  The Lightning Network (LN) has enjoyed rapid growth over recent years, and
has become the most popular scaling solution for the Bitcoin blockchain. The
security of the LN relies on the ability of the nodes to close a channel by
settling their balances, which requires confirming a transaction on the Bitcoin
blockchain within a pre-agreed time period. We study the susceptibility of the
LN to mass exit attacks in case of high transaction congestion, in the presence
of a small coalition of adversarial nodes that forces a large set of honest
users to interact with the blockchain. We focus on two types of attacks: (i)
The first is a zombie attack, where a set of k nodes become unresponsive with
the goal of locking the funds of many channels for a period of time longer than
what the LN protocol dictates. (ii) The second is a mass double-spend attack,
where a set of k nodes attempt to steal funds by submitting many closing
transactions that settle channels using expired protocol states; this causes
many honest nodes to have to quickly respond by submitting invalidating
transactions. We show via simulations that, under historically plausible
congestion conditions, with mild statistical assumptions on channel balances,
both attacks can be performed by a very small coalition. To perform our
simulations, we formulate the problem of finding a worst-case coalition of k
adversarial nodes as a graph cut problem. Our experimental findings are
supported by theoretical justifications based on the scale-free topology of the
LN.
