This book constitutes the refereed proceedings of the 26th IFIP TC 11 International Information Security Conference, SEC 2011, held in Lucerne, Switzerland, in June 2011. The 24 revised full papers presented together with a keynote talk were carefully reviewed and selected from 100 submissions. The papers are organized in topical sections on malware, information flow and DoS attacks, authentication, network security and security protocols, software security, policy compliance and obligations, privacy attacks and privacy-enhancing technologies, risk analysis and security metrics, and intrusion detection.