Block Stuffing

The core goal is often to censor specific transactions from being included in a block. This can be used to:

• Prevent a competing arbitrage or liquidations transaction from executing.
• Delay or block governance votes or contract interactions.
• Enforce regulatory blacklists by excluding addresses, though this is contentious and undermines censorship resistance.

A validator uses its privileged position as the block proposer to fill the block's gas limit with its own transactions. These are often:

• Zero-value transfers to itself.
• Calls to empty or cheap contracts.
• Transactions with just enough gas price to be valid, minimizing cost. The validator pays its own gas fees, which are rebated to itself as the block producer, making the net cost primarily the opportunity cost of forgone transaction fees from real users.

Block stuffing is a direct Maximal Extractable Value (MEV) strategy. By censoring others, a validator can:

• Secure a profitable arbitrage opportunity for itself in a subsequent block.
• Ensure it is the sole beneficiary of a liquidation event.
• Manipulate oracle price feeds by controlling which transactions are included in the critical time window. This is often coordinated with sandwich attacks or time-bandit attacks.

This practice degrades network utility for legitimate users.

• Increased Latency: Valid transactions are delayed to later blocks.
• Higher Gas Fees: As real users compete for limited space in subsequent blocks, fee auctions can drive up base fee and priority fees.
• Wasted Resources: Network throughput is consumed by meaningless transactions, reducing overall TPS (Transactions Per Second) efficiency.

A more sophisticated variant where a validator reorganizes the chain (reorg) to steal a profitable MEV opportunity from a past block. It involves:

1. Letting a profitable transaction into a block.
2. Stuffing subsequent blocks to prevent finality.
3. Creating a competing chain where the validator includes the profitable transaction for itself. This attacks the weak subjectivity of the chain and is considered more severe than simple in-slot stuffing.

Block stuffing is a Denial-of-Service (DoS) attack executed at the consensus layer. An attacker with significant staking power or mining hash rate proposes a block filled with their own, often worthless, transactions. This prevents legitimate user transactions from being included, causing delays, failed operations, and increased fees for everyone else on the network.

The most common financial motive is Maximal Extractable Value (MEV). Attackers stuff blocks to:

• Displace competing transactions to seize profitable arbitrage or liquidations.
• Censor specific transactions to manipulate market prices for personal gain.
• Create artificial network congestion to profit from users paying higher priority fees in subsequent blocks.

This attack directly harms applications and users:

• Time-sensitive transactions (e.g., liquidations, arbitrage, expiring orders) fail, leading to direct financial loss.
• User Experience degrades with unpredictable confirmation times and soaring gas fees.
• Protocol reliability is undermined, as smart contracts cannot depend on timely execution, breaking core assumptions of DeFi applications.

Block stuffing is distinct from general network spam:

• Source: It requires block production rights, not just the ability to send transactions.
• Intent: It's a targeted, adversarial act for profit or disruption, not just random activity.
• Effect: It creates localized, strategic congestion within a specific proposer's slot, whereas general spam affects the global mempool.

Networks employ several defenses:

• Proposer-Builder Separation (PBS): Separates block building from proposing, making targeted stuffing by the final proposer harder.
• Minimum Base Fee Mechanisms: Protocols like EIP-1559 create a base fee that burns with congestion, making spam costly.
• Reputation Systems: Penalizing validators for consistently producing empty or spam-filled blocks.
• Out-of-band Ordering (OBO): Using private mempools or fair ordering protocols to bypass public transaction queues.

In June 2020, the Ethereum-based decentralized exchange FCoin suffered a catastrophic failure. An attacker exploited its transaction fee mining model by stuffing the Ethereum mempool with millions of low-fee transactions. This created a backlog that prevented legitimate withdrawal transactions from being processed, effectively freezing user funds and contributing to the exchange's collapse.

• Mechanism: Spammed the network to create an artificial congestion wall.
• Impact: Highlighted the risks of DeFi protocols with complex on-chain settlement dependencies.

Solana experienced several full network outages, partially triggered by transaction flooding from bots. While not pure block stuffing in a fee-based system, the effect was similar: a flood of computationally cheap transactions (e.g., for NFT mints or arbitrage) exhausted the network's processing capacity, causing validators to diverge and the chain to halt.

• Key Difference: Solana's low, fixed fees made transaction spam economically trivial.
• Outcome: Led to core protocol changes, including the implementation of fee prioritization and QUIC protocol to manage network traffic.

A series of deliberate network stress tests, often attributed to entities like Bitcoin Unlimited, involved broadcasting hundreds of thousands of low-fee transactions to fill Bitcoin's 1MB blocks. The goal was to demonstrate perceived scalability limitations and advocate for larger block sizes, creating real congestion and spiking transaction fees for regular users.

• Context: Part of the broader Block Size Wars.
• Legacy: These events were instrumental in pushing development of Segregated Witness (SegWit) and the Layer 2 Lightning Network.

In March 2024, the Arbitrum network experienced severe congestion due to a gas token minting exploit. Bots spammed the sequencer with transactions minting worthless tokens, each consuming the minimum 21,000 gas base fee. This stuffed the L2 blocks, delaying transactions for hours and causing a spike in L1 settlement costs, demonstrating how cheap L2 execution could be exploited.

• Exploit Vector: Abused the fixed overhead cost of L2 transaction batches.
• Response: The Arbitrum team implemented a minimum base fee increase to disincentivize such spam.

Repeated events during popular Initial DEX Offerings (IDOs) or NFT collection mints have functionally acted as block stuffing. Thousands of users and bots submit transactions simultaneously with high gas fees, competing for inclusion in the next few blocks. While not malicious, the effect is identical: network congestion, skyrocketing gas prices, and failed transactions for all other network activities.

• Example: The Ethereum Name Service (ENS) domain renewal rush in 2022.
• Mitigation: Has driven adoption of private mempools (MEV-Boost), off-chain allow lists, and Layer 2 solutions.

A more sophisticated cousin to block stuffing, a Time-Bandit Attack involves mining multiple consecutive blocks filled with meaningless transactions. This is done to orphan a competing chain that contains a valuable transaction (like a large NFT sale), allowing the attacker to re-mine those blocks and potentially steal the asset. It exploits consensus finality and the cost of block production.

• Requirement: Significant hashing power (PoW) or stake (PoS).
• Defense: Mechanisms like Ethereum's proposer boost in its PoS design help mitigate this risk.

Block stuffing is a form of Maximum Extractable Value (MEV), where validators or block producers reorder, include, or exclude transactions to extract profit. It exploits the permissionless nature of block construction, turning the ability to sequence transactions into a revenue source. This is distinct from frontrunning, which targets pending transactions, whereas block stuffing directly manipulates the block's internal order and content.

A Gas Price Auction is the competitive bidding process users engage in to have their transactions included in a block. Block stuffing attacks often occur when an attacker submits a high volume of transactions with inflated gas prices, winning the auction and crowding out legitimate users. This creates a temporary, artificial spike in base fee and network congestion.

A Time-Bandit Attack is a related consensus-level attack where a validator attempts to rewrite blockchain history by mining a competing chain. While block stuffing focuses on filling a single block to disrupt a specific transaction (like a liquidation), time-bandit attacks target the canonical chain itself over multiple blocks to reverse settled outcomes, requiring significant hash power or stake.

Proposer-Builder Separation (PBS) is a protocol design, implemented in Ethereum's consensus layer, that mitigates centralization risks from MEV. It separates the roles of block builder (who orders transactions and extracts MEV) from block proposer (who selects the highest-value block). PBS aims to democratize access to MEV and reduce the incentive for harmful stuffing by creating a competitive builder market.

Block stuffing can be used to manipulate a network's base fee (EIP-1559) or similar fee mechanisms. By flooding a block with high-priority transactions, an attacker can artificially inflate the base fee for the next block. This creates a fee spiral, increasing costs for all users and can be used to trigger liquidations or disrupt time-sensitive DeFi operations that rely on predictable gas costs.