Why OT Security Works Differently Than IT Security
In classic IT, confidentiality usually comes first. In Operational Technology (OT) - controllers, fieldbuses, and drives - the order is reversed: availability takes top priority, followed by integrity, and only then confidentiality. A security patch that takes a PLC offline for a restart can cause more damage in 24/7 production than the attack it was meant to prevent. Security concepts must account for this reality rather than uncritically adopting IT standards.
Why Fieldbuses Are Inherently Insecure
PROFINET, EtherCAT, and most classic fieldbus protocols were optimized for speed and determinism - not authentication or encryption. Any device on the same network segment can generally read telegrams or inject its own. In many existing plants, the production network has also grown organically over time and is not cleanly separated from office IT or even the internet.
The Purdue Model Foundation for Network Segmentation
The Purdue model divides industrial networks into levels (Level 0 - field devices, Level 1 - controllers, Level 2 - supervisory control, Level 3 - site IT, Level 4/5 - enterprise IT). The most important practical principle: communication between levels happens exclusively through defined, monitored gateways - never through flat, unfiltered networks where a compromised office PC has direct access to a robot's PLC.
Five Concrete Measures for Existing Plants
1. Network Segmentation with Industrial Firewalls
Separate the production network physically or via VLANs from office IT. Industrial firewalls (e.g. Siemens SCALANCE S, Phoenix Contact mGuard) allow granular protocol filtering and detect protocol-specific anomalies in PROFINET or Modbus traffic.
2. Managing Remote Access
Remote maintenance access for system integrators is a common entry point. Use time-limited, logged VPN access instead of permanently open remote maintenance ports. Every access should be traceable: who, when, to what.
3. Asset Inventory
Many plant operators don't precisely know which devices are actually active on the production network. A complete, up-to-date inventory of all PLCs, HMIs, drives, and network components is the foundation of any security strategy - you cannot protect what you don't know exists.
4. Deliberate Patch Management
Unlike IT, patches in OT cannot simply be rolled out automatically - every change to a production controller requires testing and planned downtime. A structured patch cycle with defined maintenance windows is more practical than trying to close every vulnerability immediately.
5. Monitoring and Anomaly Detection
Passive network monitoring solutions (e.g. Nozomi Networks, Claroty) detect unusual traffic without actively intervening in communication themselves - a critical difference from classic IT security tools, which can cause disruptions in real-time systems.
The Human Factor Remains Decisive
The best technical segmentation is of little use if USB sticks are connected to PLC programming devices without control, or default passwords remain unchanged for years. Regular training for maintenance staff and clear policies for external service providers are just as important as the technical infrastructure.
Regulatory Context: NIS2 and IEC 62443
With the NIS2 directive, OT security becomes mandatory rather than optional for many manufacturing companies in the EU. The IEC 62443 standard provides the technical framework for implementation - from risk assessment through zone-and-conduit models to concrete requirements for component manufacturers and system integrators.
Conclusion
OT security is not a one-time project but an ongoing process that brings together network architecture, organizational processes, and regulatory requirements. Investing in segmentation, asset visibility, and structured patch management today avoids costly production outages and regulatory consequences tomorrow.