Ethereum's "composable protocol stack" enables DeFi protocols to interoperate like "money legos," fostering innovation but introducing systemic risks at the code level. Smart contracts, often open-source, allow seamless integration, yet vulnerabilities in one protocol can cascade across the ecosystem. For instance, a flaw in a widely used contract, like those in MakerDAO or Curve, could destabilize dependent protocols, amplifying losses. Historical exploits, such as the 2016 DAO hack, highlight how interconnectedness compounds risks. Additionally, composability often lacks robust cross-protocol auditing, and complex interactions increase the likelihood of unforeseen bugs. While Ethereum's modular design drives rapid development, it demands rigorous security practices to mitigate systemic threats.