The two-volume set, LNCS 11098 and LNCS 11099 constitutes the refereed proceedings of the 23nd European Symposium on Research in Computer Security, ESORICS 2018, held in Barcelona, Spain, in September 2018. The 56 revised full papers presented were carefully reviewed and selected from 283 submissions. The papers address issues such as software security, blockchain and machine learning, hardware security, attacks, malware and vulnerabilities, protocol security, privacy, CPS and IoT security, mobile security, database and web security, cloud security, applied crypto, multi-party computation, SDN security.