Flash Loans in DeFi: How to Get a Flash Loan

Key article highlights

Flash loan does not require collateral – the atomicity of the transaction in the blockchain serves as a guarantee of return.
If funds are not returned by the end of the transaction, the blockchain rolls back everything – the protocol loses nothing.
Liquidity comes from pools – providers earn a fee with each use.
Arbitrage between DEXs is the most common scenario, but it requires precise calculation of all costs.
Without Solidity programming skills, it is impossible to implement a flash loan independently.
The main risks are smart contract errors and oracle data manipulation, not the lending mechanism itself.

Understanding what a flash loan is and why people are talking about it
Is it true that no collateral is needed – and how it works
Why such loans are only possible in the blockchain
Where the money comes from and who provides it
Three ways to earn on instant loans
What is important to remember: flash loans risks
FAQ
Conclusions

What is a flash loan in DeFi in simple terms

A flash loan is a loan without collateral that must be returned before the transaction is closed. Not within a day, not in an hour – literally within a single blockchain block, in a fraction of a second.

Let's look at an analogy. Imagine you were given a million dollars, but with one condition: the money must be returned before the door closes. If you managed to use and return it – the deal took place. If you didn't – it simply doesn't count, as if nothing happened. This is exactly how the atomicity of a transaction (the "all or nothing" principle) works in the blockchain.

From a technical point of view, a flash loan is a sequence of smart contract calls within one block. The protocol transfers funds, you perform the necessary operations – and return the amount plus a fee. If the return does not happen, the chain rolls back.

The concept appeared in 2018 in the Marble protocol, but flash loans began to be used massively after the launch of Aave in 2020. Today they are available on Aave, Uniswap v3, and dYdX.

Is collateral needed for flash loans and how they are actually issued

Collateral for a flash loan is not needed. This is one of the main questions that arises for those who encounter this tool for the first time. The answer is simple: the role of collateral is performed by the transaction mechanics. The protocol does not take risks – if the money is not returned – the blockchain cancels the entire operation.

How the flash loan issuance process looks:

the user deploys a smart contract with the logic of the necessary operations;
the contract contacts the protocol (e.g., Aave) and requests an amount – even 10 million USDC;
Aave transfers funds to the contract and calls the executeOperation() function;
inside the function, the contract performs the necessary operations: swaps, purchases, liquidations;
at the end of the function, the contract returns the original amount plus a fee – 0.05–0.09% depending on the protocol;
if the return did not happen – the transaction rolls back, the pool loses nothing.

The key thing to understand here is: the risk of non–return does not exist as such – it is excluded at the level of the blockchain architecture. Therefore, the lender does not require a credit history, KYC, or collateral.

The only real requirement is the ability to write smart contracts in Solidity or work with ready–made templates. Without this skill, it will not be possible to get a flash loan – there will be no code to implement the process.

DeFi lending mechanisms: why flash loans are only possible in the blockchain

Flash loans work specifically in the blockchain – and nowhere else. There is only one reason: atomicity. In traditional finance, there is a time gap between the issuance of a loan and its return. During this interval, the borrower may not return the money. In the blockchain, there is no such gap: the smart contract checks the return even before the transaction gets into the block. Let's look at a comparison to make the difference clear:

Parameter	Flash loan	Regular DeFi loan	Bank loan
Collateral	Not needed	150–200% of the amount	Needed
Non–return risk	Architecturally excluded	Exists	Exists
Fee	0.05–0.09% one–time	1–15% per annum	5–20% per annum
Identification	Not required	Not required	Mandatory
Amount limit	Liquidity pool size	Collateral size	Income level

In the blockchain, the smart contract checks for the presence of a return within the same transaction – and after that fixes it in the block. This makes flash loans a technically unique tool.

Where the liquidity for flash loans comes from and who gives this money

Liquidity for flash loans comes from the pools of decentralized protocols – primarily dYdX, Aave, and Uniswap v3. These pools are formed by regular users: they deposit their assets and receive income from fees for each use. Liquidity providers do not take on any additional risk. The funds do not leave the protocol's smart contract for longer than the time of one transaction. The mechanics guarantee a return at the code level.

According to Dune Analytics data, by February 2026, Aave passed the historical milestone of 1 trillion dollars in total volume of all loans issued throughout its existence. Liquidity providers earn on fees without even noticing that their funds are involved in arbitrage operations. The income structure for providers is simple: when someone takes a flash loan from the pool, a fee (0.05–0.09% of the amount) is distributed among all participants proportionally to their share. On large volumes, this forms a stable passive income without active participation.

If you are interested in how liquidity pools work in general and what DeFi tools are available for work – we recommend referring to our blog.

How to earn on flash loans: main methods

In practice, flash loans are used in three main scenarios: arbitrage between decentralized exchanges, self–liquidation of own debts, and collateral recapitalization. All three require technical knowledge and precise calculation of transaction economics – otherwise, the operation will go into the red before you have time to earn anything.

Cryptocurrency arbitrage using flash loans between exchanges

Cryptocurrency arbitrage using flash loans is the most common way to use instant loans. The essence: buy an asset cheaper on one DEX (decentralized exchange) and sell it dearer on another, using borrowed funds that you don't initially have. Let's look at a specific example – hypothetical but realistic:

you take a flash loan of 500,000 USDC via Aave;
you buy ETH on Uniswap v3 at a price of 2,950 USDC per 1 ETH;
you sell ETH on SushiSwap, where the price is 2,980 USDC per 1 ETH;
you return USDC + fee ~250 USDC (0.05%) to Aave;
you pay the transaction gas – around 20–80 USDC depending on the network load;
net profit – a few hundred dollars, or zero if gas ate the difference.

There is an important point here: price discrepancies between DEXs exist for seconds. Bots monitor the mempool (transaction queue) and close arbitrage windows faster than manual execution. Successful DeFi arbitrage requires automated scripts and optimized gas – without this, it is impossible to compete with bots.

The profitability calculation must include: the protocol fee for the flash loan, gas cost, slippage on each swap, and the difference in liquidity depth of the pools. If the sum of these costs exceeds the price spread – the operation is unprofitable even before the start.

Self–liquidation of debts in lending protocols

If a position in Aave or Compound (lending protocols) is close to the liquidation threshold, the standard scenario is to wait until a liquidator bot forcibly closes it, taking a 5–15% penalty. A flash loan allows you not to wait and close the debt yourself – without a penalty. The scheme looks like this: you take a flash loan for the amount of the debt, repay the position, release the collateral, sell part of the collateral to return the flash loan, and the rest of the collateral is returned to you. Everything happens in one transaction, in seconds. This is especially valuable during periods of sharp volatility when every minute counts.

Quick replacement of one collateral with another (Collateral Swap)

This is convenient if you want to change the asset in the collateral without closing the position. For example, you have ETH, but you expect it to fall and want to replace it. A Collateral Swap is useful when the market falls quickly. It allows you to instantly "change gears" into a protective asset without spending time on withdrawing funds and re–depositing collateral.

The mechanics of a Collateral Swap via a flash loan look like this: you take an instant loan to fully repay the current debt, which automatically unlocks your ETH collateral. Next, the released ether is deposited back into the protocol as new security. A new loan is opened against this collateral, the funds from which are directed to closing the flash loan. As a result, the composition of the position changes without closing the deal, and all stages are packed into one transaction.

Collateral Swap is especially useful during periods of high volatility: when one asset loses ground and another grows, collateral migration allows you to maintain the position – without additional funds and without closing the deal.

Main risks of using flash loans

Flash loans themselves are a neutral tool. Risks arise from how they are used and from vulnerabilities in other protocols that instant large capital allows exploiting more effectively. Let's look at the main risks of using flash loans:

Errors in smart contracts. A bug in the executeOperation() logic can lead to a failed transaction. Gas is not returned in this case – every unsuccessful attempt costs real money.
Price oracle manipulation. An oracle is a source of price data for a smart contract. A number of high–profile attacks were built on the fact that an attacker used a flash loan to sharply change the price in a pool and then exploited erroneous oracle data in another protocol.
Gas cost. With high network load, gas can eat up all the profit. Complex transactions with multiple swaps are expensive.
MEV and frontrunning. Bots monitor the mempool and can insert their transaction before yours, intercepting the arbitrage opportunity.
High entry barrier. Without knowledge of Solidity and an understanding of the DeFi protocol architecture, it is impossible to write a working flash loan contract.

In the attacks on bZx and Harvest Finance protocols in 2020, attackers withdrew a total of tens of millions of dollars precisely through oracle manipulation using flash loans. The flash loan mechanism is not a vulnerability – the protocols themselves turned out to be vulnerable as they were not designed to work with suddenly large capital within a single transaction.

FAQ

Can I do a flash loan without programming?

Directly – no. A flash loan requires a smart contract in Solidity. There are visual platforms like Furucombo or DeFi Saver, but they greatly limit the flexibility of scenarios and are only suitable for basic operations.

Flash loans – what are they from a legal standpoint?

Flash loans are a completely legal DeFi tool. Specific actions using them can be illegal: market manipulation, exploitation of vulnerabilities in other protocols. The lending mechanism itself is neutral.

What is the minimum flash loan size?

Theoretically – any. In practice, taking less than a few thousand dollars makes no sense: the gas fee and protocol fee (0.05–0.09%) will make the operation unprofitable.

How can a beginner earn on flash loans?

Start by studying Solidity and test networks (Sepolia). Deploy a basic contract based on the Aave template, work out the arbitrage logic without real funds. Only after successful tests – move to the mainnet.

How do flash loans affect the cryptocurrency market?

Arbitrage operations equalize prices between platforms and improve market efficiency. During periods of instability, they can temporarily increase volatility – especially in pools with low liquidity.

What happens if a flash loan is not returned?

The transaction rolls back. The protocol does not lose funds. The user only loses the gas spent on the attempt.

Which protocols support DeFi lending mechanisms via flash loans?

The main ones are Aave V2/V3, Uniswap v3 (flash swaps), dYdX, Balancer. Each has its own fee structure and API for interaction. Aave is the most popular option for getting started.

Conclusions

Flash loans are a tool that became possible solely due to the blockchain architecture. Collateral–free credit, instant return, zero risk for the protocol – all this works because the atomicity of the transaction makes non–return technically impossible. Arbitrage between DEXs, self–liquidation of positions, collateral replacement are real earning scenarios, but each requires technical preparation and precise calculation.

If you want to dive deeper into DeFi tools – start with the materials in our blog. And if you are ready for practice – go to the main page and explore the available services for working with decentralized finance.

Flash Loans in DeFi: Mechanisms and Use Cases for Arbitrage