What is a flash loan?

A flash loan lets you borrow any amount of crypto from Aave's liquidity pools with no collateral, as long as you return the full amount plus a fee within the same Ethereum transaction. That is the entire mechanism. Borrow, use, and repay, all in a single atomic operation that takes around 13 seconds to complete.

The first time most people hear this, it sounds like a free lunch. It is not. The constraint is brutal: if you do not repay within that same transaction, the whole thing reverts as if it never happened. The lender never loses anything because no credit is actually extended. It is closer to a guarantee than a loan.

Aave introduced flash loans in January 2020. Nothing as it had existed in traditional finance. It is one of the genuinely new financial primitives that DeFi created.

Why does the same-transaction repayment remove credit risk

Traditional lending requires credit checks, collateral, or both because there is always a risk that the borrower will not repay. Flash loans solve this differently. The entire borrowing and repayment happens inside a single transaction on the Ethereum blockchain.

Ethereum transactions are atomic, meaning they either complete fully or revert entirely. There is no in-between. If your flash loan transaction cannot repay Aave before the transaction closes, the EVM discards the entire sequence. The funds go back to the pool, the borrow never happened from the protocol's perspective, and no one loses anything.

This is why you can borrow millions with no collateral. The code enforces repayment. Trust is irrelevant.

Common use cases: arbitrage, liquidations, collateral swaps

Flash loans exist because they are useful for specific on-chain operations that require large amounts of capital for a very short time.

Arbitrage is the most common use case. If ETH trades at $2,000 on Uniswap and $2,010 on Curve, a flash loan lets you borrow a large amount, buy on the cheaper exchange, sell on the more expensive one, repay the loan with the fee, and pocket the difference, all in one transaction. The profit is small per unit, which is why the size of the loan matters.

Liquidations work similarly. When a borrower's collateral falls below the required level on Aave, anyone can liquidate their position and receive a bonus. A flash loan lets a liquidator seize that opportunity without needing capital of their own.

Collateral swaps let users change their collateral without closing a position. If you are using ETH as collateral and want to switch to wBTC, a flash loan can fund the swap in a single transaction, avoiding the need to repay and reborrow manually.

Famous flash loan exploits and what we learned

Flash loans have also been used as attack vectors. This is worth understanding because it changed how DeFi protocols think about security.

In February 2020, the bZx protocol was exploited twice using flash loans, losing around $1 million in total. The attacker borrowed large amounts to manipulate price oracles and profit from the distortion. In October 2021, Cream Finance lost $130 million to a flash loan attack that exploited a reentrancy vulnerability. In April 2022, Beanstalk lost $182 million when an attacker used a flash loan to acquire enough governance tokens in a single block to pass a malicious proposal and drain the treasury.

The lesson from these exploits was not that flash loans are dangerous. It was that price oracles and governance systems need to be designed with flash loan attacks in mind. Protocols that rely on single-block price snapshots or that allow instant governance execution are vulnerable. Most serious protocols now use time-weighted average prices and governance delays specifically to prevent this.

Flash loan fees and how to use them

Aave charges 0.05%, which is 5 basis points, per flash loan. On a $1 million loan, that is $500. The fee goes to the liquidity pool, so suppliers benefit every time a flash loan is taken.

Using a flash loan requires writing a smart contract in Solidity that calls Aave's flash loan function, executes your desired logic, and returns the funds plus the fee. It is not a consumer-facing feature with a button on a website. It requires development skills and an understanding of how Ethereum transactions work. Several third-party tools and templates exist to make the process easier, but it remains a tool primarily for developers and sophisticated DeFi users.

Flash Loans on Aave and How Uncollateralised Borrowing Works