security zones and conduits

Security zones and conduits is a core concept in industrial cybersecurity, particularly in the IEC 62443 series of standards. It provides a structured way to segment operational technology (OT) and related IT systems and to control how they communicate.

Security zones

A security zone is a logical grouping of assets that share similar security requirements and risk characteristics. In an industrial or manufacturing environment, a zone typically contains systems that:

• Have similar criticality (for example, safety-critical control vs. monitoring only)
• Require similar security levels or protections
• Are under a common administration or trust boundary

Zones commonly include combinations of:

• Controllers and I/O (PLCs, RTUs, safety systems)
• Engineering workstations and HMIs
• Plant historians, MES interfaces, and local servers
• Network devices that primarily serve that zone

Zones do not have to match physical areas or existing network subnets, although they are often aligned for practicality. A single production line might contain multiple zones, such as a safety instrumented system zone and a basic process control zone.

Security conduits

A security conduit is a defined communication path that connects two or more zones and provides the necessary protection for traffic that crosses zone boundaries. It is not just a cable or a single device; it is the combination of:

• Communication channels (for example, specific VLANs, routes, or links)
• Security functions (for example, firewalls, VPNs, application proxies)
• Rules and configurations that constrain and monitor traffic

In practice, conduits often correspond to:

• Firewall rule sets between the control network and the corporate network
• VPN tunnels used for remote vendor access to OT assets
• Strictly controlled links between a process control zone and a safety system zone

Each conduit is designed and documented so that the risks of inter-zone communication are understood and addressed.

How zones and conduits are used

Within IEC 62443 and similar approaches, security zones and conduits are used to:

• Structure risk assessments around groups of assets instead of individual devices
• Assign required security levels to zones based on consequence and threat
• Define which communications are allowed between zones, and under what controls
• Support lifecycle management, change control, and documentation for OT networks

For example, a plant may define separate zones for field I/O, basic control, safety systems, MES integration, and corporate IT, with conduits handling specific flows such as production reporting from control to MES or remote maintenance from vendor networks into a dedicated support zone.

Common confusion

• Zones vs. VLANs or subnets: A security zone is a logical and risk-based construct. It may map to one or more VLANs or subnets, but they are not synonymous.
• Conduits vs. single devices: A conduit is the secured path and its configuration, not just the firewall or router. Multiple devices and rules can participate in a single conduit.
• Perimeter-only thinking: Zones and conduits apply inside the plant network as well as at the enterprise perimeter. They are not limited to a DMZ or a single “OT boundary”.

Link to risk methodologies

In risk assessment approaches aligned with IEC 62443, security zones and conduits are used as the reference objects for identifying threats, evaluating consequences, and selecting controls. Existing corporate or enterprise IT risk methodologies often need to be extended to incorporate zone/conduit modeling for OT and industrial control systems.