The Cost of Trust in MEV-Boost's Outsourced Block Production

MEV-Boost's centralization trade-off was a necessary evil for post-Merge survival. It outsourced block building to a competitive market of specialized builders like Flashbots and bloXroute, but it required validators to trust these third parties to deliver valid blocks.

The trust is not cryptographic. Validators blindly sign block headers proposed by builders, creating a relayer cartel that acts as a single point of censorship and failure. This is a regression from Ethereum's original trust-minimized design.

Evidence: Over 90% of Ethereum blocks are built via MEV-Boost. The proposer-builder separation (PBS) model it popularized is now the industry standard, but its trusted relay layer is the protocol's most glaring vulnerability.

MEV-Boost centralizes trust in a handful of professional builders like Flashbots, bloXroute, and Builder0x69. Validators outsource block construction to these entities to maximize profits, creating a trusted third-party dependency for the entire network's security.

The system creates a cartel where builders control transaction ordering and can censor or front-run. This outsourced block production model reintroduces the exact centralization risks that proof-of-stake was designed to mitigate.

Users pay a hidden tax through extracted MEV and higher gas fees. The value that should accrue to stakers and users is instead captured by the builder-searcher supply chain, making Ethereum more expensive and less fair.

Evidence: Over 90% of Ethereum blocks are built via MEV-Boost. This monoculture dependency means a bug or malicious actor in a major builder like Flashbots could compromise chain liveness, as seen in past builder-level censorship incidents.

Relays are centralized trust points. MEV-Boost outsources block building to specialized searchers and proposers rely on a handful of relay operators to deliver valid blocks. This creates a single point of censorship and failure, contradicting Ethereum's core ethos.

Censorship risk is the primary vector. Relays like BloXroute and Flashbots enforce OFAC compliance, filtering transactions from sanctioned addresses. This regulatory capture creates a two-tiered network where some users are excluded from base-layer inclusion.

Liveness risk is an economic attack. A malicious or faulty relay can withhold blocks, causing a validator to miss its slot and lose rewards. The economic incentive to avoid penalties forces validators to trust relay uptime implicitly.

Centralization risk creates cartel dynamics. The top three relays consistently control over 90% of MEV-Boost blocks. This market concentration allows for potential collusion on fee extraction or the creation of exclusive order flow deals, mirroring issues in traditional finance.

Evidence: In Q1 2024, over 95% of Ethereum blocks were built via MEV-Boost, with the majority flowing through fewer than five major relay operators, creating a fragile and centralized supply chain for block production.

Relayers are trusted intermediaries. They see the full block contents before builders, creating a single point of failure for censorship and front-running. This architecture mirrors the trusted operator model in intent-based bridges like Across or CoW Swap.

Builders centralize capital. The competitive advantage in PBS accrues to builders with the most capital and sophisticated algorithms, leading to dominance by entities like Flashbots and bloXroute. This creates an oligopoly in block production.

Proposer-Builder Separation (PBS) is incomplete. The current implementation outsources trust but not verification. A validator must still trust the relay's attestations about block validity and payment, unlike a fully-encrypted mempool design.

Evidence: As of 2024, over 90% of Ethereum blocks are built by just five entities, and three relays facilitate the vast majority of this flow. This concentration is the direct, measurable cost of the trust model.

MEV-Boost's trust model is a temporary scaffold. It outsources block production to a competitive but opaque market of builders and relays, creating a centralization vector that contradicts Ethereum's core ethos. The current system trades decentralization for short-term efficiency gains.

The endgame is permissionless PBS. Protocols like SUAVE (Single Unified Auction for Value Expression) aim to decentralize the block building market itself. This shifts trust from a few entities to a cryptoeconomic mechanism, where builders compete in a transparent, on-chain auction.

Builder centralization is the primary bottleneck. Today, a duopoly of builders (e.g., beaverbuild, Titan) consistently wins the majority of blocks. This concentration risks censorship and creates a single point of failure that future designs must structurally dismantle.

Evidence: In Q1 2024, the top two MEV-Boost builders produced over 60% of post-merge blocks. This metric quantifies the trusted third-party risk that permissionless PBS and protocols like EigenLayer restaking for relay security are designed to mitigate.