  Similar to what happens between humans in the real world, in open multi-agent
systems distributed over the Internet, such as online social networks or wiki
technologies, agents often form coalitions by agreeing to act as a whole in
order to achieve certain common goals. However, agent coalitions are not always
a desirable feature of a system, as malicious or corrupt agents may collaborate
in order to subvert or attack the system. In this paper, we consider the
problem of hidden coalitions, whose existence and the purposes they aim to
achieve are not known to the system, and which carry out so-called underhand
attacks. We give a first approach to hidden coalitions by introducing a
deterministic method that blocks the actions of potentially dangerous agents,
i.e. possibly belonging to such coalitions. We also give a non-deterministic
version of this method that blocks the smallest set of potentially dangerous
agents. We calculate the computational cost of our two blocking methods, and
prove their soundness and completeness.
