analysis of Insecure Output Handling, a critical application security vulnerability distinct from insecure input handling, emphasizing the need to never trust data sent to an interpreter. It details the diverse and severe consequences of this flaw, including client-side attacks like Cross-Site Scripting (XSS) and server-side threats such as Remote Code Execution (RCE), providing a comparative table to highlight the differences between input and output vulnerabilities. The document then examines the attack surface across various application architectures, from traditional web applications to modern APIs and the emerging risks posed by Large Language Models (LLMs), before presenting statistical data and real-world case studies to quantify its pervasive impact. Finally, it outlines a multi-layered defense strategy, advocating for a zero-trust approach, robust validation and context-aware output encoding, and the integration of both automated and manual testing methodologies throughout the Software Development Lifecycle (SDLC).