This book constitutes the refereed proceedings of the Second International Symposium on Cyber Security, CSS 2015, held in Coeur d'Alene, ID, USA, in April 2015. The 9 revised full papers presented were carefully reviewed and selected from 20 papers. The papers reflect four areas of scholarly work: permissions and trust evaluation, implementation and management; cloud and device security and privacy; social implications of networked and mobile applications; system and process assessments for improved cybersecurity.