This book constitutes the revised selected papers from the 12th International Conference on Risk and Security of Internet and Systems, CRISIS 2017, held in Dinard, France, in September 2017. The 12 full papers and 5 short papers presented in this volume were carefully reviewed and selected from 42 submissions. They cover diverse research themes, ranging from classic topics, such as vulnerability analysis and classification; apps security; access control and filtering; cloud security; cyber-insurance and cyber threat intelligence; human-centric security and trust; and risk analysis.