What is Back-Running?
Back-running is an MEV extraction technique where a searcher places their transaction immediately after a target transaction to profit from the state changes it creates. Unlike front-running (which anticipates and precedes impact), back-running exploits the aftermath of another transaction. The back-runner needs to execute after the target, not before.
How Back-Running Works
Back-runners monitor the mempool or block contents for transactions that create profitable post-execution opportunities. They construct transactions that capitalize on the new state and compete for the block position immediately following the target.
The key is positioning: back-runners want the very next slot after the target transaction. They may pay high priority fees or use MEV-specific submission channels (Flashbots bundles) to secure this position atomically with the target.
Back-Running Use Cases
Arbitrage is the classic application. A large swap creates a price difference between venues. Back-runners immediately arbitrage this difference, buying where it's cheap and selling where it's expensive, restoring price equilibrium and capturing profit. This actually benefits markets by maintaining price consistency. Liquidation back-running occurs when a price oracle update makes positions liquidatable. Bots back-run the oracle update transaction to claim liquidation opportunities before others can react. The oracle update is the trigger; liquidation is the back-run. Post-deployment opportunities: When new contracts or pools are deployed, bots may immediately interact with them to claim first-mover advantages or seed liquidity.Back-Running in Sandwich Attacks
In sandwich attacks, the third transaction (selling after the victim's trade) is technically a back-run. The attacker back-runs the victim's swap to realize profits from their front-running position, completing the sandwich.
Is Back-Running Harmful?
Unlike front-running, back-running often provides positive externalities. Arbitrage back-running improves price efficiency across venues. Liquidation back-running protects lending protocols from bad debt accumulation. The target transaction creator is typically unharmed. The opportunity exists regardless of who captures it.
However, back-running concentrates value extraction among sophisticated searchers with technical infrastructure.