Explore best practices for securing SCADA systems in remote sites. This article addresses threats and corresponding implementation measures with a focus on SCADA RTU installations and the processes they monitor and control.
In 2001, Vitek Boden, a disgruntled former employee of the contractor who installed a computer system for the Maroochy Shire Council near Brisbane, Australia, hacked into the system. According to a court statement, "He applied for a job with the council but was rejected and later hacked into the council's sewage control computers, using radio transmissions to alter pump station operations.
"Up to one million litres of raw sewage flowed into the grounds of the Hyatt Regency Resort at Coolum and nearby Pacific Paradise, where it ended up in a storm water drain." The court statement detailed a great deal of environmental damage the attacks caused.1
Could this sort of attack happen to your system?
SCADA Security Standards
Significant recent developments in SCADA security include the release of two key standards, ANSI/ISA-99 Part One and NERC CIP.
"Terminology, Concepts and Models"-Part One of ANSI/ISA-99-00-01-2007, Security for Industrial Automation and Control Systems-lays groundwork for upcoming standards on establishing and operating a security program and technical security requirements. Approved on October 29, 2007, it introduces significant common ground in definitions of security-related concepts, assets, risks, threats and vulnerabilities.
The NERC (North American Electric Reliability Council) CIP (Critical Infrastructure Protection) cyber security standards CIP-002-1 through CIP-009-1 (formerly known as the 1300 standards) were approved January 17, 2008. CIP-002-1 through CIP-009-1 require compliance for numerous provisions.
One important concept in ANSI/ISA-99 Part One is the Reference Architecture, which includes a security zone model. The model recognizes that various portions of a control system or SCADA system, whether logical or physical, vary in terms of risks, vulnerabilities and security requirements. Aligning security zones with physical areas or zones can be advantageous-for example, aligning a control center with a control security zone can provide the control center with extra security.
ANSI/ISA-99 Part One also defines zone characteristics. Each zone has a set of unique characteristics and security requirements, including:
- Security policies
- Asset inventory
- Access requirements and controls
- Threats and vulnerabilities
- Consequences of a security breach
- Authorized technology
- Change management process
For a SCADA system, Part One of ANSI/ISA-99 defines various, logical zones including the Enterprise Zone, which is generally considered the IT system, and the SCADA Zone, which includes the subsystems normally associated with a SCADA system:
- Control center zone or primary and backup control center zones
- Serial or IP network
- Control zones, which are the remote sites normally associated with RTU installations
ANSI/ISA-99, Part One includes two versions, one of which encloses the entire SCADA system in a single security zone. The other is the separate zones model.
In the separate zones model, control center zones and control zones are defined differently. The control zones are the locations, which are usually remote from the control centers and include the RTU equipment. One control zone could conceivably have varying characteristics from another. For example, one location could be classified as more vulnerable or have higher risks than another.
NERC CIP-005-1 requires an electronic security perimeter for critical cyber assets. While it is not explicitly stated in CIP-005-1, the electronic security perimeter concept does apply to ANSI/ISA security zones. General consistency exists between the two standards in definitions of assets and other terms. CIP-006-1 provides physical security requirements and is consistent with ANSI/ISA-99 Part One. This article will describe measures in terms of applicability to ANSI/ISA-99 as well as NERC CIP as much as possible.
We will focus on the control zones and their interfaces to the wide area network. Remote sites provide numerous characteristics, which differ significantly from those associated with the enterprise zone or control center zones. Since the latter two have been explored thoroughly elsewhere, we will focus on control zones. In addition, the wide area network in SCADA systems presents an interesting set of characteristics, as it is typically outside of any of the operator's security zones.
Securing RTU Devices at Remote Sites
Having control zones in remote areas, away from control center zones, presents a number of unique problems, which are notably different from control centers and plant processes. We will consider both the cyber and physical threats and offer measures for preventing and monitoring intrusions.
The term RTU will be used for the electronic monitoring and control device at these locations. Please keep in mind that the device could actually be a PAC, PLC, etc.
RTU Cyber Threats
Prevention
In many systems, it is simply too easy to gain access via an RTU local serial port or-even worse-a dial-up, radio or other network link that makes the RTU accessible worldwide. Recall that Vitek Boden targeted the remote stations by using a radio to access serial ports and operate the pumps.
RTU ports can be local or remote. Local ports are wired directly to nearby equipment such as analyzers, flowmeters, pressure transmitters and a PC or other HMI device. Wireless interfaces are becoming more popular for local links, e.g., wireless HART between an RTU and pressure transmitter and Bluetooth between a laptop PC and the RTU.
If the RTU is not in a physically secure zone, a major risk is that anyone can plug into-or wirelessly access-the local port intended for configuration, taking readings and other local operations via a PC.
Unfortunately, it is too simplistic to state that the RTU must be physically secure. Today's trend toward wireless communications, even for "local" functions, reintroduces the risk of intrusion because the radio range can extend beyond the physically secure zone. A wireless local link shares a major risk with a remote port, which is defined as one with a modem, radio or other physical connection to a wide area network.
In some cases, protocol standards are being amended to adopt authentication, the cyber security provision of choice when it comes to remote port access. The DNP Users Group Steering Committee has recently ratified a security extension that mandates the authentication of master devices through one-way cryptographic hash functions that employ a shared key to access critical DNP functions. These critical functions include write, select, operate, direct operate, cold restart, warm restart, initialize application, start application, stop application, enable unsolicited responses, disable unsolicited responses, record current time and activate configuration. Authentication ensures that messages arriving at the RTU come from the control center or other legitimate SCADA system asset.
Bill Rush of the Gas Technology Institute once proposed SCADA message encryption to address the possibility of eavesdropping. As Rush noted, if someone can eavesdrop and learn to recognize messages, that party can likely also practice spoofing, or injecting commands that can operate process equipment or corrupt proprietary information.
The SCADA encryption standardization effort was originally proposed as American Gas Association (AGA) Report No. 12. Since then, the technical standards community has favored authentication over encryption primarily because it is less resource-intensive and can more reasonably be retrofitted in existing systems.
Encryption standardization efforts still continue, and encryption appears in new installations. Some data communication devices, such as radios, offer it as an option. Many IP-based systems use encryption and, for those users replacing direct-wire local links with wireless, it is also a feature of Bluetooth.
Monitoring and Detection
At a minimum, the RTU must be able to log all activity on local or modem ports and report it to operators on the SCADA network. NERC CIP-005-1 requires 24/7 logging at all access points to the electronic security perimeter.
The Simple Network Management Protocol (SNMP) is emerging as a vehicle for security monitoring in SCADA networks. Traditionally used by IT to monitor components such as routers, servers and switches, SNMP is now also used to monitor remote sites and their characteristics, including control zone parameters like main power status, battery voltage, cabinet temperature and door switch status.
Similarly, SNMP can report activity on RTU serial ports. That information can be used for intrusion detection. SNMP operates over TCP/IP links and can function concurrently with other SCADA protocols. While DNP3 or IEC60870-5 protocols are used to transfer process or operational information between the SCADA server and RTUs, SNMP is used over the same physical network in a background mode, transferring "shadow data" used for system health monitoring and security.
RTU Physical Threats
Prevention
The first step to securing the physical RTU installation is locating it in a physically secure area. Risk is significantly decreased if the RTU is installed in a location with access control.
Ensure RTU location information is secure. Risk is significantly decreased if few people know the location of the RTU.
Similarly, power and network cabling-and their routing and termination locations-should be kept secure and out of sight.
In case of a main power failure, the RTU should include adequate battery backup to continue all operations for as long as you feel it could take to restore main power. Note that this does not mean how long it could take for operators to find out about the problem. The alarm system must inform operators of a main power failure immediately (see next section). Typical RTU backup times are between eight and 72 hours-the latter taking three-day holiday weekends into consideration.
The backup batteries should be secured inside a locked cabinet with ventilation. For outdoor locations, the most appropriate rating is Nema 3R or IP14. You must periodically maintain the batteries on a schedule provided by the battery supplier. Expect a maximum five-year lifetime from lead acid cell batteries, but check them at least once per year. In areas where temperatures are often at the extremes of the operating range, battery lifetime is significantly reduced. The RTU should continually monitor the batteries and set an alarm if they lose their charge. If their condition is in doubt, replace the batteries.
Include line filters and surge suppression on the power input. Accidentally or not, and battery-backed or not, power problems should not shut down the RTU.
Always keep RTU cabinet doors closed and secured. Once the door is opened, problems can easily occur
If the RTU is not in a physically secure area, then keep keypads, push buttons and switches secured. Users should have to open up a door secured by access control-which could be as simple as a key lock-to access these devices.
For most existing installations, it is feasible to secure the room or building in which the RTU is located. When securing the building is impossible, secure the RTU inside a locked cabinet or put a gate around it. Ideally, both the room and the RTU enclosure are secured. However, you may have to settle for one or the other.
Finally, be aware of other methods of disabling the RTU. In other industries, computer equipment has been disabled with fire extinguishers, chemical spray, excessive dust or sand, flooding, sprinkler systems, radio interference and surges on wiring. Vulnerability assessments must include such scenarios, even though they present low risk. Best practices in terms of locating and physically securing the RTU should prevent these problems.
Monitoring and Detection
The RTU should detect entry into the physical secure zone-when a door or gate is opened-via an access control device and alert operators via an alarm.
The RTU should continually monitor main power and report an alarm on main power failure.
The RTU must be able to report that a user has plugged a handheld device or PC into the local port-or gained access via Bluetooth or other local wireless link. This could be an alarm, but some users simply log it as an event.
Log an event when the user signs on by entering a password and each value change the user makes. Operators must be aware if value changes are being made locally. Log an event when the user signs off and either log an event or clear/reset the alarm when the user unplugs the handheld device or PC. If the user forgets to sign off, the RTU should automatically do this after a set time.
Alarm clear/reset when the door closes. If the user forgets to close the door, the original alarm, set upon opening of the door, should continue display as a live alarm. As a further provision, consider escalating that alarm after a certain time.
Coordinate alarms with operating procedures. These procedures should include schedules for site visits and ways to keep operators informed regarding them. Don't disable alarms just because operators know that a site visit is occurring. Maintaining active alarms reinforces procedures and allows the alarms to be kept in a history.
The RTU should not only report alarms over the SCADA network on a priority basis, it should also keep a date-and-time-stamped record of all alarms and events locally in memory. The memory must be nonvolatile. RAM must be backed up by a battery or Flash, which does not require battery backup.
Many of today's RTU products incorporate data-logging capability, including maintenance of an alarm/event log. In the gas flow computer business, this is known as the "audit trail."
One problem with an alarm/event log is a "noisy" alarm condition with recurring messages. Not only is this annoying, but meaningful messages also drop out and are permanently lost. In most cases, it is simple to automatically filter out these transitions or disable the alarming characteristic of the misbehaving input.
The alarm/event log is an excellent backup in case of problems with the SCADA host or network, which could cause alarm reports and loss of event logs. Typically, it allows the user to access all such information locally. In addition, many RTUs will allow the audit trail, as well as historical averages and totals, to be transmitted to the SCADA host once communication is restored.
Many of the security tactics in this section involve use of the RTU for alarm reporting. Please be aware that a common problem with SCADA alarm systems is that engineers are tempted to define too many points as alarms. These quickly become "nuisance" alarms, which are ignored. You should avoid this situation, because the alarm system should never lose credibility with operators for any reason. Far worse, it creates a situation in which an overloaded operator can overlook an important development. It is even possible that a security violation can occur because operators are decoyed by a deliberate overload.
Your alarm system design should define alarm points as sparingly as possible. It should use alarm management as a further measure to reduce the quantity of alarms generated from any process or zone.
Finally, for remote site security, using the RTU to report alarms for fire, smoke, water spray or water flooding is also feasible. The RTU can also be put in the security loop through interfaces with access control devices and video cameras.
Design Practices in Case of Failures
Best practice system design calls for provisions in case of various failures (or breaches) of the SCADA system.
If the host computer or network fails, the RTU should independently monitor and control the process. Remote processes should not depend on the availability or performance of the network.
The RTU should continue operating even if the network is jammed or one or more ports are busy. While this would amount to a denial-of-service attack on the RTU, we have seen many cases in which the SCADA network was simply overloaded. The multitasking kernels in today's RTUs prioritize tasks and allow the measurement and control functions to continue even with heavy activity on the network.
Consider a redundant network. Competition in the communications industry has resulted in decreasing pricing for hardware that includes cellular radio, licensed radio, spread spectrum radio and wireless Ethernet. I know some users will scoff at this because they've found that selecting even one network is difficult enough!
Users are increasingly installing redundant SCADA networks. Most SCADA software will automatically switch over to a standby network if the primary network fails. At the RTU, the standby network uses a separate communication port unaffected by problems on the primary network port.
To detect tampering with process equipment, you can use sanity limits or sanity condition tables to validate commands or process conditions. Even though no RTU includes expert system software, you can still put your expertise in the RTU program, whatever the programming language. If you know that all three influent pumps should not be on when the settling basin is at 12-ft, put that in the RTU. The RTU should know that the chlorinator shouldn't be set on maximum when the flow is only 0.4-mgd.
While these additions may seem like adding too much complexity to the RTU, some languages make the programming almost as easy as making the statement. If access control is violated and someone manually changes a process equipment setting, the RTU could detect it and report an alarm.
Finally, best practices for system design call for provisions in case of RTU failure, regardless of security issues. What happens to the control outputs upon failure, with or without power, is a basic design issue. If power remains available, many devices allow selection of a "safe mode" for the outputs. Process equipment continues to run in a reasonable manner. You also need a separate provision to cover the case in which the RTU fails and all power is lost. Equipment run using backup power must have a "safe" default setting.
Many users have procedures for activity at the sites in response to any failure or security breach in the SCADA system. You need to be in this category.
SCADA Security Checklist
Prevention
- Use authentication (e.g., Secure DNP3) on all remotely accessible serial ports.
- Use encryption if available, e.g., on Bluetooth and IP connections.
- Note that password security is a minimum measure, which does not eliminate cyber risks.
- Locate the RTU in a physically secure area with access control.
- If the RTU is not in a physically secure area, then keep keypads, push buttons and switches physically secured, e.g., behind a locked door.
- Always keep RTU cabinet doors closed and locked.
- Keep information about RTU locations secured.
- Power and network cabling must be secure and out of sight.
- Keep information on cable routing and termination locations secured.
- Use battery backup in case of main power failure and consider backup times up to 72 hours.
- Backup batteries must be physically secured.
- Maintain battery maintenance.
- Include line filters and surge suppression on the power input.
- Vulnerability assessments must consider risks from chemical spray, windblown dust or sand, flooding, sprinkler systems, radio interference and wiring surges.
Monitoring and Detection
- Log all activity on all serial ports, local and remotely accessible, e.g., SNMP reporting of "shadow data" to a risk mitigation platform.
- The RTU should detect entry into the physical secure zone via an access control device and alert operators via an alarm.
- The RTU should continually monitor main power and report an alarm upon failure.
- The RTU must be able to report an alarm or event when a user has plugged a handheld device or PC into the local port-or gained access via Bluetooth or other, local wireless link.
- Log an event when the user signs on by entering a password.
- Log an event for each value change the user makes.
- Log an event when the user signs off.
- Either log an event or clear/reset the alarm when the user unplugs the handheld device/PC or disconnects a wireless link, e.g., Bluetooth.
- Clear/reset the "door open" alarm when the door closes.
- Coordinate all alarms and events with operating procedures.
- Don't disable alarming when users are visiting a site.
- The RTU should maintain a local, date- and time-stamped, alarm/event log in nonvolatile memory as a backup of the alarm-reporting mechanism over the SCADA network.
- "Noisy" alarm or event conditions must automatically be filtered out or disabled, and the root cause must be addressed immediately.
- The alarm system design should define alarm points as sparingly as possible and use alarm management as a further measure to reduce the quantity of alarms generated from any process or zone.
- For remote site security, use the RTU to report alarms for fire, flooding, intrusion and smoke, as well as to transfer video images.
Design Practices in Case of System Failures
- The RTU should independently monitor and control the process, i.e., it should not be dependent on the host computer or SCADA network.
- The RTU should continue operating even if the network is jammed or one or more ports are kept busy.
- Users should consider a redundant network.
- Use sanity limits or sanity condition tables to validate commands or process conditions to detect tampering with or malfunctioning of process equipment.
- Best practices for system design call for provisions in case of RTU failure.
References
Industrial Defender maintains a list of "critical infrastructure incidents" on their website, http://www.industrialdefender.com/.
Pumps & Systems, September 2008
