This module examined the security implications of converging Ethereum’s EVM with the Cosmos SDK via dual‑stack architectures like EVMOS/Ethermint. We explored how message routing, gas‑refund mechanics, and decorator chains work—and how gaps in these systems enabled real exploits such as Cronos fee theft and bypassing Ethermint ante handlers. By understanding these vulnerabilities and the mitigation strategies (e.g., RejectMessagesDecorator, enhanced Authz checks), developers can better secure hybrid chains. As projects like evmos pivot to full EVM alignment, it’s vital to balance interoperability with robust, simplified security designs.