Standards for SCADA security

One of the fundamentals of industrial cyber security is defense in depth, which should be the core of any security program.


ISS SourceEditor’s Note: This is an excerpt from Eric Byres’ Practical SCADA Security blog at Tofino Security.

One of the fundamentals of industrial cyber security is defense in depth, which should be the core of any security program.

However, there is another foundation concept, which goes hand-in-hand with defense in depth, and that is using ANSI/ISA99 Standards to improve control system security.

There are two opposing trends having an impact on control network design today:

  1. 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. Isolating 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 of isolated control systems.

I don’t believe isolation really exists today, except, perhaps, if you are a highly defended nuclear facility. Rather, what Stuxnet showed us is there are multiple pathways to the control system, and they don’t require a connection to the Internet.

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 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/ISA99 security standards provides a framework for network segmentation.

Zones and conduits

ANSI/ISA99 Standards introduce the concept of of “zones” and “conduits” as a way to segment and isolate the various sub-systems in a control system.

A zone is 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.

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 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 security zones

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 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 Conduits

Once 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.
    • They 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.
    • They 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 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/ISA99 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 proven strategy in the IT community.

I 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.

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