Flash Loans: How to Borrow $200M With Zero Collateral

Let me tell you about the most insane thing in crypto.

Someone borrowed $200 million dollars. No collateral. No credit check. No questions asked.

They used that money. Made some trades. Returned it 12 seconds later.

Profit? $1.2 million.

And here's the kicker - this is completely legit. This is a flash loan.

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Wait, how is that even possible?

I know what you're thinking. This sounds like a scam. Or magic. Or some kind of glitch in the matrix.

But it's none of those things. It's just really, really clever use of how blockchains work.

See, here's the thing about blockchain transactions that most people don't get: they're atomic.

What does that mean? It means everything in a transaction either happens together, or nothing happens at all. There's no in-between.

Think about it like this:

You send a transaction that says:

1. Borrow $1 million
2. Do something with it
3. Return $1 million plus a small fee

If step 3 fails? The whole thing rewinds. Like it never happened. The protocol gets its money back. You lose nothing except gas fees.

The debt literally exists for about 12 seconds. Then it's either paid, or it never existed.

Mind = blown, right?

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But why would anyone lend money like this?

Here's where it gets beautiful.

The lender (let's say Aave) has zero risk. None. Zilch.

Think about it. If you don't pay back? Transaction reverts. They never actually gave you the money. It's like the whole thing was a dream.

Worst case scenario for them? You waste some gas trying to do something that fails. And guess what - you pay that gas, not them.

Meanwhile, they charge 0.09% on every flash loan. Billions of dollars flow through flash loans every month. That's millions in fees. Risk-free.

It's genius.

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Okay but what do people actually DO with this?

Let me give you a real example. Say you're a trader and you notice something:

• On Uniswap, ETH is trading at $2,000
• On Sushiswap, ETH is trading at $2,050

That's a $50 difference. Free money, right? Buy low, sell high.

But there's a problem. To make any real profit, you need capital. Lots of it.

Enter flash loans.

Here's what you do:

1. Flash loan $1 million from Aave
2. Buy 500 ETH on Uniswap at $2,000
3. Sell 500 ETH on Sushiswap at $2,050
4. You now have $1,025,000
5. Pay back $1,000,900 (loan + fee)
6. Keep $24,100 profit

The whole thing takes one transaction. About 12 seconds.

And if at any point something goes wrong - if the price moved, if there's not enough liquidity, if anything fails - the whole thing just... doesn't happen. You lose nothing.

You just tried to make $24k with zero risk and zero capital.

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The dark side

Now here's where things get spicy.

Flash loans aren't just for arbitrage. They're also the weapon of choice for hackers.

Let me paint you a picture.

Some DeFi protocol (let's call them "Victim Protocol") makes a rookie mistake. They use the current price on Uniswap as their source of truth. Their oracle, if you will.

Bad move.

Here comes the attacker:

1. Flash loan 10,000 ETH
2. Dump all of it into Uniswap's ETH/USDC pool
3. Price of ETH in that pool crashes from $2,000 to like $20
4. Victim Protocol looks at Uniswap: "Oh wow, ETH is worth $20!"
5. Victim Protocol lets attacker liquidate everyone's positions for pennies
6. Attacker buys everyone's ETH for almost nothing
7. Attacker buys back their 10,000 ETH from Uniswap
8. Pays back flash loan
9. Walks away with millions in stolen ETH

All in one transaction. 12 seconds.

The victim wakes up the next morning: "Wait, where's my money?"

This isn't theoretical. Mango Markets lost $110 million this way. Cream Finance lost $130 million. Harvest Finance lost $34 million.

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Is this... legal?

Honestly? It's complicated.

"Code is law" is the crypto mantra. If the code lets you do it, you can do it. That's the whole point of permissionless systems.

But try explaining that to a judge.

Some of these attackers have been caught and charged. Others are chilling on a beach somewhere counting their millions.

The line between "clever arbitrage" and "theft" is blurry as hell in DeFi.

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Why does this only work in crypto?

This is the part that always blows my mind.

Traditional finance can't do this. It's literally impossible.

Why? Because settlement takes days. When you buy stocks, the actual transfer of ownership takes T+2 - two business days.

You can't "undo" a bank transfer if something goes wrong. Once money moves, it moves.

But on a blockchain? Everything is instant. And everything is atomic. And smart contracts can enforce rules automatically.

Flash loans are only possible because of these properties. They're the most "crypto-native" thing that exists.

TradFi literally cannot copy this. Even if they wanted to.

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The bottom line

Flash loans are wild.

They let anyone access unlimited capital for a few seconds. No credit check. No collateral. No permission needed.

They power legitimate arbitrage that keeps markets efficient.

They also power attacks that have stolen hundreds of millions.

They're beautiful and terrifying at the same time.

And they're only possible because someone looked at how blockchains work and said "wait, we can do WHAT with this?"

That's crypto for you. Always finding new ways to break your brain.

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Want to go deeper? The code is actually pretty simple. Aave's flash loan interface is like 50 lines. But that's a story for another article.