  Off-chain transaction networks can mitigate the scalability issues of today's
trustless electronic cash systems such as Bitcoin. However, these peer-to-peer
networks also introduce a new attack surface which is not well-understood
today. This paper identifies and analyzes, a novel Denial-of-Service attack
which is based on route hijacking, i.e., which exploits the way transactions
are routed and executed along the created channels of the network. This attack
is conceptually interesting as even a limited attacker that manipulates the
topology through the creation of new channels can navigate tradeoffs related to
the way it attacks the network. Furthermore, the attack also highlights a
fundamental design tradeoff for the defender (who determines its own routes):
to become less predictable and hence secure, a rational node has to pay higher
fees to nodes that forward its payments. We find that the three most common
implementations for payment channels in Bitcoin (lnd, C-lightning, Eclair)
approach routing differently. We begin by surveying the current state of the
Lightning network and explore the routes chosen by these implementations. We
find that in the current network nearly 60\% of all routes pass through only
five nodes, while 80\% go through only 10 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. 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.
  We discuss the differences between implementations and eventually derive our
own suggested routing policy, which is based on a novel combination of existing
approaches.
