Probabilities – do they have a place in cyber security?

People frequently (heh, heh) define risk as the product of consequences and probabilities of uncertain or stochastic events:


This has led to a range of risk metrics and ways to compare risk with acceptance criteria. For technical safety risk, there is typically an acceptable frequency defined per consequence category. In a risk analysis, catastrophic events may for example be acceptable with a frequency no higher than one event per one million years. Recently, cyber security risk assessments have become more common also for operators of technical systems, such as waste water treatment plants, intelligent homes or oil and gas production rigs. Obviously, cyber threats are real and intruders can hit you just as hard as a random hardware failure – such as the steel mill in Germany that was hit by an advanced cyber attack originating on an external network. Asset owners have slowly come to realize this – and some of them are trying to use their established risk management frameworks also for cyber threats – only to discover that is really hard to do that. Why is this not a good approach?

Consider the following event scenario: intruder manages to steer your update call to a rogue patch server by a fake internal DNS server, pushing a corrupt Windows patch to your system, thereby gaining admin access to your engineering workstations… how do you assess the frequency of this scenario? Once every 1 million years? Weekly? Obviously, applying the concept of “frequency” as a measure of probability is not the best of ideas in this domain. This has led many people to think “probabilities cannot be used for cyber risks”. This again begs the question – if you do not know if a scenario is likely or extremely unlikely to occur – how do you decide how to allocate your resources when designing counter measures?

What if an attacker can take control over all your port cranes? When deciding which counter-measures to take, you would most likely like to know if such an attack is highly credible or merely a remote probability (all puns intended.. J )! (Photo taken in the harbor of Hamburg, May 2015).

A more reasonable concept is maybe to consider how “credible” a scenario is – rather than a frequency of occurrence. This is also how academics tend to look at the topic; we can rank threat scenarios according to how credible they are. An interesting thing that separates risks related to targeted cyber attacks from typical technical risk analysis is that the credibility (or probability) of an attack taking place is not independent of the consequence of this event; the reason the attack occurs is that somebody wants to hurt you. They may be more motivated if the consequences are worse for you! Things to consider when deciding if an attack or a threat is credible are:

  • Who is the attacker?
  • What is the motivation of the attacker?
  • Does the attacker have the necessary resources (money, people, technical equipment, etc)?
  • Does the attacker have a positive cost-benefit relationship (as seen from the bad guy’s point of view)?
  • Does the attacker have the necessary skills or access to the necessary skills to perform the attack?

Armed with this, you should be able to form a reasonably well informed estimate of the probability – or credibility – of an identified attack such as the one above.

I will be taking more about this at the next ESREL conference, together with colleagues from LR! Maybe I’ll see you in Zürich in September?

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