Decentralization of Ethereum's Builder Market

Blockchains protect an ecosystem worth more than $500bn with their strong security properties derived from the principle of decentralization. Is today's blockchain really decentralized? In this paper, we empirically studied one of the {\em least decentralized} parts of Ethereum -- the most used blockchain system in practice -- and shed light on the decentralization issue from a new perspective. To avoid centralization caused by Maximal Extractable Value (MEV), Ethereum adopts a novel mechanism that produces blocks through a {\em builder market}. After two years in operation, however, the builder market has evolved to a highly centralized one with three builders producing more than 90% of blocks. {\em Why does the builder market centralize, given that it is permissionless and anyone can join?} Moreover, {\em what are the security implications of a centralized builder market to MEV-Boost auctions?} Through a rigorous empirical study of the builder market's core mechanism, MEV-Boost auctions, we answered these two questions using a large-scale auction dataset we curated since 2022. Unlike previous works that focus on {\em who} wins the auctions, we focus on {\em why} they win, to shed light on the {openness, competitiveness, and efficiency} of MEV-Boost auctions. Our findings also help identify directions for improving the decentralization of builder markets.