Two More Attacks on Proof-of-Stake GHOST/Ethereum.

Ethereum, the world's second largest cryptocurrency with a market capitalization exceeding 120 billion USD as of this writing, aims to switch from Proof-of-Work (PoW) to Proof-of-Stake (PoS) based consensus later in the year 2022 (`the Merge'). Yet, so far, the proposed PoS consensus protocol lacks in rigorous security analysis. We present two new attack strategies targeting the 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 'proposer boosting', 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.