// GLOSSARY -- DEFI

What is Front-Running?

1 min read Updated

Front-running in blockchain is the practice of placing a transaction ahead of a known pending transaction to profit from the anticipated price movement — enabled by the public nature of the mempool.

WHY IT MATTERS

Front-running is the canonical MEV attack. A searcher sees your pending swap in the mempool, submits the same trade with higher gas (to be included first), profits from the price impact your trade will cause, then sells after your trade executes.

In traditional finance, front-running by brokers is illegal. In crypto, it's a consequence of transparent mempools and permissionless block building — anyone can see and act on pending transactions.

Protections include private transaction submission (Flashbots), MEV-aware routing (CoW Protocol's batch auctions), and L2s with sequencer-ordered transactions that reduce front-running opportunities.

FREQUENTLY ASKED QUESTIONS

How does front-running work technically?
Monitor mempool for large swaps → calculate expected price impact → submit identical trade with higher gas → your trade executes first → original trade pushes price further → you sell at profit.
Can front-running be prevented?
Not fully eliminated on public mempools. Mitigations: private mempools, batch auctions (process all orders at same price), commit-reveal schemes, and encrypted mempools.
Is front-running legal in crypto?
The legal status is murky. Unlike traditional markets, blockchain front-running isn't clearly regulated. Some jurisdictions are beginning to address it, but enforcement is challenging.

FURTHER READING

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