Crypto Gloss

Backrunning

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 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:

  1. 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
  2. Price discrepancy exists: After the large trade executes, Uniswap will have ETH at $3,200 while Binance shows $3,100 — a $100 spread
  3. Backrunner acts: The searcher submits a transaction that buys cheap ETH from other sources and sells into the now-overpriced Uniswap pool, profiting the spread
  4. Position: The backrunner’s transaction is placed after the victim’s transaction using priority gas or PBS/MEV-Boost bundles
  5. 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.

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:

  1. A price oracle update transaction is spotted in the mempool
  2. Bots calculate: “if this price update goes through, position X at Aave will be liquidatable”
  3. Bots submit liquidation transactions to execute immediately after the oracle update
  4. The first bot included after the oracle update captures the liquidation bonus

NFT Backrunning

NFTs with fixed-price listings create backrun opportunities when a trait or rarity discovery creates immediate value:

  1. A new NFT collection is minted (reveal transaction in mempool)
  2. Bots calculate rarity from the reveal data
  3. Bots immediately submit buy transactions for the rarest NFTs at mint price
  4. 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; 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 is a form of profit extraction from protocol activity by specialized actors.

History

  • 2015–2018 — Gas price wars: early Ethereum bots submit transactions with incrementally higher gas to be included first; crude precursor to modern backrunning
  • 2020 — “Flash Boys 2.0” paper by Daian et al. formalizes MEV taxonomy including backrunning; raises awareness of the practice
  • 2021 — Flashbots launches; MEV-Boost bundles replace gas wars for backrunning execution; backrunning becomes systematic and professionalized
  • 2022 — Ethereum’s Merge; MEV-Boost adoption grows to cover the majority of Ethereum blocks; backrunning infrastructure matures
  • 2022–2024 — Backrunning extends to Layer 2s (Arbitrum, Optimism, Base) and alternative L1s; searcher competition intensifies

Common Misconceptions

  • “Backrunning is the same as frontrunning.” — Frontrunning executes before the target transaction to profit from an anticipated price move, harming the victim. Backrunning executes after, profiting from a discrepancy the victim already created, with no effect on the victim’s outcome.
  • “Backrunning is unambiguously harmful.” — Arbitrage backrunning actually improves market efficiency by correcting price discrepancies faster. It is widely considered the most legitimate form of MEV — the benefit to price efficiency is real, even if the practice extracts value from the system.

Social Media Sentiment

  • r/ethereum / r/ethfinance: Backrunning is discussed as part of broader MEV conversations; generally treated as the acceptable face of MEV relative to sandwich attacks.
  • X/Twitter: MEV researchers (Flashbots, EigenPhi) frequently post data on backrunning volumes; crypto traders occasionally discuss being backrun without realizing it.
  • Discord (DeFi / MEV): Searcher communities discuss backrunning strategies in detail; liquidation backrunning competition is a frequent topic.

Last updated: 2026-04

Related Terms

See Also

  • Sandwich Attack — the more harmful MEV strategy that wraps a victim transaction both before and after
  • Searcher — the bots and operators who execute backrunning and other MEV strategies
  • Block Builder — the entities who receive backrunning bundles and order transactions within blocks

Sources