Retrieval-augmented generation (RAG) extends large language models (LLMs) with external knowledge, but this access path also introduces security risks that existing work often conflates with inherent LLM flaws. We frame secure RAG as securing external knowledge access and organize the literature with SLOT, a taxonomy along four axes: the attack Surface (S) where an adversary acts, the defense Layer (L) that controls the same point, the Objective (O) it breaks following the CIA properties, and the Target (T) it pursues, from a single known query (T1) to target-claim manipulation across a query distribution (T2). Mapping attacks, defenses, remediation, and evaluation onto a six-stage knowledge-access pipeline, we expose two structural mismatches. Finally, we discuss directions for more realistic targets, no-blind-spot and adaptively evaluated defenses, stronger confidentiality, and evaluation for multimodal and agentic RAG. The curated paper list for RAG security is in: https://github.com/TreeAI-Lab/Awesome-RAG-Security.