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How to Gain More from your Connection to an OT Network
Published Aug 03 2021 11:08 AM 14K Views

How to Gain More from your Connection to an OT Network

One of the most productive and non-intrusive tools in the Cyber Security Engineer’s bag is passive Network Traffic Analysis (NTA).  Providing network maps, inventory, and firmware information among other benefits provides insights that are not generally known any other way.  Manual inventory collection methods are error-prone and expose this information to interception over corporate email networks, shared file folders, etc.  But how do we implement this kind of system without causing any bumps in the road for real-time processes?  What are the risks?  Which methods are best?  The best sensor does no good unconnected and is of little value connected in the wrong part of the network. 


To discuss this, I will use a diagram that was developed for my last blog post Designing a Robust Defense for Operational Technology Using Azure Defender for IoT (  This diagram (below) shows an example OT network monitored by Azure Defender for IoT. Defender for IoT is an agentless passive Network Traffic Analysis tool with strong roots in Operational Technology, now expanding to IoT. Defender for IoT discovers OT/IoT devices, identifies vulnerabilities, and provides continuous OT/IoT-aware monitoring of network traffic.  The recommended locations for Azure Defender for IoT  (AD4IoT) are shown in red color.  Why have these locations been chosen?  To explain this, we will break this network into pieces and address these issues for each type of traffic.







Starting with the lower portion of this sketch, let’s look at traffic flows around the PLCs. 






1. The first arrow shows traffic between a PLC and its ethernet-connected Input/Output (I/O) modules.  This traffic utilizes simplistic protocols and is very structured and periodic.  It can be leveraged as a threat to the overall OT system and is more vulnerable when I/O is remote from the PLCs in unsecured areas.  Malicious applications could perform inappropriate control actions and/or falsify data.  Firmware problems in I/O modules often go unpatched unless some form of undesirable behavior is experienced. In certain families of PLCs or controllers, the Defender for IoT can provide data on firmware levels and types of I/O modules if this data is requested by an HMI or historian. 


The mechanism to monitor this traffic is to span switches used in the I/O subsystem as shown here.  If they are unmanaged switches, taps may be located at the connection to the PLC or controller.


2. The second arrow identifies traffic from Variable Frequency Drives or similar equipment often interfaced with the PLCs or Controllers.  This communication may be Modbus, Rockwell Protocols, or CIP.  Equipment could be damaged or destroyed by inappropriate commands sent to such devices.  Good engineering practice would put bounds of reasonability around all potential setpoints, but this may not be the case.  These protocols are well understood and in the public domain.  A man-in-the-middle attack could affect this type of equipment.  Monitoring these communications can identify inappropriate function calls, program or firmware changes, and parameter updates. As above, switch span or taps are the mechanisms to monitor this traffic.


3. Custom engineered systems may utilize well-known, open OT protocols such as Modbus, OPC, or others.  This traffic should be monitored even if it is not fully understood as the behavior patterns should be very predictable.  It is common for these systems to utilize unusual functions and atypical ranges for data.  This is the result of a developer reading a protocol spec with no actual field experience with the protocol.  Custom alerts can be configured and tuned based on the nature of the data.  Since such systems are engineered to order for a specific purpose, the damage could have long-term implications on plant production.


4. Traffic crossing OT Access-level switches should always be monitored.  This is the primary point at which PLCs or controllers communicate with HMIs, engineering stations, and sometimes historians.  The problem here is that these switches carry the actual OT control traffic.  Any action that could compromise this traffic affects the reliability of the OT system.  Many switches at the I/O and access layers may be unmanaged devices.  By unmanaged, I mean that they are not configurable and therefore cannot support a SPAN (or mirror) session. 


Unmanaged switches is not an insurmountable hurdle.  Two possible paths may be followed from this point.  The least intrusive is to install network taps.  The security engineer should consult with the OT engineer on the most valuable locations for taps.  Since a stand-alone tap monitors only one data stream, the most valuable assets (compromise targets) should be monitored. These would normally be at least the engineering station, historian and/or alarms server (if appropriate), and HMIs, particularly those with engineering tools installed.  If it is necessary to monitor all traffic, a tap aggregator may be used.


Another approach would be to replace the unmanaged switches with managed switches.  This may sound daunting but usually is not.  Most managed switches are configured to “wake up” in a basic mode which approximates an unmanaged switch.  So replacement, while requiring a system shutdown, can be accomplished rather quickly and have the system up and functioning again.  Once this is done, the configuration can be added to provide basic security and copy traffic to a SPAN or mirror port.  Make sure these configurations are saved as most switches make changes to operating memory which is not stored on power reset.  It is generally recommended to discuss this change with your OT support personnel and/or OEM service engineers.  They probably have some standard switch configurations that they apply when a customer requests managed switches.  Additionally, they should be able to provide you with approximate bus speeds needed to support OT traffic with mirroring.


What are the risks? In the case of switch SPAN (SwitchPort ANalyzer), or mirror sessions, the only concern of serious significance is the current traffic level on the switch.  If a SPAN session is added to a heavily loaded switch, the SPAN may drop packets because the SPAN session is a lower priority than actual switching traffic. This could mean that some packets might slip through unmonitored.  However, it does not affect the normal functioning of the switch for ICS traffic.  Some switches, if they are greatly overloaded can revert to ‘flood mode’ in which they act as a network hub.  This situation is extremely rare.  If switch SPANning is chosen as a method, it is wise to monitor network traffic on the switch prior to adding the session.  Assume that a full switch span will double the switch backbone traffic. 


If network taps are installed, the risks are insignificant.  Passive taps should of course be chosen.  Passive means that the tap continues to pass control traffic even if it loses power.  Passive taps are simply inserted in-line with the existing traffic, see sketch below.  Installation needs to be coordinated with OT engineers to limit the impact on operating processes. 



Network Tap.jpg


Next, we will discuss special equipment including analysis devices and robotics.  This portion of the overall diagram is shown below.  




Network traffic to analyzers typically looks like normal PC traffic using common IT protocols.  Most analyzers have some form of controller that is designed for a specific function.  Sometimes the PC is the controller, utilizing specialized I/O boards included in the machine. Some analyzers or groups of analyzers may be managed by mini computers.   In any case, from a network security perspective, these devices appear on the network as computers, not analyzers per se.  Patching of these customized machines often lags behind the upgrade strategies used for standard IT equipment.  Upgrades to analysis systems must be approved by, and often be implemented by the OEMs which may be expensive and involve downtime. Because of infrequent patching and/or OS upgrades, this equipment can become a security liability on a lab network. Ideally, lab equipment should be separated either physically onto separate networks or via VLANs, but such changes may require extensive planning and testing and still can be disruptive to ongoing lab processes.

Most major medical laboratories utilize either a LIMS (Laboratory Information Management System) or a middleware server to collect analytics data from these devices and forward that data to a patient information database managed either locally or in the cloud (see sketch below).  Hence, the traffic to/from the analyzer will be most easily recognized by the ultimate destination at the middleware or LIMS.  Since these potentially vulnerable machines may process interactions with users on the lab network for input data or maintenance functions, they should be monitored more closely than fully patched IT machines.  This presents a challenge to lab IT managers who may want to gain a handle on this type of OT equipment in their network but may not have good inventory information. 

Since medical testing facilities utilize normal switched networks, monitoring should be installed at an appropriate location to ‘see’ all the traffic from analyzers to the middleware or LIMS server.  This could be either core or distribution level switches depending on the network design.  Standard SPAN or mirror traffic can be used.


Dual-homed machines present special security challenges since they could be converted to active routers by malware.  It is common for expensive lab or analysis equipment to be leased.  OEM terms and conditions specify how this equipment may be used and what service it requires to achieve contracted performance.  This is often monitored via a ‘secure’ datalink to the manufacturer’s support site.  These may or may not be bi-directional.  These links are generally firewalled, either by the OEM, by the customer or by both.  Bi-directional links are inherently a threat.  Remote access to a computer on the lab network can put much more than that computer in jeopardy. 




In robotic applications, the primary issue is the speed of response.  The control systems are complex, utilizing high-level programming toolsets.  The low-level communication may not utilize standard ethernet framing.  Robot protocols vary widely and include Ethernet/IP, DeviceNet, Profibus-DP, Profinet, CC-Link, and EtherCat protocols.  Physical media may be Cat5/6, but RG-6 coaxial, twisted pair, RS-485, and fiber are also used.  Monitoring the low-level communication between controllers and robots requires careful coordination with the equipment designer and should not be attempted casually.  Network monitoring should utilize taps. Switch SPAN, or mirroring is not recommended. 


As described above, most industrial robots are programmed using a computer workstation.  Downloading and selection of programs may be manual or automated using standard network protocols. So, monitoring should focus on the programming workstations and the source of robot program selections.  Robot program file downloads may be transferred from a central server.  These could occur over SFTP, FTP, SMB, or other methods. 


Finally, we would like to address the OT interface to the business (Enterprise) network.  This can be a gateway for potential threats to OT systems.  Some vulnerabilities that may be unsuccessful in the IT network space may cause severe problems in the OT space because the machines may not be patched.  Out of date and unsupported operating systems may be in use.  As a result, traffic that enters from the Enterprise network and ultimately reaches the OT network should be monitored. 


Generally, good practice prevents any direct traversal of the DMZ.  For instance, remote desktop sessions should be hosted by a RAS server in the DMZ which is then used to open a remote desktop session into an OT machine with different credentials. Elaborate credential systems with short password lives attempt to increase the challenge for attackers attempting to gain control.  Well designed implementations keep all machines in the DMZ patched up-to-date which should limit the effect of known vulnerabilities. 


Zero day vulnerabilities will always be a threat prior to discovery.  So, monitoring sessions entering the DMZ from the Enterprise and those leaving the DMZ for the OT network are an important part of a security design.  Similarly, monitoring traffic from the OT network to a Historian server and Enterprise connections to that same server could uncover issues.  Since these sessions are often encrypted, efforts should focus on the legitimacy of the Enterprise hosts, times of access, data rates, and other indicators to validate these externally generated sessions.


The DMZ is also used as a connection point for a variety of other facility systems such as IP phones; perimeter security systems; weather stations; contracted supply systems like water purification, compressed air supply and the like; wireless devices; etc.  In most cases, these various systems are assigned separate VLANs and subnets.  By monitoring all the VLANS in this zone, suspicious traffic can be identified and managed.  Traffic originating from any of these devices to the ICS network should not normally exist. 


Subnet-to-subnet traffic could be cause for concern.  This is another area where Defender for IoT can help.  By mapping the assets, assigning them to VLANs, subnets, and user assigned subsystems, communication between the various device groups can be easily seen greatly aiding efforts to perform or monitor network segregation.  


The visual network map produced by Defender for IoT in conjunction with the filtering capabilities on the map make it easy to identify interconnections between various plant control systems. Having a powerful visual of group-to-group communication makes the effort of segmentation much easier.  This process is a long and tedious one using arp tables on switches.  Also, if this effort is underway, the map will show areas that may have been overlooked.



TRITON asset map 1.png




Well-engineered connections to ICS networks can yield valuable results, including accurate inventories, network maps, and improved security with no risk to the reliability of the underlying OT systems.  This information can be combined, in Azure Sentinel or other SIEM/SOAR solutions, with agent-based Defender for endpoint data to produce a complete picture of OT networks.  Custom-designed playbooks can assist your analysts in responding to OT or IoT issues.  


Teamwork between OT engineers and IT security personnel can yield benefits for both groups while presenting a more challenging landscape to potential intruders.



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