Why No Single Defense Works Against LLM Prompt Injection—And What Actually Does

LLMs fundamentally blur the boundary between instructions and data, making them inherently vulnerable to prompt injection attacks that simple keyword filters cannot stop. This analysis reveals why each defensive layer—from input filtering to semantic judges—fails individually and why organizations must adopt layered, defense-in-depth strategies. For any team deploying LLMs with sensitive information, understanding these weaknesses is essential to building resilient guardrails.