Flash Loan

A flash loan is a unique DeFi primitive that allows users to borrow unlimited amounts of cryptocurrency without collateral, provided the entire loan is repaid within the same blockchain transaction. If the borrower cannot repay by the end of the transaction, the entire operation reverts as if it never happened — the blockchain's atomic transaction property guarantees the lender faces zero risk.

Flash loans enable strategies previously reserved for well-capitalized institutions: arbitrage across exchanges (buying cheap on one DEX and selling higher on another), collateral swapping in lending protocols (changing collateral without closing positions), self-liquidation (paying off loans more efficiently), and complex multi-step DeFi interactions. A single flash loan transaction might interact with 5-10 different protocols in a chain of operations.

However, flash loans have also been used extensively in protocol attacks. Attackers use flash-borrowed funds to manipulate price oracles, drain liquidity pools through economic exploits, or execute governance attacks. Major incidents include the $130M Cream Finance exploit and numerous smaller attacks totaling hundreds of millions in losses. This has driven improvements in oracle design (time-weighted average prices), protocol security audits, and economic attack modeling. Flash loans represent one of crypto's most powerful and double-edged innovations — democratizing access to capital while creating new attack vectors.