Alt Mempools bypass public scrutiny. Traditional public mempools broadcast transactions for anyone to see, but private relay networks like BloXroute and Eden Network create a separate, opaque transaction layer. This creates a two-tiered transaction system where priority access is a paid service.
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Why the Alt Mempool Creates a New Censorship Vector
ERC-4337's separate UserOperation mempool introduces a critical, overlooked risk: centralized bundlers and relays can filter transactions based on opaque policies, creating a new attack surface for censorship and undermining Ethereum's core value proposition.
introduction
THE CENSORSHIP VECTOR
Introduction
The alt mempool is a new, unregulated transaction layer that enables sophisticated censorship.
Censorship is now a product. Validators and block builders can programmatically filter transactions based on origin, content, or sender, a service previously requiring manual intervention. This turns transaction inclusion into a policy decision, not a neutral protocol function.
Evidence: The OFAC compliance tools used by Flashbots Protect and Tornado Cash sanctions demonstrate the technical capability for automated filtering. This infrastructure is now repurposed for commercial exclusion within alt mempools.
key-insights
THE CENSORSHIP VECTOR
Executive Summary
The Alt Mempool, while solving MEV and latency, introduces a new centralization risk by concentrating transaction flow through a small set of privileged actors.
01
The Problem: The Dark Forest of Public Mempools
Public mempools expose all pending transactions, creating a toxic environment of frontrunning and sandwich attacks. This forces users to either accept ~$1B+ annual MEV losses or avoid on-chain activity entirely, a form of economic censorship.
$1B+
Annual MEV
100%
Tx Exposure
02
The Solution: Private Order Flow Auctions
Protocols like Flashbots SUAVE, CowSwap, and UniswapX create private channels for transaction submission. This solves frontrunning by routing orders through a sealed-bid auction where searchers compete for inclusion without seeing the transaction contents first.
~0ms
Frontrun Window
>90%
MEV Reduction
03
The New Vector: Centralized Gatekeeping
The 'solution' creates a bottleneck. A handful of block builders (e.g., via mev-boost) and relay operators now control which private transactions enter the chain. This creates a single point of failure for regulatory pressure and transaction filtering, replicating TradFi's choke points.
3-5
Dominant Relays
>80%
Block Share
04
The Architectural Flaw: Intent-Based Abstraction
Systems like UniswapX and Across push users to submit intents (desired outcome) instead of transactions (specific execution). While user-friendly, this abstracts away execution control, handing ultimate censorship power to the solver network that fulfills the intent.
1
Solver Decision
N/A
User Control
05
The Mitigation: Decentralized Solving & Force Inclusion
Countering this requires decentralized solver networks (no single entity can censor) and credibly neutral force inclusion lists (e.g., Ethereum's PBS proposals). The goal is to separate the economic right to build from the moral right to censor.
100+
Solver Nodes
~0%
Censor Power
06
The Bottom Line: A Necessary Trade-Off
The alt mempool is a Pandora's Box—opened to solve MEV, releasing a new censorship demon. The industry's next battle is decentralizing this infrastructure layer. Protocols that own their execution stack (e.g., dYdX Chain) have a temporary advantage, but the long-term solution is political, not technical.
Trade-Off
MEV vs Censorship
Next Battle
Infra Layer
thesis-statement
THE CENSORSHIP VECTOR
The Core Contradiction
The alt-mempool's promise of permissionless access creates a new, centralized point of censorship controlled by searchers and builders.
Searcher-Builder Cartels become the new gatekeepers. In a PBS world, the block builder is the ultimate censor, not the validator. Searchers and builders like Jito Labs and Flashbots control transaction ordering, creating a centralized filtering layer before the chain.
Permissionless Access is Illusory. While anyone can submit a bundle, economic exclusion is the real barrier. A retail user's transaction cannot compete with a searcher's MEV-extracting bundle, guaranteeing censorship through economic irrelevance.
The OFAC Compliance Problem shifts upstream. Validators can claim neutrality, but professional builders running Flashbots Protect or similar services will filter sanctioned addresses to protect their block revenue, enforcing blacklists at the infrastructure layer.
Evidence: Over 90% of Ethereum blocks are built by just five entities. This builder centralization proves the alt-mempool creates a bottleneck more centralized and accountable to regulators than the validator set.
CENSORSHIP VECTORS
Bundler Market Concentration & Risk Profile
Comparison of censorship risks and market dynamics across different mempool and block-building architectures.
| Risk Metric / Feature | Alt Mempool (e.g., SUAVE, Flashbots Protect) | Public Mempool (Status Quo) | Enshrined PBS (Proposer-Builder Separation) |
|---|---|---|---|
Primary Censorship Vector | Bundler/Relayer Cartel Formation | Validator/MEV Searcher Collusion | Builder Monopoly via Exclusive Orderflow |
Market Concentration Risk (Current) | High (P2P.org, BloXroute, etc. dominate) | Medium (Jito Labs, bloXroute, etc.) | Theoretical (No live implementation) |
Time-to-Censor Threshold | < 1 block (via transaction withholding) | 1+ blocks (requires validator complicity) | 1 block (builder-level exclusion) |
User Opt-Out Feasibility | False (Alt mempool is a systemic choice) | True (Users can choose RPC endpoints) | False (System-level architecture) |
Required Colluding Parties | 1 (Dominant Bundler/Relayer) | 2+ (Searcher + Proposer/Validator) | 1 (Dominant Builder) |
Mitigation via CR (Credible Neutrality) | False (Profit motives dominate) | Partially (via MEV-Boost relay diversity) | True (If enshrined and permissionless) |
Historical Precedent | Flashbots dominating >80% MEV-Boost relay market | OFAC sanctions compliance by major validators | N/A |
Key Dependency for Security | Relayer/Bundler decentralization (unproven) | Validator set decentralization (partially proven) | Builder set decentralization (untested) |
deep-dive
THE CENSORSHIP VECTOR
Anatomy of a Filtered Operation
The alt-mempool's permissioned entry point transforms builders into centralized gatekeepers, creating a new, protocol-level censorship vector.
Builder-as-Censor: The alt-mempool's design mandates a single, permissioned builder to receive and order transactions. This centralizes the censorship decision point, moving it from the decentralized validator set to a single entity like Flashbots SUAVE or Jito Labs.
Protocol-Enforced Filtering: Unlike public mempool exclusion, which users can bypass via direct RPC, alt-mempool censorship is structurally enforced. The protocol's own rules prevent non-compliant transactions from ever entering the ordering process, making resistance futile.
Regulatory Capture Incentive: Builders face immense pressure to comply with OFAC sanctions lists or jurisdictional demands. The economic model of proposer-builder separation (PBS) incentivizes compliance over neutrality to maintain block revenue and avoid legal risk.
Evidence: Ethereum's post-Merge censorship peaked with over 70% of blocks being OFAC-compliant, driven by dominant builders like Flashbots and Relayooor. This demonstrates the latent power of the builder role that alt-mempools codify.
case-study
THE NEW CENSORSHIP VECTOR
Real-World Censorship Precedents
The Alt Mempool isn't theoretical; it's a new attack surface proven by historical MEV and compliance actions.
01
The OFAC Compliance Slippery Slope
Since August 2022, major builders like Flashbots have censored OFAC-sanctioned transactions, filtering ~50% of Ethereum blocks. An Alt Mempool controlled by a single entity creates a permanent, protocol-level compliance chokepoint.
- Centralized Control: A single sequencer can blacklist any address or contract.
- Regulatory Capture: Becomes the easiest vector for legal pressure, beyond just builders.
- Precedent: Tornado Cash sanctions show protocol-level targeting is already happening.
~50%
OFAC Blocks
1 Entity
Chokepoint
02
MEV-Boost's Builder Monopolies
MEV-Boost created a builder market where ~90% of blocks come from three entities. This centralization allowed for soft censorship; an Alt Mempool with exclusive order flow hardens it.
- Economic Capture: The entity with the best MEV extraction wins, creating a natural monopoly.
- Exclusionary Lists: Builders already exclude certain transactions to maximize profit; an Alt Mempool formalizes this.
- Network Effect: More users → more data → better MEV → more centralization. It's a vicious cycle.
~90%
Builder Share
3 Entities
Dominant
03
The Miner Extractable Value (MEV) Wars
Front-running and sandwich attacks are a $1B+ annual industry. An Alt Mempool controlled by profit-maximizing searchers inherently censors fair, vanilla user transactions in favor of lucrative arbitrage.
- Profit-Driven Censorship: Non-MEV transactions are deprioritized or excluded, increasing failure rates.
- Data Advantage: The sequencer has perfect visibility into pending transactions, the ultimate front-running tool.
- User Harm: This isn't just about sanctions; it's about everyday users getting worse prices and failed swaps.
$1B+
Annual MEV
0 Priority
For Users
counter-argument
THE NEW CENSORSHIP VECTOR
The Rebuttal: "It's Just Economics"
An alt-mempool's economic incentives create a systemic vulnerability, not just a neutral marketplace.
Economic incentives create censorship. An alt-mempool is a profit-maximizing entity. Its operators will naturally prioritize transactions that maximize their extractable value, which includes complying with regulatory demands to block certain addresses. This is not a bug; it is the rational economic outcome of a centralized order flow auction.
It centralizes regulatory pressure. On the public mempool, validators face diffuse pressure. An alt-mempool like Flashbots Protect or a private RPC endpoint becomes a single point of control. Regulators and litigants target the centralized order flow manager, not the decentralized validator set, creating a far more effective censorship vector.
Evidence: The OFAC compliance rate on Ethereum post-Merge, facilitated by relay filtering, demonstrates this dynamic. Services like BloXroute's regulated relays explicitly offer compliance, proving the business model. The alt-mempool formalizes and monetizes this compliance layer.
FREQUENTLY ASKED QUESTIONS
FAQ: The Builder's Dilemma
Common questions about how alternative mempools like SUAVE, Flashbots Protect, and private RPCs introduce new censorship vectors for block builders.
The builder's dilemma is the conflict between a block builder's profit motive and their duty to include all valid transactions. Builders using private mempools like Flashbots Protect or SUAVE can censor transactions to maximize MEV extraction, creating a new, centralized point of control that undermines permissionless access.
takeaways
CENSORSHIP VECTORS
Architectural Imperatives
The rise of private order flow and intent-based architectures creates a new, systemic risk: the alt mempool.
01
The Problem: The Dark Forest of Private Order Flow
Seekers like Flashbots Protect and BloXroute's BackRunMe create off-chain, permissioned channels. This fragments liquidity and creates a two-tiered system where only privileged users access optimal execution.
- Centralization Risk: ~80% of Ethereum MEV flow is routed through a few dominant builders.
- Opaque Pricing: Fees are negotiated privately, obscuring true market costs.
- Exclusionary: Retail and smaller protocols are relegated to the slower, censored public mempool.
~80%
MEV Flow
2-Tier
System
02
The Solution: Credibly Neutral Public Goods
Protocols must enforce neutrality at the infrastructure layer. SUAVE (Single Unified Auction for Value Expression) aims to be a decentralized block builder and mempool. CowSwap's CoW Protocol and UniswapX use batch auctions to mitigate frontrunning.
- Universal Access: A single, open market for block space and order flow.
- Transparent Pricing: Auction-based fee discovery replaces backroom deals.
- Intent Standardization: User declarations (e.g., "swap X for Y") are matched by solvers, not manually ordered.
Batch
Auctions
Solver-Net
Architecture
03
The Implementation: Enforcing Fair Sequencing
The final defense is at the consensus layer. Fair Sequencing Services (FSS) or Leader Election mechanisms can order transactions by arrival time, not fee. This is a core research area for Ethereum's PBS (Proposer-Builder Separation) and L2s like Arbitrum.
- Time-Ordered Blocks: Prevents transaction reordering for extractive MEV.
- Builder Regulation: Requires builders to prove they processed the public mempool.
- L2 Advantage: Rollups can implement FSS more easily than L1, creating a censorship-resistant niche.
FSS
Mechanism
PBS
Framework
04
The Consequence: Protocol Design Debt
Ignoring this vector creates systemic fragility. Protocols that rely on simple P2P mempool broadcast (e.g., many early DeFi apps) are vulnerable to total censorship by searcher-builder cartels. This isn't hypothetical; OFAC-sanctioned Tornado Cash transactions were excluded.
- Execution Risk: Users face unpredictable slippage and failed transactions.
- Legal Liability: Builders complying with sanctions law create de facto blacklists.
- Innovation Tax: New protocols must design for alt-mempool-first execution, adding complexity.
100%
Censorable
OFAC
Precedent
05
The Entity: SUAVE's All-or-Nothing Bet
Flashbots' SUAVE is the most ambitious attempt to solve this. It proposes a separate chain dedicated to decentralizing the entire MEV supply chain: mempool, block building, and relay. Its success would render private order flow obsolete.
- Full-Stack Decentralization: Aims to replace the current builder/relay oligopoly.
- Cross-Chain Native: Designed to be the mempool and block builder for multiple chains.
- Existential Threat: If it fails, the alt-mempool fragmentation becomes a permanent, exploitable layer.
Full-Stack
Approach
Cross-Chain
Scope
06
The Metric: Measuring Censorship Resistance
CTOs must audit their stack. Key metrics include: % of transactions routed through private channels, builder diversity index, and public mempool inclusion latency. Tools like EigenPhi and mevboost.pics provide data.
- Quantifiable Risk: Move from theoretical to measurable censorship exposure.
- Builder Selection: Protocols can choose builders based on proven commitment to public inclusion.
- User Transparency: Frontends should display execution pathway guarantees.
3 Metrics
To Track
EigenPhi
Tooling
ENQUIRY
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