  Online advertising is currently the greatest source of revenue for many
Internet giants. The increased number of specialized websites and modern
profiling techniques, have all contributed to an explosion of the income of ad
brokers from online advertising. The single biggest threat to this growth, is
however, click-fraud. Trained botnets and even individuals are hired by
click-fraud specialists in order to maximize the revenue of certain users from
the ads they publish on their websites, or to launch an attack between
competing businesses.
  In this note we wish to raise the awareness of the networking research
community on potential research areas within this emerging field. As an example
strategy, we present Bluff ads; a class of ads that join forces in order to
increase the effort level for click-fraud spammers. Bluff ads are either
targeted ads, with irrelevant display text, or highly relevant display text,
with irrelevant targeting information. They act as a litmus test for the
legitimacy of the individual clicking on the ads. Together with standard
threshold-based methods, fake ads help to decrease click-fraud levels.
