The 15th International Conference on Network and System Security (NSS 2021) was held on October 23, 2021, in Tianjin, China. The conference ended up being a hybrid conference (in-person and via Zoom) due to the ongoing COVID-19 pandemic. The conference accepted 16 full papers and 8 short papers from a total of 62 submissions, giving an acceptance rate of 38.7%. The papers ranged in subjects both theoretical and practical but with a focus on network and system security. Papers included results on authentication, access control, privacy, and confidentiality, as well the larger question of the dependability and sustainability of computer networks and systems. The keynote presentations were particularly interesting showing both the overwhelming power that attacks on IoT devices have as well as the frankly absurd amount of information mobile and smart devices are actually able to make use of.