There are some fun systemic probabilistic attacks based on peer discovery. One such principle which doesn't need many attacking nodes, rely on peers self-clust

There are some fun systemic probabilistic attacks based on peer discovery. One such principle which doesn't need many attacking nodes, rely on peers self-clustering themselves over time is the following : You divide the peer network in 2 groups. Using a few number of attacking nodes, when a peer from one group ask you for a node you know, you answer with a peer from the same group. Then each peer from each group is more likely to recommend a peer from the same group. Over time bias accumulate, and cluster forms, with very few bridges between the two groups which you control. Obviously it can be mitigated easily by adding some hard-coded "central" nodes or monitoring the network. Afaik robust peer discovery in p2p network is still not solved.