BitTorrent is a widely deployed P2P file sharing protocol, extensively used to distribute digital content and software updates, among others. Recent actions against torrent and tracker repositories have fostered the move towards a fully distributed solution based on a distributed hash table to support both torrent search and tracker implementation. In this paper we present a security study of the main decentralized tracker in BitTorrent, commonly known as the Mainline DHT.We show that the lack of security in Mainline DHT allows very efficient attacks that can easily impact the operation of the whole network. We also provide a peer-ID distribution analysis of the network, so as to adapt previous protection schemes to the Mainline DHT. The mechanisms are assessed through large scale experiments on the real DHT-based BitTorrent tracker.