This book constitutes the proceedings of the 21st International Conference on Detection of Intrusions and Malware, and Vulnerability Assessment, DIMVA 2024, held in Lausanne, Switzerland, during July 17–19, 2024. The 22 full papers and 6 short paper presented in this volume were carefully reviewed and selected from 110 submissions. The papers are organized in thematical sections named: vulnerability detection and defense; malware and threats; mobile and web application security; AI for security; hardware and firmware security; cyber physical systems and IoT.