Security intrusions as mechanisms

The practice of security often revolves around figuring out what (malicious act) happened to a system. This historical inquiry is the focus of forensics, specifically when the inquiry regards a policy violation (such as a law). The results of forensic investigation might be used to fix the impacted system, attribute the attack to adversaries, or build more resilient systems going forwards. However, to execute any of these purposes, the investigator first must discover the mechanism of the intrusion.

As discussed at an ACE seminar last October, one common framework for this discovery task is the intrusion kill chain. Mechanisms, mechanistic explanation, and mechanism discovery have highly-developed meanings in the biological and social sciences, but the word is not often used in information security. In a recent paper, we argue that incident response and forensics investigators would be well-served to make use of the existing literature on mechanisms, as thinking about intrusion kill chains as mechanisms is a productive and useful way to frame the work.

To some extent, thinking mechanistically is a description of what (certain) scientists do. But the mechanisms literature within philosophy of science is not merely descriptive. The normative benefits extolled include that thinking mechanistically is an effective heuristic for searching out useful explanations; mechanisms provide the most coherent unity to complex fields of study; and that mechanistic explanation is necessary to guide selection among potential studies given limited experimental resources, experiment design decisions, and interpretation of statistical results. I previously argued that capricious use of biological metaphors is bad for information security. We are keenly aware that these benefits of mechanistic explanation need to apply to security as and for security, not merely because they work in other sciences.

Our paper demonstrates how we can cast the intrusion kill chain, the diamond model, and other models of security intrusions as mechanistic models. This casting begins to demonstrate the mosaic unity of information security. Campaigns are made up of attacks. Attacks, as modeled by the kill chain, have multiple steps. In a specific attack, the delivery step might be accomplished by a drive-by-download. So we demonstrate how drive-by-downloads are a mechanism, one among many possible delivery mechanisms. This description is a schema to be filled in during a particular drive-by download incident with a specific URL and specific javascript, etc. The mechanistic schema of the delivery mechanism informs the investigator because it indicates what types of network addresses to look for, and how to fit them into the explanation quickly. This process is what Lindley Darden calls schema instantiation in the mechanism discovery literature.

Our argument is not that good forensics investigators do not do such mechanism discovery strategies. Rather, it is precisely that good investigators do do them. But we need to describe what it is good investigators in fact do. We do not currently, and that lack makes teaching new investigators particularly difficult. Thinking about intrusions as mechanisms unlocks an expansive literature on good ways to do mechanism discovery. This literature will make it easier to codify what good investigators do, which among other benefits allows us to better teach sound methodological practices to incoming investigators.

Our paper on this topic was published in the open-access Journal of Cybersecurity, as Thinking about intrusion kill chains as mechanisms, by Jonathan M. Spring and Eric Hatleback.

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