Industrial Control Systems (ICS) security is extremely important because ICS devices are ubiquitous throughout critical infrastructure and services. To emphasise this, unlike core IXPs or DFZ routers which are ancillary, ICS devices are directly involved in critical processes:
- water purification
- petrochemical processing
- agriculture
- electric grid management
- etc.
If ICS devices are compromised then critical services are immediately disrupted.
What do ICS devices actually do?
ICS devices provide a range of control and observability to human operators. A common ICS device are Programmable Logic Controllers (PLCs) which are essentially embedded computers running a proprietary RTOS hooked up to hardware peripherals (e.g. sensors, actuators, valves, etc.). PLCs will often provide remote services for an operator to observe and modify the state of the PLC (and its attached peripherals). The modification of state allows an operator to manipulate the physical environment.
PLCs can be grouped together under a Sypervisory Control and Data Acquisition (SCADA) system to control and observe fleets of PLCs and other remote ICS devices. With this, a human operator can control a complex process involving a variety of devices (e.g. an entire production chain).
Real-World Example
The Siemens S7-1200 PLC is a popular (although now dated) choice for operators because it boasts a huge range of remote protocols: TCP, ISO-on-TCP, Step7, MODBUS, HTTP(S), SNMP, LLDP, NTP, ARP, etc. In addition, the S7-1200 CPUs can connect to multiple peripherals (e.g. 1212C has 8 and 6 digital inputs and outputs, respectively and 2 analog inputs).
An operator can write a Ladder-Logic program for the PLC then attach hardware periphals to available connectors (S7-1200 can add modules with more connectors, memory, DSPs, etc). Then the operator can remotely interface with the PLC individually or setup a SCADA system to poll the device on its state.
PLC Honeypots
Honeypots are popular in academia because they allow researchers to understand how attackers probe for and interact with ICS devices. The honeypots generally aim to emulate a subset of protocols then record all network ad protocol interactions. Popular honeypots include:
- GasPot (succeeded by XPOT)
- Conpot (succeeded by S7commTrace)
- Honeyd
- HoneyPLC (succeeded by ICSpot)
- HoneyPhy (succeeded by ICSpot)
- MiniCPS
- CryPLH
Honeypots can be viewed as having three core capabilities which we can tag honeypots as implementing (excluding honeypots with successors):
| Capability | Honeypot(s) |
|---|---|
| Stateful Protocols | S7commTrace, ICSpot, Honeyd |
| Network Emulation | MiniCPS |
| Physical Simulation | ICSpot, CryPLH, MiniCPS, XPOT |
Stateful Protocols
Common protocols implemented by honeypots are HTTP(S), SNMP, S7, MODBUS, and ENIP. Of these, honeypots will attempt to provide all original functionality of the target device (e.g. all hidden webpages are accessible). However, a key problem with implementing these protocols are that an interactive attacker will expect the same state transitions as well as the same implementation behaviour.
Network Emulation
Honeypots are reachable over a network. A high-level protocol can be correctly implemented but the lower level protocols may rely on the honeypot host rather than an accurate emulation (e.g. TCP segmentation when sending HTTP requests). However, the more hops between the target and the user, the more unreliable these characteristics become (i.e. a network hop might reconstruct packets). Despite this, an attacker can heuristically determine details about the network stack/environment of the peer.
Physical Simulation
PLCs do not sit idle; PLCs are supposed to be doing stuff. If an attacker is able to inspect a device with no running logic or no peripherals, the device is either suspicious or useless from an attacker POV. An attacker will struggle to verify details of the physical simulation without gleaning details off of running logic or other channels. However, an attacker can assume that the simulation behaves realistically (i.e. constant increments in sensor readings).
Implementations are fundamentally flawed
All honeypot implementations make tradeoffs over the accuracy of their emulation against the ease of configuration. This results in behaviour being correct but inaccurate. As an example, a common approach to support the proprietary S7 protocol is to use Snap7, a library implementing a subset of S7. This is unacceptable because Snap7 does not attempt to implement the entire protocol nor does it attempt to mimic a real device. An attacker can rely on undocumented features or implementation details of the legitimate S7 stack to detect differences. This issue extends to other ICS protocols where the implementors choose 3rd-party dependencies which are not perfectly identical: Modbus, ENIP, ISO-on-TCP, HTTP, etc.
Attempting to emulate the network layer is also exceedingly difficult because the networking stack of PLCs are proprietary and run on custom hardware with a custom RTOS. You cannot use a different TCP stack then hope to reconstruct packets to mimic a target device without a very tedious configuration process and a prayer that you correctly implemented all quirks. This also extends to side-channels like processing delays, impacts on other tasks, debugging interfaces tracking processor metrics, etc.
Deployments are fundamentally flawed
The first two lists are well explored in literature (with the low-level characteristics being frequently hand-waved away by implementors). However, the issues with deployment are less explored (aside from having a natural host appearance). From reading a variety of papers, all honeypots were hosted on commercial cloud providers or university networks which are trivial to tag as suspicious by looking at the target ASN.
Instead, we should try put the honeypot on an ASN owned by a business ISP which is used by industrial sites. In addition an attacker can look at the routing path to see if deployers have installed a network proxy, router, or switch which can be suspicous because a PLC should not have a huge network path if it is on the public Internet.
However the biggest flaws with all honeypot research I have read was the duration of the study. The majority of honeypots were deployed for under 180 days with none spanning over 2 years. Consequently, an attacker can ignore all peers until they have been consistently online for over 8 months. An additional issue was that operators would often deploy several instances of the same honeypot which can be an indicator that the honeypots are either:
- Part of the same OT network.
- All honeypots run by the same group.
The first scenario can be ruled out using the other networking characteristics. The final issue I see when looking at deployments are network neighbours. A network neighbour is a device on a nearby address (e.g. X.X.X.1 and X.X.X.2). On a large network, you might expect a range of addresses to be advertised to the wider internet. Using this assumption, an attacker can see if the surrounding hosts are hosting unrelated services (e.g. PLC neighbouring with someone’s 90s-style Internet homepage).
My 2¢
Honeypots are junk. A sophisticated attacker will detect anything less than genuine which is relevant because anyone targeting ICS devices will be sophisticated. Even worse, the attacker will be sophisticated and patient because ICS attackers will be state actors or state sponsored actors whose motivations (the motivations of the state) are political. Instead, more effort should be focused on adding instrumentation to real devices then putting those devices into real OT networks. Overall interactions are significantly less likely but the interactions that do occur will be extremely revealing in how an actual ICS attacker behaves.
To elaborate on this, ICS attackers are not like traditional IT attackers. The attackers will have complex intentions (i.e. not aiming to ransomware for a quick buck), be experienced with working on PLCs, and will have access to real devices to testbed exploits and compare responses. I do not think trying to deceive this cohort is productive because you cannot win.