Decentralized finance (DeFi) has revolutionized traditional financial paradigms by enabling innovative, permissionless financial transactions. Among these, flash loans represent a significant breakthrough, offering rapid liquidity without collateral requirements. However, the very features that make flash loans appealing also expose DeFi ecosystems to severe security threats. This paper presents a systematic analysis of flash loan attack methodologies, their implications, and potential countermeasures. We formalize the problem via a game-theoretic model, delineating the interactions between malicious actors and security mechanisms. Through detailed case studies of major flash loan attacks, we illustrate common exploit strategies and vulnerabilities within smart contracts. Furthermore, we propose a comprehensive, multilayered security framework that integrates real-time anomaly detection, enhanced smart contract verification, decentralized governance improvements, and cross-platform intelligence sharing. Empirical analysis leveraging blockchain security datasets underscores the viability of these mitigative measures. Our findings contribute to the broader discourse on DeFi security by providing a structured approach to mitigating the systemic risks associated with flash loans, thereby enhancing the resilience of decentralized financial systems.