From data leaks of sensitive information to the degrading or halting of complete systems upon which integral societal process rely, there are daily reminders of the precariousness of networked institutions. This form of vulnerability extends beyond services to also threaten foundational values, such as privacy, movement, and free speech. Yet, given the awareness of vulnerability to cyber disruption, little is known about how massive cyber failures might disrupt the lives of ordinary people at a micro-scale. Because technology is imbued in everyday life, and people use internet systems in bundled, complex ways to accomplish a myriad of unknown activities, a complicated web of cyber-vulnerability likely exists. Thus, the goal of this paper is to explore one dimension of this vulnerability as it pertains to public security. Terrorism is predicted to remain an urban phenomenon, and terrorists are increasingly exploiting cyber-systems. The aims of terrorism to disrupt democratic systems are perfectly achieved by cyber disruption, and this paper explores how those goals were facilitated during the Boston Marathon Bombing in 2013. By drawing on interviews with public safety officials and ordinary people who relied on the internet and linked technology during that emergency, I demonstrate the complex ways that cyber-systems limited the effectiveness of public security in times of terror. The results suggest a rethinking of the social amplification of risk paradigm that dominates in risk hazards research as well as several policy interventions in security communication and information dissemination, population management during crisis, and resilience.