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Front-Running

The practice of placing a transaction before a known pending transaction to profit from or alter its expected execution.

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Definition

The practice of placing a transaction before a known pending transaction to profit from or alter its expected execution.

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Why it matters

MEV concepts explain how transaction ordering can create economic opportunities and risks.

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How it works

Bots constantly scan the pending transaction pool (mempool) for large trades. When a large trade is detected, the bot sends a transaction to buy the same asset, paying a higher gas fee to be included in the same or previous block. Once the user’s trade pushes the price up, the bot sells the asset for a profit.

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Real-world example

Uniswap users often experience ‘sandwich attacks,’ where a bot buys before a user’s trade and sells immediately after, trapping the user in a profit-stealing loop.

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Advantages

  • Provides market efficiency for arbitrageurs
  • Incentivizes faster transaction processing
  • Highlights the need for better trade protections

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Limitations

  • Harms retail user experience
  • Increases transaction costs through gas wars
  • Can lead to high slippage for users

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Common misconceptions

  • Front-running is not always technically illegal in decentralized finance, but it is considered an extraction of user value.
  • Users believe paying higher gas fees always prevents front-running, but bots simply adjust their fees to remain ahead.
Canonical knowledge ID: glossary:front-running