Front Running / Frontrun

Front running is a practice where someone observes a pending transaction and rushes to execute a similar transaction first, profiting from the price movement the original transaction is about to cause. In traditional finance, front running by brokers who secretly act on client orders before executing them is illegal. In public blockchains, it is technically possible because all pending transactions sit in a publicly visible waiting area called the mempool before they are included in a block.

When you submit a trade on a decentralized exchange, your transaction sits in the mempool for a moment. A bot — or a sophisticated trader — can see that you are about to buy a large amount of a token, which will raise the price. The bot can pay a higher transaction fee to jump ahead of you in line, buy the token first at the current low price, let your transaction push the price up, and then immediately sell at the higher price — a practice also described as a “sandwich attack” when the bot also places a sell order right after yours.

This is possible because of how blockchains work: miners (or validators) typically order transactions in a block by fee amount, so paying more gets you processed first. This has spawned an entire field of study called Maximal Extractable Value (MEV), which quantifies all the profit that can be extracted by reordering, inserting, or censoring transactions in a block. Various solutions exist, including private transaction relays, commit-reveal schemes, and fair ordering protocols, but front running remains an active challenge in the DeFi ecosystem.

Example: You are in a shop and loudly announce you are about to buy the last 100 units of a hot toy at the listed price of $10 each. Someone standing nearby sprints to the counter, buys all 100 first, then turns around and offers to sell them to you for $12 each. They did nothing but listen and move faster.