  In shilling attacks, an adversarial party injects a few fake user profiles
into a Recommender System (RS) so that the target item can be promoted or
demoted. Although much effort has been devoted to developing shilling attack
methods, we find that existing approaches are still far from practical. In this
paper, we analyze the properties a practical shilling attack method should have
and propose a new concept of Cross-system Attack. With the idea of Cross-system
Attack, we design a Practical Cross-system Shilling Attack (PC-Attack)
framework that requires little information about the victim RS model and the
target RS data for conducting attacks. PC-Attack is trained to capture graph
topology knowledge from public RS data in a self-supervised manner. Then, it is
fine-tuned on a small portion of target data that is easy to access to
construct fake profiles. Extensive experiments have demonstrated the
superiority of PC-Attack over state-of-the-art baselines. Our implementation of
PC-Attack is available at https://github.com/KDEGroup/PC-Attack.
