An attack against the Helios election system that exploits re-voting

Election systems must ensure that representatives are chosen by voters. Moreover, each voter should have equal influence. Traditionally, this has been achieved by permitting voters to cast at most one ballot. More recently, this has been achieved by counting the last ballot cast by each voter. We show that the Helios election system fails to achieve this, because an adversary can cause a ballot other than a voter’s last to be counted. Moreover, we show how the adversary can choose the contents of such ballots, thus the adversary can unduly influence the selection of representatives.