To satisfy security requirements, software architects often adopt security tactics. These architectural tactics provide mechanisms for resisting, detecting, reacting to and recovering from attacks. Consequently, flaws in the implementation of security tactics or their deterioration during software evolution and maintenance can introduce severe vulnerabilities that could be exploited by attackers. However, we currently lack an in-depth understanding of the types and impact of vulnerabilities related to security tactics. Therefore, in this paper, we conduct a first-of-its-kind in-depth case study involving three large-scale open-source systems. We investigate the most common types of vulnerabilities associated with security tactics, how frequently they may occur over time, and how fixing them differs from fixing vulnerabilities that are not related to security tactics. Key findings are (i) most tactic-related vulnerabilities were related to the tactics “Validate Inputs” and “Authorize Actors”, (ii) vulnerabilities related to tactics have a similar distribution over time and software releases as vulnerabilities that are not related to tactics, (iii) fixing tactic-related vulnerabilities is not necessarily more complex than fixing vulnerabilities that are not related to security tactics. This study highlights the importance of ensuring an appropriate implementation of security-related design decisions in code to avoid vulnerabilities rooted in the architecture.