What is Front-Running?
Front-running is the practice of seeing a pending transaction in the mempool and submitting your own transaction to execute first, profiting from knowledge of the upcoming trade's impact. By observing what others intend to do before they do it, front-runners position themselves to benefit. Often at the original trader's expense.
How Front-Running Works
The public mempool exposes pending transactions before confirmation. Anyone can monitor this stream. Front-runners identify profitable opportunities and submit competing transactions with higher gas prices (priority fees) to ensure they execute first in the same block, or get included in an earlier block.
Common front-running scenarios include: buying tokens before a large purchase (knowing the purchase will push prices up), executing arbitrage before another searcher's arbitrage transaction, claiming DeFi rewards or airdrops before intended recipients, and minting NFTs ahead of high-demand public mints.
The front-runner profits from information asymmetry. Knowing what is about to happen before it does, using public (but not widely monitored) information.
Front-Running in DeFi
DEX trades are particularly vulnerable. A large pending swap will move prices; anyone seeing it can profit by trading in the same direction first. This is the "front" half of sandwich attacks.
Liquidations are heavily front-run. When positions become liquidatable, multiple bots race to claim the liquidation bonus. The fastest (highest priority fee) wins. NFT mints, airdrops, and any first-come-first-served opportunity attract front-running competition.
Legal and Ethical Context
Front-running using non-public order flow is illegal in traditional finance. Blockchain mempools are public by design, creating ambiguity. While the technical capability is clear, ethical implications are debated. Using public information vs. Extracting value from less sophisticated users.
Mitigation
Private transaction submission (Flashbots Protect, MEV Share, MEV Blocker) hides transactions from public view. Batch auctions (CoW Swap) eliminate ordering advantages. Commit-reveal schemes require two transactions, preventing observation of intent. Understanding front-running helps users protect themselves through appropriate tools.