TRISIS revisited

For this blog I like to go back in time, to 2017 the year of the TRISIS attack against a chemical plant in Saudi Arabia. I don’t want to discuss the method of the attack (this has been done excellently in several reports) but want to focus on the potential consequences of the attack because I noticed that the actual threat is underestimated by many.

The subject of this blog was triggered after reading Joe Weiss’ blog on the US presidential executive order and noticing that some claims were made in the blog that are incorrect in my opinion. After checking what the Dragos report on TRISIS wrote on the subject, and noticing a similar underestimation of the dangers I decided to write this blog. Let’s start with summing up some statements made in Joe’s blog and the Dragos report that I like to challenge.

I start with quoting the part of Joe’s blog that starts with the sentence: “However, there has been little written about the DCS that would also have to be compromised. Compromising the process sensors feeding the SIS and BPCS could have led to the same goal without the complexity of compromising both the BPCS and SIS controller logic and the operator displays.” The color high lights are mine to emphasize the part I like to discuss.

The sentence seems to suggest (“also have to be compromised”) that the attacker would ultimately also have to attack the BPCS to be effective in an attempt to cause physical damage to the plant. For just tripping the plant by activating a shutdown action the attacker would not need to invest in the complexity of the TRISIS attack. Once gaining access to the control system at the level the attackers did, tens of easier to realize attack scenarios were available if only a shutdown was intended. The assumption that the attacker needs the BPCS and SIS together to cause physical damage is not correct, the SIS can cause physical damage to the plant all by it self. I will explain this later with a for safety engineers well known example of an emergency shutdown of a compressor.

Next I like to quote some conclusions in the (excellent) Dragos report on TRISIS. It starts at page 18 with:

Could This Attack Lead to Loss of Life?

Yes. BUT, not easily nor likely directly. Just because a safety system’s security is compromised does not mean it’s safety function is. A system can still fail-safe, and it has performed its function. However, TRISIS has the capability to change the logic on the final control element and thus could reasonably be leveraged to change set points that would be required for keeping the process in a safe condition. TRISIS would likely not directly lead to an unsafe condition but through its modifying of a system could deny the intended safety functionality when it is needed. Dragos has no intelligence to support any such event occurred in the victim environment to compromise safety when it was needed.

The conclusion that the attack could not likely lead to the loss of life, is in my opinion not a correct conclusion and shows the same underestimation as made by Joe. As far as I am aware the part of the modified logic has never been published (hopefully someone did analyze) so the scenario I am going to sketch is just guessing a potential objective. It is what is called a cyber security hazard, it could have occurred under the right conditions for many production systems including the one in Saudi Arabia. So let’s start with explaining how shutdown mechanisms in combination with safety instrumented systems (SIS) work, and why some of the cyber security hazards related to SIS can actually lead to significant physical damage and potential loss of life.

A SIS has different functions like I explained in my earlier blogs. A little bit simplified summary, there is a preventative protection layer the Emergency Shutdown System (ESD) and there is a mitigative layer, e.g. the Fire & Gas system detecting fire or gas release and activating actions to extinguish fires and to alert for toxic gases. For our discussion I focus on the ESD function, but interesting scenarios also exist for F&G.

The purpose of the ESD system is to monitor process safety parameters and initiate a shutdown of the process system and/or the utilities if these parameters deviate from normal conditions. A shutdown function is a sequence of actions, opening valves, closing valves, stopping pumps and compressors, routing gases to the flare, etc. These actions need to be done in a certain sequence and within a certain time window, if someone has access to this logic and modifies the logic this can have very serious consequences. I almost would say, it always has very serious consequences because the plant contains a huge amount of energy (pressure, temperatures, rotational speed, product flow) that needs to be brought to a safe (de-energized) state in a very short amount of time, releasing incredible powers. If an attacker is capable of tampering with this shutdown process serious accidents will occur.

Let’s discuss this scenario in more detail in the context of a centrifugal compressor, most plants have multiple so always an interesting target for the “OT certified” threat actor. Centrifugal compressors increase the kinetic energy of for example a gas into a pressure so a gas flow through pipelines is created either to transfer a product through the various stages of the production process or perhaps to create energy for opening / closing pneumatic driven valves.

Transient operations, for example the start-up and shutdown of process equipment, always have dangers that need to be addressed. An emergency shutdown because there occurred in the plant a condition that demanded the SIS to transfer the plant to a safe state, is such a transient operation. But in this case unplanned and in principle fully automated, no process operator to guard the process and correct where needed. The human factor is not considered a very reliable factor in functional safety and is often just too slow. SIS on the other hand is reliable, the redundancy and the continuous diagnostic checks all warrant a very low failure on demand probability for SIL 2 and SIL 3 installations. They are designed to perform when needed, no excuses allowed. But this is only so if the program logic is not tampered with, the sequence of actions must be performed as designed and is systematically tested after each change.

Compressors are controlled by compressor control systems (CCS), one of the many sub-systems in an ICS. The main task of a compressor control system is anti surge control. The surge phenomenon in a centrifugal compressor is a complete breakdown and reversal of the flow through the compressor. A surge causes physical damage to the compressor and pipelines because of the huge forces released if a surge occurs. Depending on the gas this can also lead to explosions and loss of containment.

An anti surge controller of the CCS continuously calculates the surge limit (which is dynamic) and controls the compressor operation to stay away from this point of danger. This all works fine during normal operation, however when an emergency shutdown occurs the basic anti surge control provided by the CCS has shown to be insufficient to prevent a surge. In order to improve the response and prevent a surge, the process engineer has two design options called a hot bypass method or a cold bypass method recycling the gas to allow for a more gradual shutdown. The hot bypass is mostly used because of its closeness to the compressor which results into a more direct response. Such a hot bypass method requires to open some valves to feed the gas back to the compressor, this action is implemented as a task of the ESD function. The quantity of gas that can be recycled has a limit, so it is not just opening the bypass valve to 100% but opening it with the right amount. Errors in this process or a too slow reaction would easily result into a surge, damaging the compressor, potentially rupturing connected pipes, causing loss of containment, perhaps resulting in fire and explosions, and potentially resulting in casualties and a long production stop with high repair cost.

All of this is under control of the logic solver application part of the SIS. If the TRISIS attacker’s would have succeeded into loading altered application logic, they would have been capable of causing physical damage to the production installation, damage that could have caused loss of life.

So my conclusion differs a bit, an attack on a SIS can lead to physical damage when the logic is altered, which can result in loss of life. A few changes in the logic and the initiation of the shutdown action would have been enough to accomplish this.

This is just one example of a cyber security hazard in a plant, multiple examples exist showing that the SIS by itself can cause serious incidents. But this blog is not supposed to be a training for certified OT cyber terrorist so I keep it with this for safety engineers well known example.

Proper cyber security requires proper cyber security hazard identification and hazard risk analysis. This has too little focus and is sometimes executed at a level of detail insufficient to identify the real risks in a plant.

I don’t want to criticize the work of others, but do want to emphasize that OT security is a specialism not a variation on IT security. ISA published a book “Security PHA Review” written by Edwar Marsazal and Jim MgClone which addresses the subject of secure safety systems in a for me far too simplified manner by basically focusing on an analysis / review of the process safety hazop sheet to identify cyber related hazards.

The process safety hazop doesn’t contain the level of detail required for a proper analysis, neither does the process safety hazop process assume malicious intent. One valve may fail, but multiple valves at the same time in a specific sequence is very unlikely and not considered. While these options are fully open to the threat actor with a plan.

Proper risk analysis starts with identifying the systems and sub-systems of an ICS, than identifying cyber security hazards in these systems, identifying which functional deviations can result from these hazards, and than translate how these functional deviations can impact the production system. That is much more than a review of a process hazop sheet on “hackable” safeguards and causes. That type of security risk analysis requires OT security specialists with a detailed knowledge on how these systems work, what their functional tasks are, and the imagination and dose of badness to manipulate these systems in a way that is beneficial for an attacker .

Sinclair Koelemij

2 thoughts on “TRISIS revisited”

  1. Good explanation Sinclair,

    We need more people in the field of ics security with the amount of hands on experience you provide. I’m not saying Dragos/Joe are devoid of experience, but they cannot provide the same level of nuance.

Geef een reactie