Front-running in cryptocurrency is the practice of placing a transaction ahead of another known pending transaction to gain a financial advantage from the anticipated price impact. It is one of the most common forms of MEV (Maximal Extractable Value) extraction in decentralized finance.
In traditional finance, front-running is illegal and refers to a broker executing trades based on advance knowledge of client orders. In crypto, it operates in a legal gray area because blockchain mempools are public — anyone can see pending transactions before they are confirmed.
Front-running in crypto typically works like this:
- A bot monitors the mempool for large pending transactions on decentralized exchanges.
- When a profitable opportunity is detected, the bot submits the same trade with a higher gas fee to ensure it gets processed first.
- The bot's transaction executes before the original, moving the price in a favorable direction.
- The original transaction executes at a worse price, and the bot profits from the difference.
Front-running is a key component of sandwich attacks, where it is combined with back-running to maximize profit extraction.
To mitigate front-running, several solutions have emerged:
- Private mempools — services like Flashbots allow users to submit transactions directly to block builders, bypassing the public mempool.
- Batch auctions — DEXs like CoW Swap execute trades in batches, eliminating ordering advantages.
- Encrypted transaction ordering — emerging protocols that hide transaction details until after ordering is finalized.
Understanding front-running is crucial for anyone trading on decentralized exchanges, as it directly affects execution quality and trading costs.