This book constitutes the refereed proceedings of the 21st  International Symposium on Research in Attacks, Intrusions, and Defenses, RAID 2018, held in Heraklion, Crete, Greece, in September 2018.The 32 revised full papers were carefully reviewed and selected from 145 submissions. They are organized in the following topical sections: attacks; intrusion detection and prevention; DDoS attacks; passwords, accounts, and users; machine learning for computer security; hardware-assisted security; software security; malware; IoT/CPS security; security measurements; and defenses.