Ed. Note: The ANSI/ISA-99 Standards are now the ISA IEC 62443 Standards. The zones and conduits model described in this article remain the same.

Recently I wrote about one of the fundamentals of industrial cyber security, which is the concept of Defense in Depth.Today I am going to write about another foundation concept, which goes hand-in-hand with Defense in Depth, and that is using ANSI/ISA-99 Standards to improve control system security.

Factors that have degraded Control Network Security

There are two opposing trends impacting control network design today:

  1. The trend toward greater "interconnectedness" of control systems with enterprise systems as organizations seek increased business productivity and as they increase the use of Ethernet-TCP/IP technology.
  2. The trend to isolate control networks as an attempt to block advanced malware threats such as Stuxnet.

How does a controls engineer deal with the conflicting requirements of more integration and more isolation? My advice is to accept and plan for high integration with business systems, and to dismiss the idea that control systems can be isolated.

I am on the record as saying I don't believe isolation really exists today, except, perhaps, if you are a highly defended nuclear facility. Rather, what Stuxnet showed us is that there are multiple pathways to the control system, and they don't require a connection to the Internet. This is discussed more in the White Paper you can download at the end of this document, or, for extensive details on this, see How Stuxnet Spreads.

If isolation is not an effective security measure, then how can you protect your control system? One way you can make significant improvements in your facility's cyber security posture is to improve network segmentation. Many control networks have remained "flat", even though more and more devices have been connected to them. Flat networks mean that a cyber intrusion or network incident that originates in one part of the network can quickly spread to other areas.

The "zone and conduit" model included in the ANSI/ISA-99 security standards provides a framework for network segmentation.

Zones and Conduits

ANSI/ISA-99 standards introduce the concept of of "zones" and "conduits" as a way to segment and isolate the various sub-systems in a control system.


Figure 1: Security Zone Definition

A zone is defined as a grouping of logical or physical assets that share common security requirements based on factors such as criticality and consequence. Equipment in a zone has a security level capability. If the capability level is not equal to or higher than the requirement level, then extra security measures, such as implementing additional technology or policies, must be taken.

Any communications between zones must be via a defined conduit.


Figure 2: Conduit Definition

Conduits control access to zones, resist Denial of Service (DoS) attacks or the transfer of malware, shield other network systems and protect the integrity and confidentiality of network traffic.

Typically, the controls on a conduit are intended to mitigate the difference between a zone's security level capability and its security requirements. Focusing on conduit mitigations is typically far more cost effective than having to upgrade every device or computer in a zone to meet a requirement.

Defining the Security Zones


A zone might be a group of physical assets that have common security requirements.

Zone and conduit design starts with the facility or operation being analyzed to identify groups of devices that have common functionality and common security requirements. These groups are the zones that require protection.

For example, a facility might first be divided into operational areas, such as materials storage, processing, finishing, etc. Then within these areas it could be further divided into functional layers, such as Manufacturing Execution Systems (MES), Supervisory Systems (i.e. operator HMIs), primary control systems (e.g. DCS Controllers, RTUs and PLCs) and safety systems.

Each zone is defined with not only its boundaries, assets and risk analysis, but also its security capabilities. In other words, the security capability of a zone full of Windows 2008 servers is very different than that of a zone of Windows NT servers or a zone with PLCs. This security capability, along with the security risk faced by the zone, drives the security function requirements for conduits that connect the zone to other zones.

Defining the Security Conduits


The next step is to discover the pathways in the system through which data is passed between these zones; these are the network "conduits".

Each conduit should be defined in terms of the zones it connects, the technologies it utilizes, the protocols it transports and any security features it needs to offer its connected zones.

Typically, determining the information transfer requirements between zones over the network is straight forward. Tools like traffic flow analyzers or even simple protocol analyzers can show which systems are exchanging data and the services they are using.

It is also wise to look beyond the network, to determine the hidden traffic flows. For example, are files ever moved via USB drive between the lab and the primary control systems? Do people remotely connect to the RTUs using a dialup modem? These flows are easy to miss, but can result in serious security issues if not managed carefully.

Securing the ConduitsOnce the conduits and their security requirements are defined, the final phase is to implement the appropriate security technologies. There are two popular options for this stage:

1. Industrial Firewalls

These devices control and monitor traffic to and from a zone. They:

  • compare the traffic passing through to a predefined security policy, discarding messages that do not meet the policy's requirements
  • they are typically configured to pass only the minimum traffic that is required for correct system operation, blocking all other unnecessary traffic.
  • filter out high risk traffic, such as programming commands or malformed messages that might be used by hackers to exploit a security hole in a product
  • are designed to be very engineer-friendly and are capable of detailed inspection of SCADA protocols such as DNP3, Ethernet/IP and Modbus/TCP

2. VPNs (Virtual Private Networks)

These are networks that are layered onto a more general network using encryption technology to ensure "private" transmission of data and commands.

VPN sessions tunnel across a transport network in an encapsulated format, making them "invisible" to devices that don't have access to the VPN members' secret "keys" or "certificates".

ANSI/ISA-99 and Defense in Depth

The zone and conduit approach helps implement a strategy of "defense in depth", that is multiple layers of defense distributed throughout the control network. This is a strategy that has been proven in the IT community.

I strongly recommend you become proficient with segmenting control networks for zones and conduits, and with appropriate industrial security solutions. Doing so will greatly assist your organization to mitigate against threats from "interconnectedness" and "Son-of-Stuxnet" malware.

Is your network segmented with zones and conduits? If not, do you think this is a good approach? If it is, are there any tips you would like to share with others? Let me know.

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