In mid-November 2017, the Dragos, Inc. team discovered ICS-tailored malware deployed against at least one victim in the Middle East. The team identifies this malware as TRISIS because it targets Schneider Electric’s Triconex safety instrumented system (SIS) enabling the replacement of logic in final control elements. TRISIS is highly targeted and likely does not pose an immediate threat to other Schneider Electric customers, let alone other SIS products. Importantly, the malware leverages no inherent vulnerability in Schneider Electric products. However, this capability, methodology, and tradecraft in this very specific event may now be replicated by other adversaries and thus represents an addition to industrial asset owner and operators’ threat models.