms, insufficient network segmentation, insecure OT protocols, and insecure remote access points. Each of these attack vectors becomes more dangerous in environments where asset visibility remains incomplete.

The Role of OT Taxonomy in Asset Management

CISA introduces the concept of OT taxonomy as a categorization system that organizes and prioritizes OT assets to facilitate risk identification, vulnerability management, and incident response. This taxonomy provides several critical benefits:

Enhanced Organization and Management: A well-structured OT taxonomy enables effective categorization and organization of various assets, processes, and data, making it easier to manage and retrieve information.

Improved Communication: Standardizing terminology and classifications ensures teams speak the same language, reducing misunderstandings and improving collaboration between different departments.

Better Decision Making: Clear understanding of relationships and dependencies between different assets and processes enables more informed decisions about resource allocation, maintenance planning, and upgrade implementation.

Cost Savings: By optimizing asset management and reducing inefficiencies, an OT taxonomy drives significant cost savings, minimizes downtime, and improves overall operational efficiency.

Data Analytics and Insights: A structured taxonomy enables better data analytics by providing a clear framework for organizing and analyzing data, leading to valuable insights that drive continuous improvement.

From Asset Discovery to Modern Defensible Architecture

The Five-Step Process for OT Asset Inventory Development

CISA outlines a systematic five-step approach that transforms asset discovery into actionable security architecture:

Step 1: Define Scope and Objectives Organizations must identify governance over asset management, assign roles and responsibilities for data collection and validation, and define clear boundaries for the program. This includes determining what constitutes an "asset" for inventory purposes.

Step 2: Identify Assets and Collect Attributes This involves conducting physical inspections and logical surveys to gather detailed digital and network-based information about system components. CISA prioritizes collecting 14 high-priority attributes including asset criticality, communication protocols, IP addresses, and physical locations.

Step 3: Create a Taxonomy to Categorize Assets The guidance emphasizes two classification approaches: criticality-based (where assets are classified by their importance to operations, safety, and mission) and function-based (where assets are grouped by their roles within the OT environment).

Step 4: Manage and Collect Data Organizations must establish centralized databases or asset management systems while implementing security controls to ensure data protection and resilience against cyber threats.

Step 5: Implement Life Cycle Management This final step involves defining asset life cycle stages and developing policies for managing assets throughout their operational life, including maintenance schedules, replacement plans, and backup strategies.

Bridging Asset Inventory and Network Segmentation

The connection between comprehensive asset inventory and effective network segmentation becomes clear through CISA's emphasis on the ISA/IEC 62443 series of standards. These standards organize assets into Zones and Conduits:

Zones represent groupings of logical or physical assets that share common security requirements based on factors such as criticality and consequence. For example, all machines controlling a production line might exist in one Zone because they require the same protection level.

Conduits consist of cyber assets dedicated exclusively to communications, sharing the same cybersecurity requirements. They ensure only authorized data or traffic passes between Zones, acting as controlled communication pathways.

This zone-based approach provides the foundation for identity-based microsegmentation, where security policies follow device identity rather than network location.

Implementing OT Microsegmentation with Comprehensive Asset Intelligence

How Identity-Based Microsegmentation Enhances Asset Protection

Traditional network segmentation approaches—VLANs, firewall rules, and ACLs—fail in dynamic OT environments where devices frequently change location, IP addresses, and network connections. Identity-based microsegmentation represents a fundamental shift from infrastructure-centric to identity-driven security.

Modern microsegmentation platforms discover every user, workload, and device across the network, correlating metadata and adding context through comprehensive identity graphs. This approach enables security policies that persist regardless of network location, eliminating the brittleness of IP-based controls.

Aaron Weismann, CISO at Main Line Health, explains the network security transformation: "We deployed [the identity-based microsegmentation platform] at two of our sites in less than an hour, and by the next day we were confidently implementing policies. This made [the solution] an indispensable part of our network security strategy across our manufacturing sites."

The benefits extend beyond deployment speed. Identity-based approaches enable:

• Continuous Asset Discovery: Platforms automatically discover both managed and unmanaged devices, eliminating blind spots in asset inventories
• Dynamic Policy Enforcement: Security policies adapt automatically based on device identity, risk scores, and contextual information
• Simplified Management: No additional hardware, agents, firewalls, or VLANs required for policy implementation
• Rapid Incident Response: Real-time containment capabilities that can isolate compromised devices within minutes

IEC 62443 Zones and Conduits: From Theory to Practice

Implementing IEC 62443 compliance through identity-based microsegmentation transforms theoretical zone concepts into practical security controls. Organizations can create granular security zones at the workload level, surpassing traditional network segmentation capabilities.

The approach enforces least-privilege access policies between zones based on identity and context, restricting lateral movement while providing visibility into communication patterns to identify and isolate critical assets per IEC 62443 requirements.

Manufacturing organizations particularly benefit from this approach. Edmond Mack, former CISO at Haleon and current SVP & Chief Information Security Officer at Cencora, said, "Now you're able to lean in and say, Hey, we can actually input policies in place that truly protect and reduce risk. The visibility that [identity-based microsegmentation platform] brings to the table allows for that."

Sector-Specific OT Asset Inventory Best Practices

Oil and Gas OT Asset Classification Strategies

CISA's sector-specific guidance for oil and gas organizations demonstrates practical taxonomy implementation. The framework categorizes assets into three criticality levels:

High-Criticality Assets require the most stringent security measures, including network segmentation and role-based access control. These include primary production systems (drilling rigs, wellheads, subsea equipment), safety systems (emergency shutdown systems, fire and gas detection, blowout preventers), control systems (DCS, PLCs for critical processes), and power systems (backup generators, UPS).

Medium-Criticality Assets require robust monitoring and regular updates, encompassing processing equipment (separators, compressors, heat exchangers), monitoring systems (condition monitoring sensors, data historians), communications systems (SCADA systems, RTUs), and networking equipment (switches and routers for process control networks).

Low-Criticality Assets maintain basic security measures while remaining included for completeness, covering auxiliary systems (HVAC, lighting), non-critical monitoring (environmental tracking, non-essential data logging), and peripheral devices (operator workstations, non-critical HMIs).

Electricity and Water/Wastewater Taxonomy Implementation

The electricity sector taxonomy follows similar criticality-based classifications, emphasizing protection of primary equipment (power transformers, circuit breakers, switchgear), protection systems (protection relays, fault detection mechanisms, voltage regulators), control systems (DCS, PLCs, SCADA), and power supply systems (backup generators, UPS).

Water and wastewater organizations focus on protecting primary treatment systems (pumps, screens, clarifiers), secondary treatment systems (aeration systems, biological treatment reactors), safety and environmental systems (emergency shutdown systems, chemical dosing, spill containment), control systems (SCADA, DCS), and power systems (backup generators, UPS).

The commonality across sectors highlights how identity-based microsegmentation provides consistent security frameworks regardless of specific industrial processes, enabling standardized policy templates that adapt to sector-specific requirements.

Advanced OT Asset Management with Modern Microsegmentation Platforms

Automated Asset Discovery and Classification

Modern identity-based microsegmentation platforms eliminate manual asset discovery limitations through automated discovery engines that achieve 99% coverage within hours of deployment. These platforms integrate with existing technology stacks via APIs, aggregating user, workload, and device data from multiple sources to create comprehensive device intelligence.

The automated classification capability addresses one of CISA's key challenges: maintaining accurate, up-to-date asset inventories in complex OT environments. Platforms continuously correlate device metadata, communication patterns, and risk indicators to automatically assign devices to appropriate policy groups.

Main Line Health's experience demonstrates this capability's impact. Aaron Weismann reports: "Deploying [identity-based microsegmentation] has really minimized our attack surface, at least gaps in our attack surface. We've been able to use both data sets to really get a pretty granular view of what devices we have where and how they're operating, which is absolutely fantastic for general security hygiene."

Real-Time Policy Enforcement Based on Asset Context

The integration of comprehensive asset inventory data with dynamic policy enforcement creates unprecedented security capabilities. Policies automatically adjust based on device identity, risk scores, communication patterns, and contextual factors without requiring manual intervention.

This approach addresses CISA's emphasis on continuous monitoring and real-time response capabilities. Security teams can implement policies that automatically block devices exhibiting suspicious behavior, restrict communications based on asset criticality, and maintain least-privilege access regardless of network location.

The "no-fear" policy creation capability enables rapid security improvements without operational disruption. Organizations can simulate policy changes before implementation, ensuring business continuity while strengthening security posture.

Measuring Success: KPIs for OT Asset Inventory and Microsegmentation Programs

Asset Coverage and Classification Accuracy Metrics

Successful OT asset inventory programs require measurable outcomes that demonstrate both completeness and accuracy. Key performance indicators include:

• Discovery Coverage Rate: Percentage of assets automatically discovered versus manually identified, with targets of 95% or higher within 24 hours of deployment.
• Classification Accuracy: Percentage of devices correctly classified by automated systems, validated through manual verification processes.
• Time to Inventory Completeness: Duration from initial deployment to comprehensive asset visibility across all network segments.
• Inventory Freshness: Frequency of asset data updates and time lag between device changes and inventory reflection.

Security Posture Improvement Through Inventory-Driven Policies

The true value of comprehensive asset inventory emerges through security improvements enabled by accurate device intelligence:

• Policy Coverage: Percentage of discovered assets covered by active security policies, with targets approaching 100% for critical assets.
• Mean Time to Containment: Average time to isolate compromised devices, with identity-based systems achieving containment in minutes rather than hours.
• Lateral Movement Prevention: Reduction in potential attack paths between network segments, measured through attack surface analysis.
• Compliance Readiness: Time required to generate compliance reports and demonstrate adherence to regulatory requirements.

Main Line Health achieved remarkable results, reporting 99% device discovery and classification within 4 hours without network disruption, 76% total cost of ownership reduction, and automated compliance with NIST, HIPAA, and HHS 405(d) requirements.

The Path Forward: Implementing CISA's Modern Defensible Architecture

CISA's 2025 asset inventory guide provides the foundation, but the implementation approach determines success. Organizations that combine comprehensive asset discovery with identity-based microsegmentation create the modern defensible architectures that CISA envisions—systems capable of adapting to evolving threats while maintaining operational efficiency.

The convergence of accurate asset inventory and dynamic security policies represents the future of OT cybersecurity outlined in CISA's guide. Rather than relying on static network constructs that break under operational pressure, modern defensible architecture leverages identity-based approaches that provide resilient security scaling with organizational growth and technological evolution.

As cyber threats targeting critical infrastructure intensify, the window for proactive security transformation narrows. Organizations that implement CISA's asset inventory guidance alongside modern microsegmentation platforms position themselves to lead rather than react to emerging security challenges.

Frequently Asked Questions: OT Asset Inventory Implementation

Q: How long does it take to complete a comprehensive OT asset inventory?

A: Using traditional manual methods, comprehensive asset inventory can take 6-18 months. However, modern automated platforms achieve 99% discovery coverage within 4 hours of deployment, with full taxonomy development completed within 2-4 weeks.

Q: What's the difference between asset inventory and asset management?

A: Asset inventory is the foundational list of what you have and where it's located. Asset management encompasses the entire lifecycle of those assets, including maintenance, updates, and decommissioning. CISA's guidance emphasizes that accurate inventory is prerequisite to effective management.

Q: Can we implement asset inventory without disrupting production operations?

A: Yes, when using agentless discovery methods. Modern platforms integrate with existing network infrastructure without requiring device modifications, agent installations, or network downtime. Aaron Weismann from Main Line Health confirms: "We deployed at two sites in less than an hour without any operational disruption."

Q: How does OT asset inventory support regulatory compliance?

A: Comprehensive asset inventories directly support multiple compliance frameworks including IEC 62443, NIST Cybersecurity Framework, HIPAA Security Rule updates, and FDA cybersecurity guidance. Automated inventory systems enable push-button compliance reporting versus weeks of manual compilation.

Q: What happens to our asset inventory as devices change locations or IP addresses?

A: Traditional IP-based inventories break when devices move. Identity-based systems track device identity regardless of network location, automatically updating inventory records as devices change positions across your infrastructure.

Q: How do we handle legacy devices that can't support modern security agents?

A: CISA specifically addresses this challenge. Agentless discovery methods identify and classify legacy devices without requiring modifications. Identity-based microsegmentation then protects these devices through network-level policy enforcement rather than endpoint agents.

Q: What's the ROI timeline for OT asset inventory investments?

A: Organizations typically see immediate benefits through improved visibility, with financial returns materializing within 3-6 months. Main Line Health reported 76% total cost reduction compared to traditional approaches, while Shaw Industries achieved full deployment across manufacturing sites within days.

Q: How often should we update our OT asset inventory?

A: CISA recommends continuous monitoring rather than periodic updates. Automated platforms provide real-time inventory updates as devices appear, disappear, or change configuration, ensuring accuracy without manual overhead.

The evidence from early adopters demonstrates clear benefits: reduced costs, improved security posture, faster incident response, and simplified compliance. The question for security leaders is not whether to implement these approaches, but how quickly they can begin transformation.

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