  Payment channel networks provide a fast and scalable solution to relay funds,
acting as a second layer to slower and less scalable blockchain protocols. In
this paper, we present an accessible, low-cost attack in which the attacker
paralyzes multiple payment network channels for several days. The attack is
based on overloading channels with requests that are kept unresolved until
their expiration time. Reaching the maximum allowed unresolved requests (HTLCs)
locks the channel for new payments. The attack is in fact inherent to the way
off-chain networks are constructed, since limits on the number of unresolved
payments are derived from limits on the blockchain. We consider three main
versions of the attack: one in which the attacker attempts to block as many
high liquidity channels as possible, one in which it disconnects as many pairs
of nodes as it can, and one in which it tries to isolate individual nodes from
the network. We evaluate the costs of these attacks on Bitcoin's Lightning
Network and compare how changes in the network have affected the cost of
attack. Specifically, we consider how recent changes to default parameters in
each of the main Lightning implementations contribute to the attacks. As we
evaluate the attacks, we also look at statistics on parameters in the Lightning
Network, which are of independent interest and compare the various
implementations of Lightning nodes. Finally, we suggest mitigation techniques
that make these attacks much harder to carry out.
