This book constitutes the refereed proceedings of the 18th Nordic Conference on Secure IT Systems, NordSec 2013, held in Ilulissat, Greenland, in October 2013. The 18 revised regular papers together with 3 short papers and one invited talk were carefully reviewed and selected from 35 submissions. The papers are organized in topical sections on formal analysis of security prools, cyber-physical systems, security policies, information flow, security experiences, Web security, and network security.