Front-Running
Placing a transaction ahead of a known pending transaction in the mempool to profit from the price movement it will cause.
Front-Running — Front-running is the practice of placing a transaction ahead of a known pending transaction to profit from the anticipated price movement. In DeFi, front-running is executed by bots that monitor the mempool, detect profitable pending trades, and submit their own transactions with higher gas fees to be processed first. This is a form of MEV extraction that imposes hidden costs on regular traders.
What Is Front-Running?
Front-running occurs when a bot observes your pending trade in the mempool and executes the same trade before yours. If you are about to buy $10,000 of a token, a front-runner buys first, pushing the price up. Your trade then executes at a higher price. The front-runner immediately sells after your trade at the elevated price, profiting from the difference.
In traditional finance, front-running by brokers is illegal. In DeFi, it is technically possible because the mempool is public and transaction ordering is determined by gas price competition rather than arrival time.
How Front-Running Works
On Ethereum, front-running bots connect to mempool nodes and scan every pending transaction for DEX swap operations. When they detect a profitable opportunity — typically a large buy that will move the price — they construct a front-running transaction with a higher gas price to ensure it is included before the victim's trade.
The most sophisticated form is the sandwich attack, which combines front-running and back-running: buy before the victim (pushing price up), let the victim's trade execute (pushing price up further), then sell after (capturing the price difference). Sandwich attacks on a $10,000 trade with 2% slippage can extract $100-$200.
Front-Running Protection
Traders can protect against front-running by: using private transaction relays (Flashbots Protect on Ethereum, Jito bundles on Solana), setting tight slippage tolerance (reducing the profit margin for attackers), using DEX features with built-in MEV protection, or trading through aggregators that route through protected pools. OpenLiquid includes anti-MEV protection in its volume generation sessions to prevent sandwich attacks from eroding session budgets.
Related Terms
Anti-MEV Protection
Tools and techniques to shield transactions from MEV bots, such as private RPC endpoints, transaction bundling, or slippage controls.
Read definition Security & PrivacyMEV (Maximal Extractable Value)
Profit extracted by block producers by reordering, inserting, or censoring transactions within a block.
Read definition Security & PrivacySandwich Attack
An MEV attack where a bot front-runs and back-runs a large swap to profit from the price impact, costing the victim extra slippage.
Read definition Launchpad & Token LaunchSniper Bot
An automated bot that buys tokens in the first seconds of a new liquidity pool opening, often before regular traders can react.
Read definitionFrequently Asked Questions
Common questions about Front-Running in cryptocurrency and DeFi.
Ethereum traders lose an estimated $2-5 million per week to front-running and sandwich attacks. Individual losses range from a few cents on small trades to thousands of dollars on large swaps with generous slippage tolerances. The total cost depends on market volatility and trading volume.
Yes, but through a different mechanism. Solana does not have a traditional mempool, but Jito Labs provides a tip-based system where searchers can pay validators for priority transaction inclusion. Sandwich attacks on Solana are common on popular trading pairs, particularly during high-volatility periods.
Compare your transaction's execution price to the market price at the time. If the price jumped significantly between your submitted price and execution price, and you can see a buy transaction from an unknown wallet immediately before yours in the same block, you were likely front-run. MEV tracking tools like Eigenphi visualize sandwich attacks.
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