Off-chain transaction networks can mitigate the scalability issues of today’s trustless blockchain systems such as Bitcoin. However, these peer-to-peer networks also introduce a new attack surface which is not yet fully understood. This paper identifies and analyzes a novel type of Denial-of-Service attack which is based on attracting routes, i.e., which exploits the way transactions are routed and executed along the channels of the network in order to attract nodes to route through the attacker. This attack is conceptually interesting as it highlights a fundamental design tradeoff for the defender (who determines its own routes): to become less susceptible to hijacking, a rational node has to pay higher fees to nodes that forward its payments. We focus on the Lightning network, and we investigate both the structure of the network using real data collected over an extended period of time, and the routing algorithms of different implementations. We find that the three most common implementations (lnd, C-lightning, Eclair) approach routing differently. We then explore the routes chosen by these implementations in the current Lightning network. We find that very few nodes route most of the traffic: nearly 60% of all routes pass through only five nodes, while 80% go through only 15 nodes. Thus, a relatively small number of colluding nodes can deny service to a large fraction of the network. We then turn to study an external attacker who creates links to the network and draws more routes through its nodes by asking for lower fees. While finding the optimal set of links to create is NP-complete, we show that using a greedy attack strategy, an attacker can obtain a 1 − 1/e approximation. Given this strategy, we find that just five new links are enough to draw the majority (65% - 75%) of the traffic regardless of the implementation being used. The cost of creating these links is very low. Drawing this traffic, the attacker is then able to deny service to all those who route through it. We further show that newer routing algorithms recently introduced into lnd, to penalize routes that failed in the past and avoid selecting them again, do not effectively prevent such attacks. Finally, we suggest modified routing policies, which may help alleviate the problem.

Top