Greedy Attack: Breaking Finality against VeChain Proof-of-Authority Consensus Protocol

Finality gadget is a critical component in modern blockchain systems. Ever since Ethereum used the Casper finality gadget, many variants of finality approaches have been adopted to make it possible to finalize blocks, i.e., when a block is finalized, its order will never be reversed. In this work, we study the finality approach by VeChainThor, the consensus protocol of VeChain blockchain. We propose greedy attack, an attack in which a block proposed by an honest sealer can be finalized with a probability of only 0.19%. We validate our results via theoretical analysis and extensive experimentation. Our findings have been disclosed and acknowledged by the VeChain team, and we are currently working with them to improve the VeChainThor protocol.