Quick answer
A flash loan is an uncollateralised DeFi loan borrowed and repaid within the same blockchain transaction. If the funds are not returned (plus a small fee) before the transaction ends, every action in the transaction is automatically reversed — as if the loan never happened. Flash loans require no credit check, no collateral, and can be for any amount up to the pool's full liquidity.
What Is a Flash Loan?
A flash loan is an uncollateralised loan available in decentralised finance (DeFi) that must be borrowed and repaid within the same blockchain transaction. Unlike a traditional loan — which requires collateral, a credit check, and repayment over time — a flash loan requires nothing except the ability to return the borrowed amount plus a small fee before the transaction completes.
If the borrower fails to repay within the same transaction, the blockchain's smart contract reverses every action in the transaction as if the loan and everything done with it never occurred. The lender never risks any capital, because the loan only exists for the duration of a single transaction block.
Flash loans were pioneered by Aave in 2020 and are now available across multiple DeFi protocols. They were designed for developers and arbitrage bots to execute complex multi-step operations with large amounts of capital without any collateral — as long as the operation is profitable enough to repay within a single block.
How Flash Loans Work: Step by Step
- 01
Request the loan
A smart contract or bot sends a transaction to a flash loan provider (such as Aave's pool), requesting a specific amount — for example, 1,000,000 USDC.
- 02
Receive funds and execute logic
The contract releases the funds immediately. The borrower's code then executes all its operations — for example, buying a token cheaply on one DEX and selling at a higher price on another.
- 03
Repay the loan plus fee
Before the transaction ends, the borrower's code must repay the full loan plus a fee (typically 0.05–0.09% on Aave). The repayment must happen within the same transaction.
- 04
Transaction confirms or reverts
If repayment succeeds, all actions are permanently recorded on-chain. If repayment fails for any reason, the entire transaction reverts — no funds move, no state changes, the lender keeps their capital intact.
A flash loan exists only for the duration of a single transaction block. No collateral or credit history is needed — the smart contract's atomicity guarantee is the only protection the lender requires.
Legitimate Uses of Flash Loans
- Arbitrage
- If the same token trades at different prices on two DEXs, a flash loan lets a bot borrow millions, buy cheap on one exchange, sell high on another, repay the loan, and keep the difference — all in one transaction with no upfront capital.
- Collateral swaps
- A borrower can use a flash loan to swap their collateral type on a lending protocol without closing the loan. For example: borrow enough to repay an ETH-collateralised loan, replace ETH with WBTC as collateral, repay the flash loan — all atomically in one transaction.
- Self-liquidation
- If a position is close to liquidation, the owner can use a flash loan to repay the debt, retrieve collateral, and avoid the liquidation penalty — without needing capital upfront.
- Protocol liquidations
- Liquidation bots use flash loans to close undercollateralised positions on lending protocols, earning a liquidation bonus for maintaining protocol solvency without needing to hold capital themselves.
Flash Loan Exploits: When They Are Misused
Flash loans have also been used in some of the largest DeFi exploits in history. The uncollateralised access to enormous capital makes them a powerful tool for manipulating protocols with vulnerable price oracles.
The bZx attack (2020) was among the first: an attacker borrowed 10,000 ETH via flash loan, used part to manipulate an illiquid oracle, and profited from a resulting position — approximately $1M in damage. The Pancake Bunny exploit (2021) used a flash loan to inflate BNB price, manipulate an internal oracle, and mint enormous quantities of BUNNY tokens immediately sold for approximately $45M.
In most flash loan attacks, the vulnerability is in the target protocol's price oracle design — not in flash loans themselves. A protocol using robust, manipulation-resistant oracles (Chainlink decentralised feeds or TWAP) cannot be exploited by flash loan capital alone.
Flash loan exploits attack oracle vulnerabilities in target protocols, not flash loans themselves. A well-designed protocol with decentralised price oracles is not vulnerable to flash loan manipulation.
Flash Loan Providers in 2026
| Provider | Fee | Chains Supported | Notable |
|---|---|---|---|
| Aave V3 | 0.05% | Ethereum, Arbitrum, Optimism, Polygon, Base | Largest by volume; widest token support |
| Uniswap V3 Flash Swaps | Pool fee (0.05–1%) | Ethereum + all Uniswap V3 chains | Any token in a Uniswap pool; fee goes to liquidity providers |
| Balancer | 0% | Ethereum, Arbitrum, Polygon | Fee-free flash loans; used for complex multi-pool strategies |
| dYdX | 0% | dYdX Chain (Cosmos) | Zero-fee; designed to attract arbitrage volume to the platform |
Frequently asked questions
Can anyone take out a flash loan?
Technically yes — flash loans are permissionless. However, to make use of one you need to write and deploy smart contract code specifying what to do with the borrowed funds, or use a no-code tool such as Furucombo or DeFi Saver. Simply borrowing and immediately repaying with no action in between earns nothing and costs the fee plus gas.
Are flash loans legal in the UK?
Flash loans themselves are not illegal. They are a technical financial primitive on public blockchains. Using flash loans to deliberately exploit a protocol could potentially constitute computer fraud under the Computer Misuse Act 1990, though this remains largely untested in UK courts. Arbitrage and collateral management uses are considered legitimate.
How much can you borrow in a flash loan?
Up to the full available liquidity in the provider's pool. On Aave V3, individual pools can hold hundreds of millions of dollars. There is no fixed hard cap — the limit is the pool's liquidity at the moment of the transaction.
What fee do flash loans charge?
Aave charges 0.05% of the borrowed amount. Balancer and dYdX offer zero-fee flash loans. Uniswap charges the standard pool fee (0.05%, 0.3%, or 1% depending on the pool). For a $1,000,000 Aave flash loan, the fee is $500.
What is the difference between a flash loan and a regular DeFi loan?
A regular DeFi loan (Aave, Compound) requires collateral worth more than the loan, accrues interest over time, and can remain open indefinitely. A flash loan requires no collateral, charges a one-time fee, and must be fully repaid within a single blockchain transaction — typically within seconds.