Legions

Generalized Frontrunners seek to profit by frontrunning any arbitrary transaction, whether on a DEX or in a vulnerable smart contract. Generalized Frontrunner bots monitor the pending transaction pool, copy pending transaction data, and execute those transactions in a local testing environment as if they were the transaction sender. If the tested transaction results in a profit to the bot in any way, the bot will copy the transaction and send it to the blockchain for real with a higher gas price than the original transaction. Since miners prioritize transactions with higher gas prices, miners will most likely include the bot's transaction in a block before the original transaction. Generalized Frontrunners often lead to Priority Gas Auctions, where several bots bid using high gas prices to claim a profitable transaction.

Liquidators search for arbitrage opportunities in lending/debt protocols (i.e., Compound and Aave). Borrowers in these protocols usually need to over-collateralize their positions (i.e., post $100 worth of ETH as collateral to borrow $90 USDC). In periods of extreme volatility, the value of your ETH collateral may fall below your loan value, making your position under collateralized. Liquidators can now pay off your debt, seize your collateral at a discount, and immediately sell it for an instant risk-free profit. You can learn more about liquidations on Aave here. Here is an example of a real liquidation in the wild.

Sandwichers will monitor the pending transaction pool for large DEX trades (i.e., on Uniswap or Sushiswap) and construct transaction sequences that “sandwich” and frontrun a victim's large trade. For example, a sandwich bot would (1) buy an asset before the victim, pushing up the asset's price (2) execute the victim's large trade, causing the price to increase even more (3) immediately sell the asset at the new higher price. You can find a real example of a sandwich attack in this block, starting with this transaction. Here is a great thread for more details. Finally, there has been a recent example of a sandwich attack involving crypto punks!

While Frontrunners try to get their transaction mined BEFORE some other target transaction, Backrunners use special techniques to get their transaction mined directly AFTER a target transaction. Target transactions may create profitable opportunities that did not exist before they were mined, so Backrunners capture these opportunities as soon as they become available. For example, a price oracle update transaction on Compound may cause several borrowers to become under collateralized. Backrunners will monitor these oracle updates and place liquidation transactions directly afterward, ensuring they are the first bot to capture the liquidation. There is an example of backrunning a Compound oracle update in this block, with this liquidation transaction occurring directly after this price oracle update. Finally, here is a video walking through this strategy.

In a Time-Bandit attack, a miner would rewind the blockchain to capture profitable MEV opportunities that happened in a previous block. As far as we know, there has yet to be a Time-Bandit attack on Ethereum. However, there has been an Uncle-Bandit attack, a very close cousin to the Time-Bandit. Time-Bandit attacks are a severe risk to the underlying security of Ethereum. You can learn more about Time-Bandit attacks here.