  Influencing (and being influenced by) others indirectly through social
networks is fundamental to all human societies. Whether this happens through
the diffusion of rumors, viruses, opinions, or know-how, finding the source is
of persistent interest to people and an algorithmic challenge of much current
research interest. However, no study has considered the case of diffusion
sources actively trying to avoid detection. By disregarding this assumption, we
risk conflating intentional obfuscation from the fundamental limitations of
source-finding algorithms. We close this gap by separating two mechanisms
hiding diffusion sources-one stemming from the network topology itself and the
other from strategic manipulation of the network. We find that identifying the
source can be challenging even without foul play and, many times, it is easy to
evade source-detection algorithms further. We show that hiding connections that
were part of the viral cascade is far more effective than introducing fake
individuals. Thus, efforts should focus on exposing concealed ties rather than
planted fake entities, e.g., bots in social media; such exposure would
drastically improve our chances of detecting the source of a social diffusion.
