The core issue behind the $200 million hack of Euler Finance was the failure to remember the core invariant of lending markets, which led to the exploitation of a specific function. While the Checks-Effects-Interactions pattern is helpful for function safety, it can cause developers to overlook the broader context of protocol level invariants. A better approach is the Function Requirements-Effects-Interactions + Protocol Invariants pattern, or FREI-PI, which ensures that developers focus on both function level safety and protocol level invariants. By implementing this pattern, lending markets and other DeFi protocols can be made safer and more secure.
