  The Lightning Network promises to alleviate Bitcoin's known scalability
problems. The operation of such second layer approaches relies on the ability
of participants to turn to the blockchain to claim funds at any time, which is
assumed to happen rarely. One of the risks that was identified early on is that
of a wide systemic attack on the protocol, in which an attacker triggers the
closure of many Lightning channels at once. The resulting high volume of
transactions in the blockchain will not allow for the proper settlement of all
debts, and attackers may get away with stealing some funds. This paper explores
the details of such an attack and evaluates its cost and overall impact on
Bitcoin and the Lightning Network. Specifically, we show that an attacker is
able to simultaneously cause victim nodes to overload the Bitcoin blockchain
with requests and to steal funds that were locked in channels. We go on to
examine the interaction of Lightning nodes with the fee estimation mechanism
and show that the attacker can continuously lower the fee of transactions that
will later be used by the victim in its attempts to recover funds - eventually
reaching a state in which only low fractions of the block are available for
lightning transactions. Our attack is made easier even further as the Lightning
protocol allows the attacker to increase the fee offered by his own
transactions. We continue to empirically show that the vast majority of nodes
agree to channel opening requests from unknown sources and are therefore
susceptible to this attack. We highlight differences between various
implementations of the Lightning Network protocol and review the susceptibility
of each one to the attack. Finally, we propose mitigation strategies to lower
the systemic attack risk of the network.
