Front Running

Front running is the practice of placing a transaction or trade ahead of someone else’s pending transaction by using advance knowledge of that order. In traditional finance, this is associated with trading on non-public order information and is widely considered unethical, and often illegal. In crypto, the concept carries over, but the “advance knowledge” frequently comes from observing transactions before they are finalized on-chain.

On many blockchains, transactions are broadcast to a public mempool before they are included in a block. This creates a window where other participants can see a swap, mint, or liquidation that is likely to move a token’s exchange rate or affect execution. A front runner tries to get their own transaction confirmed first, commonly by paying a higher fee or otherwise influencing transaction ordering. If successful, they can buy before a large buy, sell before a large sell, or adjust positions ahead of a liquidation event.

Decentralized exchanges are a common venue for front running because automated market makers reprice assets based on trade size. For example, if a large swap is pending, a bot can submit a buy first, then sell after the victim’s swap executes at a worse rate, capturing the difference. This behavior is closely related to maximal extractable value (MEV), a broader category of profit strategies that depend on transaction ordering, inclusion, or censorship. Some MEV is considered “toxic,” particularly when it degrades user execution.

Users and protocols attempt to reduce front running with slippage controls, private transaction relay systems, batch auctions, and intent-based trading that hides details until execution. Front running matters because it can increase trading costs, worsen price execution, and undermine perceptions of fairness and trust in on-chain markets.