What is Flash Loan?

A flash loan is an uncollateralized DeFi loan that must be borrowed and repaid within the same blockchain transaction — typically lasting just seconds — enabling users to access potentially unlimited capital with zero upfront collateral, provided the loan is repaid before the transaction finalizes. Flash loans are one of DeFi’s most novel and uniquely blockchain-native financial innovations, with no equivalent in traditional finance.

In traditional lending, borrowers must provide collateral or demonstrate creditworthiness. Flash loans eliminate this requirement through a simple but powerful mechanism: the lending smart contract issues the loan, the borrower executes their strategy, and the loan plus fees must be repaid — all within a single atomic transaction. If the repayment condition isn’t met, the entire transaction reverts as if it never happened, and the lender loses nothing.

Aave pioneered flash loans in 2020, and they remain the largest flash loan provider. The typical fee is 0.05-0.09% of the borrowed amount. Because the loan exists only within a single transaction, there is no counterparty risk — the smart contract guarantees repayment or complete reversal.

Legitimate flash loan use cases include arbitrage (exploiting price differences between DEXs without needing capital), collateral swaps (changing the collateral backing a DeFi loan without closing the position), self-liquidation (repaying a loan to avoid costly liquidation penalties), and interest rate optimization (refinancing between lending protocols in a single transaction).

However, flash loans have also enabled some of the largest DeFi exploits. Attackers have used flash-borrowed capital to manipulate oracle prices, exploit governance voting mechanisms, and drain liquidity from vulnerable protocols. Notable flash loan attacks include the bZx exploits (2020), Cream Finance ($130 million, 2021), and numerous smaller protocol drains. In these attacks, the borrowed capital is used to manipulate market conditions, extract value from a vulnerable protocol, repay the loan, and pocket the profit — all in one transaction.

The existence of flash loans has forced DeFi protocols to dramatically improve their security practices, particularly around oracle design (using time-weighted average prices instead of spot prices), governance safeguards (implementing voting delays), and economic modeling (stress-testing against flash-loan-funded attacks).

Flash loans have also democratized access to financial strategies previously available only to well-capitalized actors. An individual with zero capital can execute the same arbitrage as a hedge fund, provided they can code the transaction or use platforms like Furucombo that offer no-code flash loan interfaces.

Last updated: April 2026