The lack of clearly defined security layers drove the decision to adopt a zone-and-conduit model as defined in ANSI/ISA99.02.01 as the solution to the realization/addressing phase. The plant business network, the management network, the basic control system, the supervisory (operator) system and the safety system were each defined as a separate security zone (See Figure 4). Between these, approved "conduits" were defined for inter-zone communications. For example, only the Supervisory Zone was permitted to have direct connection to the rest of the Business Network Zone, while the Safety Zone had only a single approved conduit to the Supervisory Zone (for additional information on the use zone and conduit models see http://www.tofinosecurity.com/ansi-isa99).
Once the zone and conduits were defined, the technical aspects of the solution began. The engineering team defined appropriate safety/security controls on each of the conduits to regulate inter-zone traffic. For example, to manage all network traffic into the Safety Zone from the Supervisory Zone, Tofino industrial firewalls were placed between the two zones. These specially designed security devices are hardened for industrial environments and engineered specifically to manage MODBUS/TCP traffic, the particular network protocol used to communicate to the SIS. They were also designed to be transparent to the network on start-up so they would not disrupt refinery operations while being commissioned. Similar firewalls were also placed between the Basic Control Zone and the Supervisory Zone.
It is interesting that, while the firewalls were being installed, the engineering team noted that Windows PCs on the Supervisory network were generating a significant number of "multicast" messages on the safety and control network, even though they were of no use to the SIS controllers. Messages like these were a significant factor in another famous nuclear safety incident that occurred at Alabama's Brown's Ferry reactor in August 2003, where network traffic caused a redundant cooling water system to fail. This time the firewalls were configured to have these nuisance messages silently dropped from the network as they entered the Safety Zone. Two years later, the safety system has operated without incident, despite numerous changes to the networks around it.
While it is easy to get caught up in IT view of security and think only of hackers and viruses, for the operator of a hydrocarbon processing facility, security is much more. Security is about maintaining the reliability and safety of the entire system. As a result, the security of process systems can be significantly improved with a coordinated approach to both safety and security, starting with the initial analysis. By following a well-defined process, this analysis phase can be both cost-effective and significantly improve the reliability of the entire processing facility, providing an excellent return on investment.
John Cusimano is director of exida's security services division.
Eric Byres is the Chief Technology Officer of Byres Security Inc.