Passive network monitors, known as telescopes or darknets, have been invaluable in detecting and characterizing malware outbreaks. However, as the use of such monitors becomes commonplace, it is likely that malware will evolve to actively detect and evade them. This paper highlights the threat of simple, yet effective, evasive attacks that undermine the usefulness of passive monitors. Our results raise an alarm to the research and operational communities to take proactive countermeasures before we are forced to defend against similar attacks appearing in the wild. Specifically, we show how lightweight, coordinated sampling of the IP address space can be used to successfully detect and evade passive network monitors. Equally troubling is the fact that in doing so attackers can locate the “live” IP space clusters and divert malware scanning solely toward active networks. We show that evasive attacks exploiting this knowledge are also extremely fast, overtaking the entire vulnerable population within seconds.