While large language models (LLMs) have achieved notable success in generative tasks, they still face limitations, such as lacking up-to-date knowledge and producing hallucinations. Retrieval-Augmented Generation (RAG) enhances LLM performance by integrating external knowledge bases, providing additional context which significantly improves accuracy and knowledge coverage. However, building these external knowledge bases often requires substantial resources and may involve sensitive information. In this paper, we propose an agent-based automated privacy attack called RAG-Thief, which can extract a scalable amount of private data from the private database used in RAG applications. We conduct a systematic study on the privacy risks associated with RAG applications, revealing that the vulnerability of LLMs makes the private knowledge bases suffer significant privacy risks. Unlike previous manual attacks which rely on traditional prompt injection techniques, RAG-Thief starts with an initial adversarial query and learns from model responses, progressively generating new queries to extract as many chunks from the knowledge base as possible. Experimental results show that our RAG-Thief can extract over 70% information from the private knowledge bases within customized RAG applications deployed on local machines and real-world platforms, including OpenAI's GPTs and ByteDance's Coze. Our findings highlight the privacy vulnerabilities in current RAG applications and underscore the pressing need for stronger safeguards.

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