Abstract. Ensuring security of complex systems is a difficult task that requires utilization of numerous tools originating from various domains. Among those tools we find attack-defense trees, a simple yet practical model for analysis of scenarios involving two competing parties. Enhancing the well-established model of attack trees, attack-defense trees are trees with labeled nodes, offering an intuitive representation of possible ways in which an attacker can harm a system, and means of countering the attacks that are available to the defender. The growing palette of methods for quantitative analysis of attack-defense trees provides security experts with tools for determining the most threatening attacks and the best ways of securing the system against those attacks. Unfortunately, many of those methods might fail or provide the user with distorted results if the underlying attack-defense tree contains multiple nodes bearing the same label. We address this issue by studying conditions ensuring that the standard bottom-up evaluation method for quantifying attack-defense trees yields meaningful results in the presence of repeated labels. For the case when those conditions are not satisfied, we devise an alternative approach for quantification of attacks.

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