Backrunning is an MEV (maximal extractable value) strategy where a searcher bot spots a pending transaction in the mempool that will create a price discrepancy, and submits its own transaction with a slightly higher gas fee to execute immediately after the target transaction — capturing the arbitrage opportunity that the first transaction creates. Unlike sandwich attacks, backrunning does not hurt the original transaction’s execution. The victim’s trade goes through as normal; the backrunner simply reacts to the consequence. This makes backrunning widely considered the most “benign” form of MEV — it’s a form of arbitrage that actually helps restore efficient prices across markets after a large order temporarily moves them.
How Backrunning Works
Classic DEX Arbitrage Backrun
The most common backrunning scenario:
- Large trade spotted: A pending transaction swaps $2M USDC → ETH on Uniswap V3, pushing the ETH/USDC price on that pool above the global market price
- Price discrepancy exists: After the large trade executes, Uniswap will have ETH at $3,200 while Binance shows $3,100 — a $100 spread
- Backrunner acts: The searcher submits a transaction that:
Buys cheap ETH from other sources (Curve, Binance via bridge, etc.)
Sells that ETH into the now-overpriced Uniswap pool
Profits the spread - Position: The backrunner’s transaction is placed after the victim’s transaction using priority gas or PBS/MEV-Boost bundles
- Markets rebalance: Uniswap price is restored to the global price by the arbitrage
The victim (the large trade sender) experiences no worse execution than if the backrunner hadn’t acted. Markets are actually more efficient because the price discrepancy is corrected faster.
Backrunning vs. Frontrunning vs. Sandwich
| Strategy | Transaction Order | Effect on Victim | Mechanism |
|---|---|---|---|
| Frontrunning | Before victim | Bad: worse price | Copy victim’s trade, execute first |
| Backrunning | After victim | Neutral: no effect | Profit from price discrepancy victim created |
| Sandwich attack | Both before AND after | Bad: significantly worse price | Frontrun + backrun victim simultaneously |
Backrunning is the only one of these three that doesn’t directly harm the person being backrun.
Liquidation Backrunning
A second major use case for backrunning: liquidations.
When a borrowing position’s health factor drops below 1.0, it becomes eligible for liquidation. Multiple searcher bots monitor all lending protocol positions and race to be first to submit the liquidation transaction — the first successful liquidator claims the liquidation bonus.
Liquidation backrun pattern:
- A price oracle update transaction is spotted in the mempool
- Bots calculate: “if this price update goes through, position X at Aave will be liquidatable”
- Bots submit liquidation transactions to execute immediately after the oracle update
- The first bot to be included after the oracle update captures the liquidation bonus
This is technically backrunning — the bots are backrunning the oracle update transaction to be first to liquidate.
NFT Backrunning
NFTs with fixed-price listings create backrun opportunities when a trait or rarity discovery creates immediate value:
- A new NFT collection is minted (reveal transaction in mempool)
- Bots calculate rarity from the reveal data
- Bots immediately submit buy transactions for the rarest unrevealed NFTs at mint price
- These bots “backrun” the reveal transaction, sniping underpriced rare NFTs before humans can react
How Searchers Execute Backruns
Modern backrunning is executed through Flashbots / MEV-Boost bundles rather than naive gas bidding:
Without MEV-Boost: Submit a transaction with slightly higher gas than the victim. Risk: another bot outbids you; uncertain ordering.
With MEV-Boost bundles: Submit a “bundle” — a group of transactions that must execute in a specific order. The block builder guarantees:
- Victim’s tx executes first
- Backrunner’s tx executes immediately after
- No one can insert between them
Searchers pay the block builder a share of the profit in exchange for guaranteed ordering. Builders earn by selecting the most profitable bundle for the block.
Is Backrunning Harmful?
For the person being backrun: Neutral — their transaction executes normally.
For the broader ecosystem:
- Positive: Arbitrage backrunning restores price efficiency quickly. Without it, price discrepancies would persist longer, harming subsequent traders.
- Ambiguous: Liquidation backrunning creates fierce competition that can lead to network spam (many bots submitting the same liquidation); MEV-Boost reduces this by moving competition off-chain.
- Negative (systemic): The value extracted by backrunners represents value that could have gone to LPs, traders, or the protocol — it’s a form of profit extraction from protocol activity by specialized actors.