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.