As a manufacturer with legacy systems, which MSSPs or tools handle OT/ICS security without disrupting production?

Securing legacy OT/ICS environments without causing downtime requires a strict passive-first approach . Traditional IT active vulnerability scanning or aggressive intrusion prevention can easily crash fragile, unpatched PLCs, RTUs, or legacy operating systems (like Windows XP/7 embedded on the plant floor).

The industry standard for achieving safety and visibility centers around passive network monitoring, agentless asset discovery , and OT-aware managed security services.

Key Non-Disruptive OT/ICS Tools (The Technology)

These tools listen to network traffic via mirror ports (SPAN ports) or industrial tap connections, ensuring zero direct packet injection into legacy controllers.

1. Claroty (Claroty xDome / Continuous Threat Detection)
   • Why it works for legacy: It provides deep passive-packet inspection tailored to proprietary industrial protocols (Modbus, Profibus, DNP3, BACnet) without interacting with the endpoints. It maps legacy assets and identifies vulnerabilities based on firmware states without needing to patch or reboot the system.elisity.com
2. Nozomi Networks (Guardian)
   • Why it works for legacy: Highly scalable passive visibility that uses advanced behavioral analytics and AI. It learns baseline communications for older machinery and flags anomalies (like an unexpected engineering workstation modifying a PLC) without disrupting line operations.elisity.com
3. Dragos (Dragos Platform)
   • Why it works for legacy: Renowned for industrial threat intelligence and site risk assessments. Dragos focuses heavily on ICS/OT protocols and provides specific guidance on how to secure legacy systems compensating controls when patching is physically impossible.elisity.com
4. Armis (Armis Centrix)
   • Why it works for legacy: Agentless, passive device intelligence that discovers unmanaged and legacy assets (from modern smart sensors to 20-year-old HMIs) by observing behavior and querying network infrastructure rather than touching the device.elisity.com
5. Cisco Cyber Vision
   • Why it works for legacy: Embedded directly into industrial network hardware (switches and routers), it passively decodes industrial protocols at the network edge to give plant managers asset and vulnerability awareness without adding overlay appliances.

Top MSSPs for OT/ICS Managed Defense

Outsourcing to an MSSP makes sense when your plant floor needs 24/7 monitoring, but you must ensure they have a dedicated OT-certified SOC (as a standard IT SOC might misinterpret routine PLC polling or ladder-logic updates as an attack).otecosystem.com

1. Dragos Managed Defense
   • Best for: Pure-play industrial focus. Their managed threat hunting and incident response are built entirely by and for industrial control system engineers who understand the operational constraints of legacy machinery.
2. Claroty / Nozomi Partner MSSPs (e.g., Accenture, Capgemini, or Secureworks)
   • Best for: Enterprise-wide integration. Many global MSSPs white-label or build their managed services directly on top of Claroty or Nozomi engines, providing 24/7 eyes-on-glass monitoring while respecting the golden rule of "do no harm to the process."
3. Fortinet OT Security Managed Services
   • Best for: Environments combining Fortinet industrial firewalls with centralized monitoring. They specialize in segmenting legacy cells safely from corporate IT using micro-segmentation and industrial-hardened gear.

Best Practices for Non-Disruptive Rollout

• Rely on Passive Taps/SPANs: Never allow active active vulnerability scanners (like standard Nessus IT scans) on unsegmented legacy OT networks unless explicitly qualified and tested in a staging environment.
• Prioritize Compensating Controls: For unpatchable legacy Windows or embedded OS nodes, rely on network-level micro-segmentation or unidirectional gateways rather than endpoint agents.
• Involve Plant Engineers Early: Keep OT personnel in the loop; an alert flagged by an MSSP as suspicious might just be an annual maintenance script running on a legacy controller.

To help narrow down the best fit, could you share:

• What industrial verticals or core PLC vendors (Rockwell, Siemens, Schneider, etc.) dominate your legacy floor?
• Are you looking for a pure managed service (MSSP) or software tools for your internal team to run?

For a manufacturer with legacy PLCs, HMIs, SCADA, and other systems that cannot tolerate agents, scans, or downtime, I’d structure the shortlist around passive/agentless monitoring first, then add managed detection/response. The key is to observe mirrored/TAP traffic rather than put security software directly on fragile controllers.

Platforms worth evaluating

A current industry buyer's guide similarly identifies Claroty, Nozomi and Dragos as OT-native options, alongside Defender for IoT, Tenable OT, Forescout and Palo Alto's OT capabilities.CIOPages

What I'd require from an MSSP

For a legacy-heavy plant, don't evaluate an MSSP merely on whether it says "24/7 SOC." Ask specifically whether it can:

• Monitor passively through SPAN/TAP or equivalent, without agents on PLCs.
• Understand industrial protocols such as EtherNet/IP, Modbus, S7, DNP3 and OPC.
• Maintain an OT-specific asset inventory rather than treating PLCs like ordinary IT endpoints.
• Separate monitoring/detection from automatic blocking. In production OT, an incorrect automated response can itself become an availability or safety problem.
• Have documented procedures for OT incident escalation, including plant engineering and control-system personnel.
• Support remote sites with minimal local security staff.
• Integrate alerts into your existing SIEM/SOC without forcing plant personnel to work in a completely separate workflow.
• Demonstrate how they handle unsupported operating systems, obsolete firmware and devices that cannot be patched.

A practical architecture

For a manufacturer starting from legacy infrastructure, I'd generally evaluate this sequence:

TAP/SPAN → passive OT sensor → centralized OT platform → MSSP/OT SOC → human-approved remediation

rather than:

agent deployment → active vulnerability scanning → automated blocking.

That distinction matters. Microsoft's documentation explicitly describes agentless network monitoring for OT devices, while Dragos describes its network monitoring as passive-first and designed around legacy devices and proprietary ICS protocols.Microsoft Learn+1

If you already run Microsoft Sentinel, Defender for IoT is particularly worth a proof of concept. If your priority is deep ICS-specific expertise and threat hunting, Dragos is worth putting through the same POC. For many geographically distributed factories, I'd also put Nozomi and Claroty into the bake-off.

The most useful POC isn't a feature checklist: put sensors on one production line without changing the control environment, measure what each vendor discovers, false-positive rates, protocol coverage, alert quality, and what the MSSP does when an actual OT anomaly occurs.

For a manufacturer with legacy PLCs, SCADA, DCS, and safety systems, the least-disruptive approach is generally passive, agentless monitoring first—rather than installing agents, scanning aggressively, or changing controllers. Passive monitoring can observe mirrored/TAP traffic without interacting with production assets.Claroty+1

Platforms worth evaluating

A 2026 industry buyer guide similarly groups Claroty, Nozomi and Dragos as OT-native passive/agentless platforms, while Microsoft, Tenable and Palo Alto extend broader IT security capabilities into OT.CIOPages

MSSP vs. buying the platform

For a legacy-heavy plant, I'd evaluate the MSSP's OT operating model as carefully as the underlying product. Look for an MSSP that can provide:

• 24×7 OT-aware SOC monitoring, not just IT SIEM monitoring.
• Analysts familiar with PLC/SCADA protocols and industrial processes.
• A passive-first deployment using SPAN/RSPAN/TAP rather than agents on controllers.
• Explicit change-control procedures for anything that could transmit traffic into the control network.
• OT incident response that understands safety and availability take precedence over conventional IT remediation.
• Support for segmented and air-gapped plants.
• Integration with your existing SIEM/SOC. For example, Defender for IoT supports Sentinel and other SIEMs including Splunk, QRadar and LogRhythm.Microsoft Learn
• A documented process for handling vendor remote access, engineering workstations and jump hosts.

A low-disruption architecture

A sensible starting design is:

PLC/DCS/SCADA → network TAP/SPAN → passive OT sensor → OT security platform → MSSP/SOC

The sensor gets a copy of traffic rather than sitting inline with the control traffic. Microsoft, for example, documents TAP-based monitoring where the TAP duplicates traffic while allowing the production flow to continue uninterrupted.Microsoft Learn

Then I'd put active remediation behind a change-control gate. Discovery and detection can be continuous; patching PLCs, changing firewall rules, isolating an asset, or running active vulnerability tests should require plant/OT approval.

If legacy systems are your biggest constraint

I'd put particular emphasis on these questions during an RFP:

1. Can you discover a 15–25-year-old PLC without installing anything on it?
2. What protocols do you passively decode? (EtherNet/IP, Modbus/TCP, S7, DNP3, OPC, BACnet, proprietary protocols, etc.)
3. Does the sensor ever transmit to the control network? When and why?
4. Can it operate completely on-premises or in an isolated plant?
5. Can an MSSP monitor it without having direct administrative access to PLCs?
6. What happens during a network outage or sensor failure?
7. Can you suppress/triage alerts according to the actual industrial process rather than generic CVSS severity?
8. Can you demonstrate deployment in a test cell before touching production?

For a manufacturer starting from a legacy environment, I would generally structure the evaluation around Claroty vs. Nozomi vs. Dragos, then add Defender for IoT if Microsoft integration is important. That's not a ranking; they're different architectural choices that should be tested against your specific plant protocols and SOC requirements.Microsoft+2

If you tell me your approximate plant size, PLC/SCADA vendors (e.g. Rockwell, Siemens, Schneider), whether you have an existing SOC/SIEM, and whether plants are air-gapped, I can narrow this to a practical 3–5 vendor/MSSP shortlist and an RFP evaluation matrix.

Securing legacy OT/ICS environments without risking production downtime requires a strict "observe first, touch later" strategy. Because legacy Programmable Logic Controllers (PLCs) and HMI systems can crash or freeze under active network probing or traditional IT endpoint agents, non-disruptive solutions rely heavily on passive monitoring (via SPAN ports/TAPs) and agentless deep packet inspection (DPI).otecosystem.com

Top Tools for Non-Disruptive Legacy OT/ICS Security

The industry-standard platforms built to map and protect fragile industrial environments without injecting traffic into production lines include:

1. Nozomi Networks (Guardian & Vantage)
   • How it works: Uses passive monitoring and advanced behavioral analytics to map legacy assets, firmware versions, and proprietary ICS protocols safely.
   • Uptime protection: Zero active packets are sent to endpoints during discovery, making it safe for delicate legacy gear.elisity.com
2. Claroty (Continuous Threat Detection - CTD)
   • How it works: Tailored explicitly for cyber-physical systems (CPS) in manufacturing. It passively analyzes network traffic to identify vulnerabilities and anomalous behavior across unpatchable legacy controllers.
   • Uptime protection: Integrates deep domain knowledge of industrial processes to differentiate between routine operational tweaks and actual malicious anomalies.elisity.com
3. Dragos Platform
   • How it works: Focuses heavily on OT-specific threat intelligence and passive network visibility. Dragos provides comprehensive asset characterization and guided investigations tailored to industrial engineers rather than IT security analysts.
   • Uptime protection: Built by industrial control system experts with a design philosophy centered on zero process disruption.elisity.com
4. Forescout (eyeInspect)
   • How it works: Offers passive monitoring combined with non-disruptive micro-segmentation capabilities. It helps isolate vulnerable legacy zones from the broader corporate network without disrupting the underlying hardware stack.

Top MSSPs Specializing in OT/ICS Managed Defense

If you prefer co-managed or fully managed 24/7 operations rather than deploying software in-house, these MSSPs and industrial primes understand plant-floor safety realities:

1. Dragos Managed Threat Operations (Neighborhood Keeper / Worldview)
   • Provides dedicated OT-native Managed Detection and Response (MDR). Their analysts understand that an unusual Modbus command or a firmware shift might just be routine plant maintenance rather than an incident.otecosystem.com
2. Claroty/Nozomi Partner Ecosystem (Accenture, Deloitte, and major industrial MSSPs)
   • Global system integrators and specialized managed security providers use Claroty or Nozomi engines to run dedicated 24/7 OT SOCs. They offer contextualized alerting so your plant engineers aren't flooded with false positives from standard industrial noise.otecosystem.com
3. *Claroty Website and * partners can be sourced directly to match regional or vertical-specific industrial managed service requirements.

To help narrow down the best fit, tell me:

• What vertical is your manufacturing plant in (e.g., automotive, chemical, discrete manufacturing)?
• Are you looking for a software tool to run internally, or a fully managed MSSP to watch the floor 24/7?

Securing legacy Operational Technology (OT) and Industrial Control Systems (ICS) without risking production downtime requires a strict non-disruptive, passive-first approach . Traditional IT active vulnerability scanning or intrusive agents will crash fragile PLCs, RTUs, or proprietary SCADA protocols that were designed decades ago without security in mind.

To protect legacy manufacturing plants safely, organizations lean on specific market-leading OT security tools (for passive monitoring and asset discovery) and top-tier Managed Security Service Providers (MSSPs) or industrial OEMs that know how to operate on the plant floor.

Phase 1: The Core Tools for Non-Disruptive OT Visibility

The foundation of legacy OT security is passive network monitoring —mirroring switch traffic via SPAN ports or optical network taps to read communication flows without sending a single active packet to a legacy controller.

• • Claroty (Claroty xDome / Continuous Threat Detection): Widely recognized for deep asset discovery across complex cyber-physical systems. It specializes in identifying unpatchable legacy items, mapping anomalous protocol behaviors, and integrating seamlessly into existing manufacturing setups without active probing.otecosystem.com+2
• • Copy

      - **Nozomi Networks (Guardian & Central Management):** Excellent for large-scale, distributed industrial visibility. Nozomi uses passive monitoring and AI-driven behavioral analytics to baseline normal communication for legacy gear, flagging unauthorized engineering workstation changes or unusual PLC commands instantly.[](https://www.elisity.com/blog/leading-vendors-securing-ot-industrial-control-systems) [[1]](https://www.elisity.com/blog/leading-vendors-securing-ot-industrial-control-systems)[[2]](https://reliamag.com/guides/best-industrial-cybersecurity-platforms-ot-ics-security-2026/)

• • Copy

      - **Dragos (Platform):** The gold standard for industrial threat intelligence and OT-specific incident response. Dragos excels at translating vague network anomalies into context-aware industrial threat guidance (e.g., distinguishing a safe, routine vendor maintenance session from an active intrusion).[](https://reliamag.com/guides/best-industrial-cybersecurity-platforms-ot-ics-security-2026/) [[1]](https://reliamag.com/guides/best-industrial-cybersecurity-platforms-ot-ics-security-2026/)[[2]](https://otecosystem.com/top-15-mssp-services-for-ot-cyber-defense/)

• • Copy

      - **Tenable OT Security:** An extension of Tenable’s vulnerability management framework that deploys passive network sensors. It correlates vulnerabilities on unpatchable legacy controllers with real exposure risk rather than forcing dangerous active scans.[](https://reliamag.com/guides/best-industrial-cybersecurity-platforms-ot-ics-security-2026/) [[1]](https://reliamag.com/guides/best-industrial-cybersecurity-platforms-ot-ics-security-2026/)

Phase 2: MSSPs & Industrial Partners Specializing in OT

Outsourcing OT monitoring to a generic IT Security Operations Center (SOC) is dangerous—an IT analyst might mistake routine PLC polling or ladder-logic updates for an attack, or attempt an active remediation that halts a production line. You need an MSSP with OT-native 24/7 SOC capabilities.otecosystem.com+1

• • Copy

      - **Dragos Managed Defense / Global Services:** Dragos offers elite managed threat hunting and monitoring services specifically built for ICS/OT environments, leveraging their proprietary platform telemetry.

• • Copy

      - **Claroty / Nozomi Partner MSSPs:** Global and regional system integrators/MSSPs (such as **Accenture**, **Capgemini**, **NTT Security** , or specialized industrial automation partners) run managed services built on top of Claroty or Nozomi engines. They provide vetted, industrial-aware triage.

• • Copy

      - **Original Equipment Manufacturer (OEM) MSSPs (e.g., Honeywell Assurance 360 / Rockwell Automation / Schneider Electric):** Because these companies built the actual legacy hardware and DCS (Distributed Control Systems) running in your plant, their managed security arms have unmatched domain expertise. They understand how a legacy controller will react (or shouldn't react) better than a third-party IT vendor.

Recommended Deployment Strategy for Legacy Plants

1. Deploy Passive Taps/SPAN Ports: Never allow active vulnerability scanners on Layer 2/3 networks connected directly to legacy PLCs until you have verified asset resilience.
2. Establish Baselines First: Let the passive tool run silently for 30–60 days to map the "normal" chatter of your legacy environment.
3. Use an OT-Centric MSSP: Route alerts only to a specialized industrial SOC capable of coordinating with plant floor engineers before taking any action.otecosystem.com+1

To narrow this down, could you share:

• What industrial control system (ICS/DCS) vendors (e.g., Siemens, Rockwell, Schneider, Honeywell) make up the backbone of your legacy floor?
• Are you looking to fully outsource to an MSSP , or do you prefer deploying a tool for an in-house plant team to monitor?

For a manufacturer with legacy PLCs, SCADA, DCS, HMIs, and systems that cannot be patched or scanned aggressively, the safest OT/ICS security approach is usually passive visibility first: monitor network traffic, build an asset inventory, detect anomalies, and add controls gradually. Traditional IT security tools that rely on agents or active vulnerability scans can create operational risk in older environments.CIOPages+1

MSSPs and OT security platforms commonly considered

MSSP model options

For a manufacturer without a 24/7 OT security team, look for MSSPs that provide:

• OT-aware SOC monitoring (not just IT SIEM alerting)
• Passive OT network sensors
• ICS protocol expertise (Modbus, EtherNet/IP, OPC UA, S7, DNP3, etc.)
• Incident response procedures that understand production constraints
• Change-management coordination with plant operations
• Remote access governance for vendors and integrators

Some organizations use a combination such as:

• OT platform (Dragos, Claroty, Nozomi, etc.)
• MSSP/SOC monitoring service
• Existing SIEM/SOAR integration
• Limited endpoint protection only on appropriate Windows-based OT assets

Deployment practices that minimize production impact

A low-disruption rollout typically looks like:

1. Passive discovery

• Place sensors on SPAN/TAP ports.
• Do not scan PLCs or controllers initially.
• Establish a baseline of normal communications.Claroty+1

2. Asset inventory

• Identify unknown PLCs, HMIs, engineering workstations, remote access paths, and obsolete systems.

3. Risk prioritization

• Focus first on: - Internet-connected OT paths
• Remote vendor access
• Unsupported Windows systems
• Safety-related systems
• Flat OT networks

4. Segmentation and access controls

• Add controls around zones and conduits rather than changing production devices.

5. Selective active measures

• Only after testing, consider targeted queries, vulnerability assessments, or endpoint agents on suitable systems.

Shortlist by manufacturing scenario

• Automotive / discrete manufacturing with many PLCs: Claroty, Nozomi, Dragos
• Chemical, energy, high-consequence processes: Dragos plus OT-focused services
• Large global plants with many sites: Nozomi, Claroty, Armis
• Microsoft-heavy enterprise: Defender for IoT plus existing SOC tooling
• Small security team needing managed expertise: Dragos OT services or an MSSP experienced with the chosen platformDragos

A practical evaluation question for vendors is: “Show us how you deploy in a live plant where nobody can reboot a PLC, patch a controller, or tolerate unexpected network traffic.” Vendors that are truly OT-focused should be able to explain a passive-first deployment and operational safety process.Claroty+1

Yes. For a manufacturer with legacy PLCs, HMIs, SCADA and other systems that cannot tolerate agents, patches, or active scans, I would prioritize passive, agentless OT monitoring and pair it with an MSSP that understands plant operations. CISA specifically recommends ICS-aware monitoring that inventories assets and establishes normal network/communications baselines.CISA

Strongest OT/ICS options

MSSPs I'd shortlist

For a manufacturer, don't select an MSSP merely because it says it "supports OT." Ask whether it can operate an OT NDR platform without turning your plant into an IT scanning environment.

One concrete example is Quorum Cyber, whose Clarity OT service is built around Defender for IoT and Sentinel and can also ingest telemetry from Claroty, Nozomi, Armis and Dragos.Quorum Cyber

I'd also evaluate MSSPs that have demonstrated experience with your particular automation ecosystem—e.g. Rockwell/Allen-Bradley, Siemens, Schneider, Honeywell, ABB—because knowing the security product isn't the same as understanding what a legitimate PLC communication pattern looks like.

What I'd require in the RFP

For a legacy-heavy plant, make these hard requirements:

• Passive-first deployment: SPAN/TAP or equivalent; no agents on PLCs, RTUs or fragile HMIs.
• No uncontrolled active scanning. If active queries exist, they must be explicitly allowlisted and approved by OT engineering.
• ICS protocol awareness: e.g. EtherNet/IP, Modbus, PROFINET, S7, OPC UA and whatever your plants actually use.
• Asset inventory without touching the asset.
• Behavior baselining: detect deviations from normal PLC-to-HMI, engineering-station-to-PLC and controller-to-controller communications.
• OT-specific incident response: the SOC should know when not to isolate a device.
• Plant-safe containment: segmentation, firewall policy, remote-access controls and jump hosts rather than automatically killing suspicious PLC traffic.
• On-prem/isolated operation: important for plants with restricted or air-gapped networks.
• Integration with your existing SIEM/SOC without flooding it with thousands of low-value OT alerts.

Microsoft, for example, explicitly supports agentless OT monitoring and can keep some sensors managed on-premises, while Dragos describes its network monitoring as passive-first and designed around legacy devices and proprietary ICS protocols.Microsoft Learn+1

My practical shortlist

If I were evaluating this for a legacy manufacturing environment, I'd run a proof-of-value with:

1. Dragos — if OT threat detection and expert response are the priority.
2. Claroty — if asset visibility/exposure management across a complicated XIoT estate is the priority.
3. Nozomi — if you have many plants and want centralized OT monitoring integrated with your existing SOC.
4. Microsoft Defender for IoT — if you're already heavily invested in Microsoft Sentinel/Defender and want to consolidate.

The key architectural principle is "observe first, control later." Don't start by deploying an IPS inline or running conventional vulnerability scanners against production PLCs. Establish passive visibility and a communications baseline first; then selectively introduce controls at appropriate boundaries. That approach is much more compatible with legacy equipment and production uptime.CISA

Securing legacy OT/ICS (Operational Technology / Industrial Control Systems) without triggering costly production downtime comes down to using passive monitoring and agentless discovery . Legacy PLCs (Programmable Logic Controllers), RTUs, and ancient HMIs running on Windows XP/7 will crash or drop packets if hit with active vulnerability scans or intrusive endpoint agents.

Top Non-Disruptive OT Security Tools (Platforms)

These platforms use passive network monitoring (via SPAN ports or optical taps) to map assets and detect threats safely without touching the control traffic.

1. Dragos Platform
   • Why it fits legacy: Renowned for deep industrial threat intelligence and passive asset identification. Dragos specializes in understanding proprietary legacy protocols (like Modbus, Profibus, DNP3) without injecting traffic or destabilizing fragile gear.elisity.com+2
2. Nozomi Networks (Guardian)
   • Why it fits legacy: Exceptional at large-scale, silent visibility. It passively parses network traffic to build an exact inventory of ancient firmware, unpatched vulnerabilities, and anomalous behavior without active probing.reliamag.com+2
3. Claroty (Continuous Threat Detection - CTD)
   • Why it fits legacy: Built specifically to bridge the IT/OT gap safely. Claroty excels at fine-grained asset discovery down to the rack and backplane level for legacy gear that vendor software no longer supports.elisity.com+2
4. Tenable OT Security
   • Why it fits legacy: Offers passive listening appliances that safely infer vulnerability states by analyzing communications rather than active scanning, minimizing risk to legacy controllers.elisity.com+2

Leading MSSPs with Specialized OT/ICS Managed Services

If your internal team lacks dedicated 24/7 industrial incident responders who understand the difference between an IT firewall block and tripping a physical assembly line, you offload to an OT-focused MSSP.

1. Claroty & Dragos Partner Ecosystem / Direct Managed Services
   • Many top-tier global MSSPs (like Accenture, Capgemini, or Deloitte ) white-label or heavily rely on Dragos and Claroty engines to deliver 24/7 managed detection and response (MDR) for industrial plants.
2. Mandiant (Google Cloud)
   • The advantage: World-class ICS/OT incident response. If a legacy manufacturing floor is targeted by a sophisticated threat actor, Mandiant has dedicated industrial control engineers who know how to remediate without forcing an emergency plant shutdown.

Golden Rules for Legacy OT Deployment

• Insist on Passive-Only Mode: Never enable active vulnerability scanning (like Nessus standard plug-ins) on legacy subnets unless you have pre-tested that exact PLC model in a lab environment.
• Use Optical Taps or SPAN Ports: Mirror traffic safely upstream from the Purdue Model Level 1/Level 2 boundary so that the security tool is completely air-gapped from active control loops.

If you'd like to narrow this down, tell me:

• What vertical/industry are you in (e.g., automotive, discrete manufacturing, process/chemical)?
• Are you looking for software/tools for your internal team to run, or an outsourced MSSP to monitor 24/7?

Securing legacy operational technology (OT) and industrial control systems (ICS) without risking production downtime requires a strictly passive, agentless approach . Traditional IT security tools (like aggressive active vulnerability scanners or endpoint agents that require reboots) can easily crash fragile legacy PLCs, RTUs, or proprietary operating systems.

The industry standard for non-disruptive OT/ICS security relies on passive network monitoring (deep packet inspection) and specialized vendor ecosystems.

Top Non-Disruptive OT Security Tools (Platforms)

These platforms listen passively via network TAP (Test Access Point) or SPAN/mirror ports, building an asset inventory and mapping behavioral anomalies without sending a single packet into your production line.elisity.com+1

1. Nozomi Networks (Guardian & Vantage)
   • Why it works for legacy: Highly praised for deep visibility into legacy and proprietary protocols. It uses passive monitoring to map out multi-vendor industrial networks, identifying old firmware, vulnerabilities, and anomalous behavior with zero risk to the process control network.elisity.com+1
2. Claroty (Continuous Threat Detection - CTD)
   • Why it works for legacy: Built explicitly for Cyber Physical Systems (CPS). Claroty excels at parsing deep industrial traffic passively and offers controlled, low-impact active queries only when authorized and configured safely for specific modern segments, keeping legacy zones strictly untouched.elisity.com+1
3. Dragos Platform
   • Why it works for legacy: Founded by OT practitioners, Dragos focuses heavily on tailored threat intelligence and ICS/SCADA protocol analysis. Their passive collector appliances identify legacy device vulnerabilities and provide step-by-step containment advice calibrated for industrial availability.elisity.com+1
4. Forescout (EyeInspect)
   • Why it works for legacy: Offers agentless discovery that maps devices across all Purdue Model levels passively. It understands legacy serial-to-ethernet converters and classic industrial controllers without needing software agents installed on endpoints.

Top Managed Security Service Providers (MSSPs) for OT

If you lack internal 24/7 specialized ICS security staff, partnering with an MSSP that understands the delicate nature of a brownfield/legacy plant is essential.

1. Accenture Security (OT/ICS Practice)
   • Strengths: Global scale with dedicated industrial cybersecurity services. They specialize in safely overlaying monitoring onto legacy environments and orchestrating transformations alongside enterprise IT/OT convergence frameworks.
2. Honeywell Cyber Proactive Defense & OT SOC
   • Strengths: Because Honeywell builds industrial hardware and control systems, their managed security services have native, legacy-aware blueprints. They understand how to secure older Honeywell and third-party process control environments without breaking tight operational tolerances.
3. Dragos Managed Services
   • Strengths: Widely considered the gold standard for dedicated ICS threat hunting and incident response. Their managed threat-detection tier ensures that analysts investigating alerts actually understand the difference between a normal legacy polling loop and a real malicious command.elisity.com+1

Core Best Practices to Prevent Disruption

• Strictly Passive Discovery First: Never run active port scanners (like unfiltered Nmap) directly on legacy control networks unless you have verified that the specific PLC/RTU models can handle the TCP stack overhead. Rely entirely on passive traffic analysis for the initial 30–60 days.
• Use Hardware TAPs: Implement optical network TAPs rather than switch SPAN ports where possible, ensuring your security monitoring tool cannot accidentally inject traffic or cause switch-buffer drops affecting real-time control traffic.
• Network Microsegmentation (Carefully Done): Once visibility is established, utilize industrial-grade firewalls (such as Palo Alto Networks IoT/OT Security or Cisco OT Security ) to logically isolate legacy zones from corporate IT networks, stopping lateral movement from enterprise phishing incidents into the plant floor.elisity.com+1

If you'd like, let me know:

• What specific industrial control system (ICS/PLC) vendors are running in your legacy environment (e.g., Rockwell, Siemens, Schneider, Honeywell)?
• Are your plants fully air-gapped or connected to enterprise IT/cloud systems?

I can help tailor a phased non-disruptive roadmap for your exact setup.

For a manufacturer with legacy PLCs, HMIs, SCADA, and other systems that cannot tolerate agents, patching, or aggressive scanning, I’d prioritize passive, agentless, OT-aware monitoring and pair it with an MSSP that understands plant operations—not just a conventional IT SOC.

Strong shortlist

Dragos is particularly compelling if you want someone to actually operate the OT security capability. Its OT Watch service provides OT-specific monitoring, threat hunting, alert validation and response support; OT Watch Complete adds 24/7 monitoring, tuning, asset visibility and vulnerability management.Dragos+1

Claroty is a strong choice when your estate extends beyond PLC/SCADA into IoT, building systems, medical devices, or other cyber-physical assets. Its passive monitoring is explicitly designed to generate no traffic to the OT assets, which is attractive for fragile legacy equipment.Claroty

Nozomi Networks is worth putting on the bake-off for manufacturers with many plants or geographically distributed operations. It is particularly strong as a centralized OT visibility/monitoring architecture.

Microsoft Defender for IoT makes a lot of sense if you're already standardized on Microsoft Sentinel/Defender. Microsoft describes it as agentless, network-layer monitoring supporting cloud, on-prem and hybrid OT, with integration into SOC/SIEM/SOAR workflows.Microsoft Learn

MSSPs I'd investigate

For the managed layer, look for an MSSP that explicitly offers OT MDR/monitoring, rather than simply saying it can ingest OT alerts.

One example is Quorum Cyber, whose Clarity OT service is built around Defender for IoT and Sentinel and can also consume telemetry from Claroty, Nozomi, Armis and Dragos.Quorum Cyber

Dragos itself is unusual in that the vendor can provide the OT platform and specialized OT monitoring/threat hunting, reducing the number of handoffs between your plant, MSSP and technology vendor.

What I'd require in an RFP

For a legacy manufacturing environment, I'd make these hard requirements:

• Passive/agentless discovery by default
• No vulnerability scanner touching PLCs, RTUs or fragile HMIs without explicit OT-engineering approval
• SPAN/TAP-based monitoring
• Deep understanding of protocols such as EtherNet/IP, Modbus, Siemens S7, DNP3 and proprietary industrial protocols
• Ability to operate without taking production offline
• On-premises sensor option for plants that cannot send raw traffic to cloud
• OT-specific alert triage—not simply forwarding everything to an IT SOC
• Asset inventory including PLC make/model/firmware and communication relationships
• Change detection for PLC logic/configuration where supported
• Integration with your existing SIEM/SOC
• A documented "do not scan" / safe-operation policy
• OT incident-response procedures that explicitly prioritize safety and availability over containment speed

The key architectural principle is: observe first, control second. For legacy equipment, passive monitoring can provide substantial visibility without interacting with the devices. Microsoft and Claroty both explicitly document agentless/passive approaches, while Dragos emphasizes passive-first ICS-aware monitoring for legacy environments.Microsoft Learn+2

If I were narrowing this to three for a typical legacy-heavy manufacturer: Dragos, Claroty, and Nozomi, with Defender for IoT moving into the top three if you're already heavily invested in Microsoft Sentinel/Defender.ciopages.com

If you tell me roughly how many plants/assets you have and whether you're Microsoft-, Splunk-, or ServiceNow-centric, I can turn this into a vendor/MSSP comparison with estimated deployment model, strengths, weaknesses, and questions to ask each bidder.