Front Running on Ethereum: How MEV Bots Make Profits

Front running is one of the oldest trading strategies in financial markets, but on Ethereum it takes on a completely different form. Instead of insider knowledge, blockchain front runners use the transparency of the mempool — the public queue of pending transactions — to detect and profit from others' trades before they execute. This deep dive explains the mechanics, strategies, and economics of front running on Ethereum.

What is Front Running?

In traditional finance, front running means trading ahead of a known large order — a practice that's illegal for brokers with non-public order information. On Ethereum, front running is fundamentally different because the "insider information" is publicly available to anyone monitoring the mempool.

When you submit a transaction on Ethereum, it doesn't execute immediately. It first enters the mempool, a public waiting area where pending transactions sit until a block builder includes them in a block. During this window (typically 12 seconds between blocks), anyone running a node can see your pending transaction, decode what it does, and submit their own transaction to execute first.

Front running bots exploit this transparency by monitoring every pending transaction, identifying those that will create profitable opportunities (like large DEX swaps that will move prices), and submitting competing transactions with higher gas prices or through Flashbots to ensure they execute first.

How Front Running Works on Ethereum

The Mempool: Ethereum's Transparent Queue

Every Ethereum transaction goes through the mempool before being included in a block. The mempool is not a single location — each node maintains its own version. Front running bots run their own Ethereum nodes (or connect to specialized mempool services) to receive pending transactions as fast as possible, often within milliseconds of submission.

The bot decodes each pending transaction's calldata to understand what it does. A swap on Uniswap, a token transfer, a liquidation on Aave — the bot can interpret the function call, parameters, and potential market impact before the transaction is confirmed.

Transaction Ordering and Gas Priority

In Ethereum's post-merge architecture, block builders construct blocks by ordering transactions to maximize their revenue. Transactions that pay higher priority fees (tips) are generally placed earlier in the block. Front running bots exploit this by submitting transactions with higher tips than the target transaction.

This led to "gas wars" — competing bots bidding up gas prices trying to front-run each other. Flashbots solved this inefficiency by creating a private channel where bots submit transaction bundles with specific ordering requirements and pay the block builder directly instead of through inflated gas prices.

Flashbots and MEV Auctions

Flashbots introduced a sealed-bid auction system for MEV extraction. Instead of competing in the public mempool, bots submit bundles — ordered sets of transactions — to Flashbots' relay. Block builders select the most profitable bundles and include them in blocks.

Each bundle includes a payment to the block builder (a "bribe") that represents the bot's bid for having its transactions included in a specific order. The block builder maximizes revenue by selecting the highest-paying bundles, creating an efficient marketplace for transaction ordering.

Types of Front Running Strategies

1. Pure Front Running

The bot detects a pending transaction that will profit the original sender (like an arbitrage opportunity) and copies the transaction, submitting it with higher gas to execute first. The front runner captures the opportunity that the original trader found. This is the most aggressive form of front running and has largely moved to Flashbots-based competition.

2. Displacement

The front runner submits a transaction that replaces or displaces the target transaction entirely. For example, if a trader found an arbitrage opportunity and submitted a transaction to capture it, the front runner copies the trade logic and gets it included first, making the original transaction revert.

3. Insertion (Sandwich Attacks)

Rather than replacing the target transaction, insertion front running places transactions around it. The sandwich attack is the classic example — a front-run buy before a large swap and a back-run sell after. The target transaction still executes, but at a worse price, with the front runner extracting the difference.

4. Liquidation Front Running

DeFi lending protocols like Aave and Compound allow anyone to liquidate undercollateralized positions for a reward. Front running bots monitor these protocols for positions approaching liquidation thresholds. When a price movement triggers a liquidation, multiple bots compete to execute the liquidation first and claim the reward.

The Economics of Front Running

Front running on Ethereum is driven by clear economic incentives. The total MEV extracted from Ethereum has exceeded billions of dollars since the DeFi boom of 2020. This value comes from:

• DEX arbitrage: Price differences between pools create risk-free profit opportunities
• Sandwich attacks: Extracting value from large swaps by manipulating execution price
• Liquidation rewards: DeFi protocols pay 5-10% bonuses for liquidating unhealthy positions
• NFT sniping: Buying underpriced NFTs listed in pending transactions before others can

The competitive nature of front running means profits are shared between the bot operator and the block builder. Typically, 80-90% of MEV goes to the block builder as a payment for including the bundle, with the bot keeping 10-20% as net profit. Despite this split, the volume of opportunities makes front running highly profitable.

Front Running vs. Back Running

While front running means executing before a target transaction, back running means executing immediately after. Back running is used for arbitrage: when a large swap moves a DEX pool's price away from the market price, a back runner submits an arbitrage transaction right after to capture the price difference.

Back running is considered less harmful than sandwich attacks because it doesn't affect the target transaction's execution price. The back runner profits from the price discrepancy created by the trade, not from the trader themselves. In fact, back running helps restore market efficiency by realigning DEX prices after large trades.

The Infrastructure Behind Front Running Bots

Successful front running requires serious infrastructure investment:

• Dedicated Ethereum nodes: Running Geth or Erigon nodes with optimized configurations for mempool access and fast block processing
• Low-latency networking: Co-locating servers near major Ethereum node clusters to minimize mempool propagation delays
• Custom smart contracts: Optimized Solidity contracts that minimize gas consumption and maximize execution efficiency
• Simulation infrastructure: Local EVM forks that can simulate hundreds of potential transactions per second to evaluate profitability
• Flashbots integration: Connection to Flashbots relay for private transaction bundle submission