Term
Front-running
Front-running in crypto is when someone sees a pending transaction in the mempool and submits their own copy or reaction with a higher fee, getting theirs mined first — a form of MEV extraction that damages the original transaction's economics.
Front-running is a transaction-ordering attack. Because most blockchain mempools are public — anyone can see pending transactions before they are included in a block — an observer who spots a profitable pending transaction can construct their own reaction, submit it with a higher fee, and be mined first. The victim's original transaction still lands, but the attacker has already extracted profit ahead of it.
Common on-chain examples: buying a token just before a large user swap pushes its price up, then selling immediately after (sandwich attack); liquidating a lending position just before the target does; sniping arbitrage opportunities right after they appear. This is a major component of MEV on Ethereum and other public-mempool chains.
Mitigations include private mempools (Flashbots Protect, MEV Blocker) that hide transactions from public view until included; explicit slippage tolerances on DEX trades to bound how much can be extracted; and time-averaging or split-order strategies for large trades.
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