Front Running

Front running exploits the gap between when a transaction is broadcast and when it is confirmed. Because most blockchains process transactions in an order the block producer or validator chooses, anyone who can see a trade waiting in the mempool gains a preview of market-moving information before it takes effect.

On Ethereum and other public-mempool chains, specialized bots continuously scan pending transactions for large swaps on a decentralized exchange, then pay a higher gas fee to jump the queue. The most common version, the sandwich attack, wraps a bot's buy and sell around a victim's trade, extracting profit purely from the price impact the victim's own order creates. Liquidation bots and arbitrage bots use the same technique legitimately, competing to be first rather than manipulating a specific victim.

Front running is one form of Maximal Extractable Value, the broader profit validators and searchers can capture by reordering, inserting, or censoring transactions within a block. Chains without a public mempool, such as Solana, remove the classic observation window, but MEV persists there too through priority tips paid to block leaders. On Ethereum, tools like Flashbots Protect route orders through private channels rather than the open mempool, and some decentralized exchange aggregators batch trades or delay execution to blunt the advantage.

For traders, practical defenses include lower slippage tolerance, private transaction routing, and MEV-aware exchange aggregators rather than relying on the public mempool for large orders.