This book constitutes the proceedings of the 21st International Conference on Availability, Reliability and Security, ARES 2026, held in Linköping, Sweden, during August 2026.The 35 full papers presented in this book together with 9 short papers and 2 SoK papers were carefully reviewed and selected from 225 submissions.They covered topics such as Network and Communication Security, Traffic Analysis and Intrusion Detection, Security in Hardware, Privacy, Systems Security, Hu...