  Flash Loan, as an emerging service in the decentralized finance ecosystem,
allows users to request a non-collateral loan. While providing convenience, it
also enables attackers to launch malicious operations with a large amount of
asset that they do not have. Though there exist spot media reports of attacks
that leverage Flash Loan, there lacks a comprehensive understanding of existing
Flash Loan services. In this work, we take the first step to study the Flash
Loan service provided by three popular platforms. Specifically, we first
illustrate the interactions between Flash Loan providers and users. Then, we
design three patterns to identify Flash Loan transactions. Based on the
patterns, 76, 303 transactions are determined. The evaluation results show that
the Flash Loan services get more popular over time. At last, we present four
Flash Loan applications with real-world examples and propose two potential
research directions.
