  A diffusion auction is a market to sell commodities over a social network,
where the challenge is to incentivize existing buyers to invite their neighbors
in the network to join the market. Existing mechanisms have been designed to
solve the challenge in various settings, aiming at desirable properties such as
non-deficiency, incentive compatibility and social welfare maximization. Since
the mechanisms are employed in dynamic networks with ever-changing structures,
buyers could easily generate fake nodes in the network to manipulate the
mechanisms for their own benefits, which is commonly known as the Sybil attack.
We observe that strategic agents may gain an unfair advantage in existing
mechanisms through such attacks. To resist this potential attack, we propose
two diffusion auction mechanisms, the Sybil tax mechanism (STM) and the Sybil
cluster mechanism (SCM), to achieve both Sybil-proofness and incentive
compatibility in the single-item setting. Our proposal provides the first
mechanisms to protect the interests of buyers against Sybil attacks with a mild
sacrifice of social welfare and revenue.
