Flash Loan

A flash loan lets you borrow millions of dollars with zero collateral — as long as you pay it back before the transaction closes. Not the next day. Not in an hour. Within the same block, which takes roughly 12 seconds on Ethereum.

That sounds like a loophole, but it's actually elegant engineering. The blockchain enforces repayment automatically: if the loan isn't returned plus fees by the end of the transaction, the entire sequence of events reverts as if it never happened. The lender never actually loses funds.

How It Works

Here's a concrete example. Say ETH trades at $3,000 on Uniswap and $3,045 on another DEX. You borrow $3,000,000 in USDC via a flash loan, buy 1,000 ETH on the cheaper exchange, sell it on the more expensive one, pocket the $45,000 spread, repay the loan plus a small fee, and keep the profit. Total time: one transaction.

Flash loans also power collateral swaps and liquidation protection. If your loan is approaching its liquidation threshold — say your LTV climbs toward 80% — a flash loan can swap your collateral from one asset to another in a single atomic step, without you ever needing to close the position manually.

Aave popularized the model and charges 0.05% per flash loan. The fee is small because the risk to the protocol is near zero — the smart contract guarantees repayment or reversion. There's no credit check because there's no credit risk.

Why It Matters

Flash loans keep DeFi markets efficient. Arbitrageurs using them close price gaps across exchanges faster than any human trader could. That benefits every user on those platforms by tightening spreads.

They also democratize capital. In traditional markets, executing a $3M arbitrage trade requires either $3M or a prime brokerage relationship most retail traders will never get. A flash loan hands that same leverage to anyone who can write the code.

What to Watch

Flash loans have been the tool of choice in some of DeFi's largest exploits. Attackers borrow enormous sums, manipulate an on-chain price oracle within the same transaction, drain a protocol, and repay the loan — all before the block closes. The loan itself isn't the vulnerability, but the capital it provides can amplify one.

As a borrower or depositor on a lending platform, you don't execute flash loans — but you're exposed to them. If the protocol you're using relies on a single price oracle that can be manipulated with a large enough position, a flash loan attack can drain the pool your funds sit in. Audit reports and oracle design matter more than the APY.