Flash Loans and Uncollateralised Lending

In ordinary finance, borrowing without collateral means a lender trusts you. A flash loan takes trust out of it completely, and it can do that because of one property of blockchain transactions: they either complete in full or don't happen at all.

How does a flash loan work?

A blockchain transaction can contain many steps. A flash loan puts borrowing, using and repaying into one of them:

1. Borrow a large amount from a lending protocol's pool, with nothing put up.
2. Do something with it: trade, repay another loan, swap collateral.
3. Repay the full amount plus a small fee before the transaction ends.

The lending contract checks at the end that its pool is whole again. If it isn't, the entire transaction reverts. The borrowed money, the trades and every intermediate step are undone as if they never happened, and the borrower loses only the gas fee they paid to try.

That's why lenders can offer millions with no collateral. A defaulting borrower can't exist, because a loan that isn't repaid never happened in the first place.

What are flash loans used for legitimately?

Arbitrage. If a token is cheaper on one exchange than another, a flash loan can buy on one and sell on the other in the same transaction, repay, and keep the difference. Without a flash loan you'd need that capital already. With one, anyone who writes the contract can capture the gap, which helps keep prices across venues in line.

Collateral swaps and refinancing. Say you have a loan backed by one token and want to back it with another. Normally that takes several steps and capital you might not have. A flash loan can repay the old loan, release the collateral, swap it, open the new position and repay itself in one go.

Self-liquidation. A borrower close to forced liquidation can use a flash loan to close their own position on better terms than the protocol's liquidation penalty.

Why do flash loans show up in so many exploits?

Because they remove the one thing that used to limit attackers: needing a lot of money.

The typical pattern is oracle manipulation. A lending protocol needs to know what a token is worth, and some protocols read that price straight from a single on-chain exchange pool. If that pool is thin, a huge trade moves its price sharply. With a flash loan, an attacker borrows a fortune, uses it to distort the price in the pool, borrows against or trades with the victim protocol at the false price, puts the pool price back and repays the loan, all in one transaction.

That pattern, and variations on it, cost DeFi protocols heavily between 2020 and 2023. In the Beanstalk governance attack of 2022, borrowed tokens were used to win a governance vote and drain the treasury. Euler Finance lost around $197 million in 2023, most of which was later returned.

Note what was actually broken in each case: a manipulable price feed, a governance design that counted borrowed votes, a flaw in contract logic. The flash loan made the attack affordable. It didn't create the bug. Protocols that use time-weighted or multi-source price feeds are far harder to attack this way.

What about uncollateralised lending more broadly?

People searching for "uncollateralised lending" are often looking for something different from flash loans: a way to borrow on credit over weeks or months.

That exists in crypto, but it isn't trustless. Lending pools for institutions extend credit to vetted, identified borrowers, usually trading firms. The 2022 collapse showed the risk clearly: several large borrowers defaulted and lenders in those pools took real losses. That's ordinary credit risk wearing a DeFi interface. If someone offers you high yield for lending without collateral, the question to ask is who the borrower is and what happens if they don't pay. "It's on-chain" doesn't answer either.

What does this mean if you just hold tokens?

You'll never take out a flash loan, but they can still reach you. If you hold a protocol's governance token, deposit into a lending market or provide liquidity, a flash-loan-funded exploit on that protocol can drop the token's price or drain the pool within a single block.

You can't out-trade an exploit that completes in twelve seconds. What helps is noticing quickly and understanding what happened before deciding anything. That means alerts on abnormal moves in what you hold, and a sober read of what an exploit actually means before reacting to the first headline.