The industrial world is at a crossroads. For decades, “Operational Technology” (OT) lived in a world of physical isolation-the “air gap.” Today, that gap has evaporated. Driven by the demands of Industry 4.0, predictive maintenance, and real-time data analytics, legacy Industrial Control Systems (ICS) are being plugged into the global grid.
The problem? These assets-Programmable Logic Controllers (PLCs), Human-Machine Interfaces (HMIs), and SCADA systems-were often built twenty or thirty years ago. They speak insecure, plaintext protocols and lack the processing power for modern encryption. They are the “fragile backbone” of our critical infrastructure.
Securing these legacy assets isn’t just about software; it’s about maintaining the “safety-availability-security” triad without disrupting the physical processes that keep the world running.
The Background: Why Legacy OT is a Different Beast
Unlike IT security, where the primary goal is Confidentiality, OT security prioritizes Availability and Safety. If a laptop freezes during a patch, it’s an inconvenience; if a turbine controller freezes, it’s a potential catastrophe.
Legacy OT assets face three primary hurdles:
- Protocol Insecurity: Older protocols like Modbus or PROFIBUS lack authentication. Anyone on the network can send a “stop” command.
- Patching Paradox: Many legacy systems run on “End of Life” (EoL) operating systems like Windows XP or NT. Even if a patch exists, the downtime required to apply it is often unacceptable to the business.
- Visibility Gaps: You cannot protect what you cannot see. Many plants operate with “ghost” assets that aren’t on any official inventory.
As we move through 2025, the rise of AI-driven threats and nation-state hacktivism has made the “set it and forget it” approach to legacy OT a liability. Here are the top 10 tools designed to bridge the gap between ancient hardware and modern security requirements.
Top 10 Tools for Securing Legacy OT Assets
1. Nozomi Networks Guardian
Nozomi has long been a leader in the OT space, and in 2025, their Guardian sensors are more critical than ever. Guardian specializes in Passive Deep Packet Inspection (DPI).
Because legacy assets are often sensitive to active scanning (which can cause them to crash), Nozomi listens to the network traffic silently. It builds a real-time map of every device, identifies its firmware version, and alerts you to behavioral anomalies-like a PLC suddenly communicating with an external IP address.
2. Claroty xDome
Claroty’s xDome is a cloud-based platform that excels at Vulnerability Management for legacy environments. One of its standout features for 2025 is the ability to map assets to a “Virtual Patching” framework.
Since you often can’t patch a 20-year-old HMI, Claroty helps you implement compensating controls (like specific firewall rules) that act as a shield, neutralizing the vulnerability without touching the asset itself.
3. Dragos Platform
Written by practitioners who have defended against world-class ICS attacks (like Industroyer), the Dragos Platform is less about “general security” and more about Industrial Threat Intelligence.
It excels at detecting the specific “Tradecraft” used by attackers targeting legacy infrastructure. For a legacy site, Dragos provides the “Playbooks” needed to respond to an incident, ensuring that operators don’t accidentally make things worse during a breach.
4. TXOne StellarOne
For legacy workstations still running Windows XP or 7, you cannot use standard IT antivirus-it’s too resource-intensive. TXOne StellarOne offers “lockdown” technology.
Instead of looking for “bad” files (which changes every day), it creates a “Known Good” whitelist of applications. If a piece of ransomware tries to execute on that legacy workstation, it is blocked instantly because it isn’t on the approved list. This is the gold standard for Endpoint Protection in OT.
5. Cisco Cyber Vision
The best security is the one built into the network you already have. Cisco Cyber Vision embeds security sensors directly into Cisco industrial switches.
For legacy assets, this means you get visibility right at the “edge” where the device connects. It’s particularly useful for Micro-segmentation-grouping legacy assets into small, isolated “cells” so that if one is compromised, the infection cannot spread to the rest of the plant.
6. Fortinet FortiGate Rugged Series
While not a “tool” in the software sense, these Industrial Next-Generation Firewalls (NGFWs) are the gatekeepers of legacy zones.
The 2025 models include specialized “OT Application Control” that understands industrial protocols. It can be configured to allow a “Read” command to a legacy PLC but block a “Write” or “Firmware Update” command unless it comes from a specific, authorized engineering workstation.
7. Microsoft Defender for IoT (formerly CyberX)
If your organization is heavily invested in the Azure ecosystem, Defender for IoT provides seamless integration.
It is particularly strong at Cross-Domain Correlation. It can see an initial phishing attack on an IT email account and track the lateral movement as the attacker attempts to bridge into the legacy OT network. This “unified” view is essential for modern SOC teams.
8. Forescout Continuum
Forescout is the king of Network Access Control (NAC). In a legacy environment, the biggest risk is often a technician plugging a “dirty” laptop or a rogue USB drive into a switch.
Forescout identifies every device the moment it touches the wire. If the device doesn’t meet the security baseline, it is automatically quarantined, protecting the legacy assets from “sneakernet” infections.
9. Darktrace / OT
Darktrace applies Unsupervised Machine Learning to the industrial environment. It doesn’t use “signatures” of known attacks. Instead, it learns the “Pattern of Life” for every legacy asset.
If a legacy pump controller usually sends 50KB of data a day and suddenly starts sending 5GB, Darktrace identifies the anomaly in seconds. This is vital for detecting “Zero-Day” attacks that have never been seen before.
10. OPSWAT MetaDefender OT Security
Legacy environments often struggle with “data silos.” OPSWAT focuses on the Secure Transfer of Files.
Many legacy systems are updated via USB. MetaDefender acts as a “Media Security Kiosk.” Before a technician can use a USB drive on a legacy asset, they must plug it into the kiosk, which “sanitizes” the files, stripping out malicious code and ensuring only clean data enters the air-gapped zone.
Conclusion: The Path Forward for OT Ecosystems
Securing legacy OT assets is not a “one and done” project; it is a continuous journey of visibility and risk mitigation. In 2025, the most successful organizations are moving away from the “hard shell, soft middle” approach. They are adopting a Zero Trust mindset where every communication-even between two legacy machines-is verified.
By leveraging a combination of passive monitoring, whitelisting, and robust network segmentation, you can extend the life of your legacy assets while protecting the physical world from digital threats.