This two-volume set LNCS 15992-15993 constitutes the proceedings of the 20th International Conference on Availability, Reliability and Security, ARES 2025, in Ghent, Belgium, during August 11-14, 2025. The 34 full papers presented in this book together with 8 short papers were carefully reviewed and selected from 186 submissions. They cover topics such as: Privacy-Enhancing Technologies and Legal Compliance; Network and Communication Security; IoT and Embedded Systems Security; Machine Learning and Privacy; Usable Security and Awareness; System Security; Supply Chain Security, Malware and Forensics; and Machine Learning and Security.