Two Attacks On Proof-of-Stake GHOST/Ethereum (2203.01315v1)

Abstract: 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.

• The paper reveals how the avalanche attack exploits orphaned blocks in PoS GHOST to amplify adversarial influence.
• The research demonstrates the LMD-specific balancing attack that divides validators and stalls consensus progression.
• The findings underscore critical security flaws, urging protocol revisions to reinforce the integrity of PoS systems.

Analysis of "Two Attacks On Proof-of-Stake GHOST/Ethereum"

This paper presents a detailed analysis and critique of the Proof-of-Stake (PoS) Ethereum consensus protocol, with a particular focus on identifying vulnerabilities associated with the GHOST (Greedy Heaviest-Observed Sub-Tree) fork choice paradigm. It identifies and meticulously describes two distinct attacks—termed the avalanche attack and the LMD-specific balancing attack—that potentially undermine the security assumptions of PoS Ethereum.

Avalanche Attack

The first attack, termed the avalanche attack, highlights a fundamental conceptual incongruity between PoS systems and the GHOST fork choice rule. The core of this issue lies in how GHOST accounts for orphaned uncle blocks within the decision-making for determining the canonical chain. In a PoS setting, an adversary can exploit this by generating an excessive quantity of equivocating blocks that, although orphaned, can significantly influence the fork choice process. This attack underscores the inadequacy of integrating GHOST with PoS as it disproportionately amplifies the influence of an adversary, allowing control over the canonical chain with nominal initial stake. A proof-of-concept implementation reinforces this assertion, illustrating the vulnerability of PoS GHOST and Committee-GHOST under this attack mechanism.

LMD-Specific Balancing Attack

The second vulnerability takes advantage of the Latest Message Driven (LMD) aspect of PoS Ethereum’s fork choice rule—a mechanism specifically implemented to mitigate straightforward equivocation-based attacks. The authors introduce a balancing attack variant that incorporates LMD, effectively overcoming countermeasures such as proposer boosting that were intended to enhance protocol robustness. This attack utilizes strategic equivocation and network propagation dynamics to bifurcate the validator network into two competing subgroups, each perceiving a different blockchain as the longest. This results in a persistent split-view scenario, stalling consensus progression and thereby jeopardizing both safety and liveness guarantees of the protocol.

Implications and Future Directions

The implications of these findings are profound for both the theoretical understanding and practical implementation of PoS-based consensus protocols using GHOST. The paper questions the viability of GHOST as a fork choice mechanism in PoS settings, given the identified security loopholes, necessitating alternative approaches or significant modifications to maintain robust security properties. Additionally, the exploitation of LMD, aimed at enhancing protocol reliability, suggests that further refinements are required to preclude balancing attacks.

The authors contribute to ongoing discussions on PoS security by dovetailing their analysis with preceding works addressing similar attack vectors and vulnerabilities. This research can catalyze further exploration into the design of more resilient PoS systems, potentially driving innovation in consensus protocols beyond the confines of GHOST and LMD mechanisms.

In conclusion, the paper provides a critical evaluation of PoS Ethereum's fork choice rule's adaptability and security under adversarial conditions. It advocates for caution when adopting such paradigms and calls for continued scrutiny and enhancement of PoS systems to uphold their integrity under diverse operational scenarios. This work is a significant touchstone in ongoing research aimed at fortifying consensus mechanisms against sophisticated adversarial strategies.