  Popular Internet services in recent years have shown that remarkable things
can be achieved by harnessing the power of the masses using crowd-sourcing
systems. However, crowd-sourcing systems can also pose a real challenge to
existing security mechanisms deployed to protect Internet services. Many of
these techniques make the assumption that malicious activity is generated
automatically by machines, and perform poorly or fail if users can be organized
to perform malicious tasks using crowd-sourcing systems. Through measurements,
we have found surprising evidence showing that not only do malicious
crowd-sourcing systems exist, but they are rapidly growing in both user base
and total revenue. In this paper, we describe a significant effort to study and
understand these "crowdturfing" systems in today's Internet. We use detailed
crawls to extract data about the size and operational structure of these
crowdturfing systems. We analyze details of campaigns offered and performed in
these sites, and evaluate their end-to-end effectiveness by running active,
non-malicious campaigns of our own. Finally, we study and compare the source of
workers on crowdturfing sites in different countries. Our results suggest that
campaigns on these systems are highly effective at reaching users, and their
continuing growth poses a concrete threat to online communities such as social
networks, both in the US and elsewhere.
