This book constitutes the proceedings of the 20th International Conference on Detection of Intrusions and Malware, and Vulnerability Assessment, DIMVA 2023, held in Hamburg, Germany, in July 2023. The 12 full papers and 1 short paper presented in this volume were carefully reviewed and selected from 43 submissions. The papers are organized in thematical sections named: Side Channels Attacks; Security and Machine Learning; Cyber Physical System Security; Security Issues when Dealing with Users; Analysis of Vulnerable Code; Flow Integrity and Security.