In simple terms — A flash loan lets you borrow assets, use them, and repay them inside one transaction. If even part of the required amount is missing at the end, the blockchain cancels the entire sequence.
The loan is not “free.” It normally requires a fee, technical ability, and enough funds for gas and execution differences. Traditional collateral is not needed because the protocol never leaves debt open between two blocks: it either receives principal and fee before the transaction ends, or the state returns to what it was before the operation began.
Atomicity replaces collateral
A smart contract requests the assets, performs several operations, and authorizes repayment. All these calls form one atomic unit. The capital can be used for arbitrage between markets, collateral swaps, refinancing, or liquidating positions. Feasibility depends on liquidity, fees, operation order, and the network state at that exact moment.
Under the flashLoanSimple path, the borrower cannot keep the principal after
the transaction: the final check requires principal and fee. Aave V3's
flashLoan can instead open variable debt when sufficient collateral or credit
delegation exists; that outcome is no longer an uncollateralized loan closed
atomically. A strategy that looks profitable before execution may still fail
and consume gas if prices or other conditions change.
A neutral tool, real attacks
A flash loan is not an attack by itself. It can, however, temporarily provide the large amount of capital needed to exploit an existing flaw: a manipulable oracle, a shallow pool, a calculation error, or a vulnerable governance rule. “Flash-loan attack” therefore often describes how the operation was funded, not its primary technical cause.
Analyzing an incident requires reconstructing every call: the loan's origin, markets crossed, prices used by contracts, reserve changes, and final destination. Blaming everything on the flash loan can hide the real design failure. For developers, robust oracles, economic limits, and invariant checks matter more than merely trying to recognize the loan.
Sources
- Aave Docs — Flash Loans — Documents atomicity, repayment, fees, the execution flow, and the main applications.
- Aave Docs — Pool contract — Specifies the
flashLoanandflashLoanSimplefunctions exposed by the main contract.