This thesis explores factors influencing system administrators' security vulnerability remediation decisions. Little is known about how system administrators determine when and whether to address security vulnerabilities and how they prioritize their remediation decisions. Herein, we present a preliminary semi-structured interview study with seven system administrators and a large-scale survey study with 124 system administrators working in North America. The findings reveal that factors such as vulnerability severity, administrator's skills and experience, fix complexity, and potential impact on the system greatly influence system administrators' decisions.We also explore the idea of "vulnerability ownership," the system administrator responsible for introducing the vulnerability to the system. We found that the concept of ownership differs based on the type of vulnerability. Our results highlight the importance of system administrators collaborating with their colleagues and external vendors.First and foremost, I would like to express my deepest gratitude to my supervisor, Professor Hala Assal, for her invaluable guidance, expertise, and unwavering support throughout my Master's journey. As an international student, I am incredibly fortunate to have had Professor Assal's patience and kindness, which helped me navigate through academic life and feel welcome in my new home. Her insightful feedback and suggestions have been instrumental in shaping this thesis. I am forever grateful to Professor Assal for choosing me to join her team and for her constant support and encouragement throughout my studies. I could not have wished for a more dedicated and inspiring supervisor.I would also like to thank the distinguished members of my thesis defence committee, Professor Robert Biddle and Professor Adrian Chan, for their valuable time, attention, and invaluable feedback during my defence. Their insightful suggestions and our discussions helped me to improve the quality of this thesis and opened new avenues of research.To my dear husband, Maksym, I cannot thank you enough for your unwavering support, encouragement, and patience throughout the ups and downs of this journey.Your love and persistent belief in me were the driving force behind this achievement.Thank you for not letting me give up during the difficult times and for always cooking me dinner while I was working on this thesis. Your love and support have been my anchor.To my wonderful parents, Olena and Anatolii, and my siblings, Yevheniia and Yehor, I am forever grateful for your unconditional love, sacrifice, and support. Your unwavering belief in me and your constant encouragement have been instrumental in shaping me into the person I am today. Thank you for being my biggest cheerleaders and for always believing in me. You are invaluable to me.Last but not least, I would like to express my heartfelt appreciation to my friend and classmate, Kazma. Your friendship and intellectual curiosity have inspired me to work harder and strive for excellence. Your support an...