How Do Flash Loans Work in DeFi?

How Do Flash Loans Work in DeFi?

The world of DeFi introduces a unique loan concept where borrowers are not required to invest their own money. Instead, they must repay the lender instantly within the same transaction that provided the funds. This begs the question: what opportunities arise from a loan that needs to be repaid within seconds?

Remarkably, this type of loan can be leveraged by employing smart contracts in the same transaction. By generating additional profits using the borrowed funds, borrowers can promptly repay the loan and retain the remaining earnings. Nevertheless, it is crucial to acknowledge that this process is not as simple as it appears.


The cryptocurrency realm is abuzz with excitement over the prospect of revolutionizing the traditional financial system through blockchain technology. While skeptics may express their doubts, notable progress is being made in constructing intriguing infrastructure in this field.

The fundamental goal of decentralized finance (DeFi) is to establish a transparent, decentralized, and permissionless financial ecosystem atop blockchain networks. Cryptocurrencies have already demonstrated the feasibility of achieving this with monetary transactions. Every day, systems like Bitcoin facilitate the seamless transfer of value across the globe.

The latest wave of DeFi technologies introduces an additional layer of innovation. Presently, individuals have the opportunity to access crypto-backed loans, trustlessly exchange digital assets, and store wealth in coins that mirror the value of traditional fiat currencies.

In this piece, we will delve into a specific category of loans known as flash loans. These loans represent truly unique additions to the expanding decentralized finance landscape.

How Do Loans Work in General?

Before we delve into the comparison, let's briefly revisit the concept of a regular loan, which is familiar to most people.

Unsecured Loans

An unsecured loan does not require collateral, meaning there is no specific asset for the lender to claim if you fail to repay the loan. For instance, suppose you desire a $3,000 luxury watch. Although you currently lack the funds, you anticipate having the necessary cash when you receive your upcoming paycheck.

You discuss your desire with your friend Sarah, explaining how the watch will complement your style and boost your confidence. Sarah, being a good friend, agrees to lend you the money on the condition that you repay her as soon as you receive your paycheck. Sarah's trust in your friendship leads her to forgo charging any fees for lending you the $3,000. However, not all lenders may extend such kindness, as they may not know you well enough to trust your repayment commitment.

Typically, financial institutions offering unsecured loans conduct credit checks to assess your repayment capability. They review your credit history, particularly your credit score, to determine if you have a reliable track record of repaying loans on time. A positive credit history increases your chances of being approved for a loan.

Once approved, the loan comes with certain conditions, primarily in the form of interest rates. By accepting the loan, you acknowledge that you will be paying back a higher amount in the future. This model may resemble your experience with credit cards, where failing to pay the bill within a specified period incurs interest charges until the balance is settled, along with additional fees.

Secured Loans

In certain situations, having a good credit score alone may not suffice. Even if you have a long history of timely loan repayments, obtaining a substantial loan based solely on your creditworthiness can be challenging. In such cases, collateral becomes a necessity.

When you approach someone for a significant loan, it carries an inherent risk for the lender. To mitigate this risk, they will require you to provide collateral. Collateral can take various forms, ranging from real estate to valuable assets, and serves as an asset that the lender can claim if you fail to repay the loan. This arrangement ensures that the lender can recoup some of their potential losses. In essence, collateral acts as a form of security for the lender.

For instance, let's say you want to purchase a new boat worth $50,000. Your friend David trusts you, but he prefers not to offer an unsecured loan. Instead, he requests that you provide collateral in the form of your valuable artwork collection. By doing so, if you were to default on the loan, David would have the right to seize and sell your artwork collection to recover the outstanding amount.

How Does a Flash Loan Work?

Flash loans can be categorized as unsecured loans since they do not require collateral from the borrower. Notably, no credit check or similar assessment is necessary to obtain these loans. The process is remarkably straightforward: you simply request to borrow a specific amount, such as $50,000 in ETH, and upon approval from the lender, the funds are promptly provided without any collateral obligations.

However, there is a crucial caveat to consider. Flash loans must be repaid within the same transaction in which they are obtained. This peculiarity may appear counterintuitive when compared to conventional transactions involving the transfer of funds between parties, such as making payments or depositing tokens into an exchange.

Nevertheless, Ethereum's remarkable flexibility, often referred to as programmable money, offers a comprehensive framework for flash loans. Conceptually, the transaction sequence comprises three distinct parts: receiving the loan, utilizing the loan, and ultimately repaying the loan. Notably, this entire process unfolds within an exceptionally brief timeframe.

The seamless functionality of flash loans can be attributed to the inherent capabilities of blockchain technology. When a transaction is submitted to the network, the funds are temporarily allocated to the borrower. During the utilization phase, various actions can be performed with these funds. However, it is imperative to ensure that the funds are repaid in a timely manner to fulfill the requirements of the repayment phase. If the repayment is not completed as stipulated, the transaction is rejected by the network, effectively restoring the lender's original funds. From the perspective of the blockchain, it is as though the funds were never actually loaned.

This unique mechanism elucidates the absence of collateral requirements in flash loans. The repayment contract is enforced through robust code structures, ensuring that the loan is repaid by the predetermined terms and conditions.

What's the Point?

Flash loans may leave you wondering about their practicality. Since all the steps occur within a single transaction, it may seem challenging to achieve substantial gains. However, the key lies in the second part of the transaction, where you engage with the loan. The primary objective of flash loans is to generate profits by leveraging the funds within a smart contract or a series of contracts.

Several use cases illustrate the potential benefits. While you cannot engage in off-chain activities during the process, you can utilize DeFi protocols to amplify your returns. One prominent application is arbitrage, where you capitalize on price disparities across different DEXs.

For instance, suppose you want to invest in a promising new token that is currently trading at a lower price on one exchange compared to another. In a hypothetical scenario with zero fees, purchasing 1,000 tokens on Exchange A and selling them on Exchange B would yield a profit of $500. Although such gains may not be substantial, the potential for profit increases with larger trading volumes. If you were to acquire 100,000 tokens for $100,000 and successfully sell them for $105,000, your profit would amount to $5,000.

By obtaining a flash loan through platforms like Aave, you can seize investment opportunities on decentralized exchanges. Here's an illustrative example:

  1. Secure a flash loan of $100,000.
  2. Utilize the loan to purchase tokens on Exchange A.
  3. Sell the tokens on Exchange B.
  4. Repay the loan, including any interest incurred.
  5. Retain the generated profit.

All of this occurs seamlessly within a single transaction. However, it is crucial to acknowledge that real-world factors, including transaction fees, intense competition, interest rates, and slippage, contribute to narrow profit margins in arbitrage. Successfully capitalizing on price differences demands strategic tactics to ensure profitability. As thousands of users pursue similar opportunities, achieving substantial gains may prove challenging.

Are Flash Loans Risky?

Flash loans have been a recent addition to the DeFi landscape that has left a notable impact. However, they do come with their own risks. Hackers have demonstrated the extent to which they are willing to go to exploit vulnerabilities in flash loans. The fact that flash loans require minimal investment from attackers means that the financial deterrent is lacking. This is in contrast to traditional market manipulation, which typically required substantial amounts of cryptocurrency.

With flash loans, anyone can momentarily become a major player. It is possible to abscond with hundreds of thousands of dollars worth of cryptocurrency in mere seconds. While the flaws exploited were not inherent to flash loans themselves, the attacks that have occurred serve as valuable lessons for the wider industry. Diligent efforts are necessary to eliminate vulnerabilities in oracles, which have their own weaknesses. Despite the risks, flash loans hold promising potential for various applications in the future, particularly due to their low risks for both borrowers and lenders.


Flash loans have emerged as a recent addition to the DeFi landscape, leaving a notable impact. By introducing the concept of uncollateralized loans, governed solely by code, they have unlocked a realm of possibilities within the realm of finance. Although the current use cases for flash loans may be somewhat restricted, they have established a solid groundwork for the development of pioneering applications in the decentralized financial ecosystem.

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