  We present a way to combine security and safety assessments using Bowtie
Diagrams. Bowties model both the causes leading up to a central failure event
and consequences which arise from that event, as well as barriers which impede
events. Bowties have previously been used separately for security and safety
assessments, but we suggest that a unified treatment in a single model can
elegantly capture safety-security interdependencies of several kinds. We
showcase our approach with the example of the October 2021 Facebook DNS
shutdown, examining the chains of events and the interplay between the security
and safety barriers which caused the outage.
