  We introduce a system for automatically generating warnings of imminent or
current cyber-threats. Our system leverages the communication of malicious
actors on the darkweb, as well as activity of cyber security experts on social
media platforms like Twitter. In a time period between September, 2016 and
January, 2017, our method generated 661 alerts of which about 84% were relevant
to current or imminent cyber-threats. In the paper, we first illustrate the
rationale and workflow of our system, then we measure its performance. Our
analysis is enriched by two case studies: the first shows how the method could
predict DDoS attacks, and how it would have allowed organizations to prepare
for the Mirai attacks that caused widespread disruption in October 2016.
Second, we discuss the method's timely identification of various instances of
data breaches.
