  We present two attacks targeting the Proof-of-Stake (PoS) Ethereum consensus
protocol. The first attack suggests a fundamental conceptual incompatibility
between PoS and the Greedy Heaviest-Observed Sub-Tree (GHOST) fork choice
paradigm employed by PoS Ethereum. In a nutshell, PoS allows an adversary with
a vanishing amount of stake to produce an unlimited number of equivocating
blocks. While most equivocating blocks will be orphaned, such orphaned `uncle
blocks' still influence fork choice under the GHOST paradigm, bestowing upon
the adversary devastating control over the canonical chain. While the Latest
Message Driven (LMD) aspect of current PoS Ethereum prevents a straightforward
application of this attack, our second attack shows how LMD specifically can be
exploited to obtain a new variant of the balancing attack that overcomes a
recent protocol addition that was intended to mitigate balancing-type attacks.
Thus, in its current form, PoS Ethereum without and with LMD is vulnerable to
our first and second attack, respectively.
