Confused Deputy Attack Against Model Context Protocol
The model context protocol (MCP) has rapidly emerged as a standard framework for integrating large language models (LLMs) with external tools and resources. However, its metadata-driven and non-deterministic tool selection mechanism introduces a previously overlooked security threat. Leveraging this weakness, we uncover the confused deputy attack, where an adversarial server with subtly manipulated metadata covertly overshadows a benign one, intercepting tool invocations without exhibiting overtly malicious behavior. To systematically study this threat, we develop Puppet, the first automated security evaluation framework that: (i) enriches benign tool descriptions through selective requirement engineering to maximize semantic expressiveness, (ii) restructures them into LLM-preferred formats using description schema transformation, and (iii) applies name prioritization to introduce complementary lexical bias. Furthermore, Puppet synthesizes valid user queries to enable systematic attack evaluation. We comprehensively evaluate Puppet across 14 models from 6 providers on 2 MCP hosts, demonstrating tool selection hijacking rates up to 90.89% and end-to-end malicious payload execution rates up to 86.46%, while remaining undetectable by representative security scanners (MCP-Scan and McpSafetyScanner), which are architecturally incapable of detecting metadata-level manipulation attacks. Counterintuitively, we find that reasoning-enabled models are significantly more vulnerable than their non-reasoning counterparts. Our findings expose a critical design-level attack surface in the MCP ecosystem and highlight the urgent need for principled security safeguards.