Mitigating Taint-Style MCP Server Vulnerabilities Without Touching Server Code (SPELLSMITH)

arXiv·medium signal

'Mitigating Taint-Style Vulnerabilities in MCP Servers via Security-Aware Tool Descriptions' (arXiv:2607.07461, July 8, 2026) finds that taint-style flaws — untrusted input flowing to dangerous sinks — make up a substantial fraction of MCP-server CVEs, require large code changes to remediate, and meet slow community response. The proposed defense, SPELLSMITH, rewrites tool descriptions to be security-aware (a Description Enhancement Module plus an LLM self-reflection loop) so the model avoids triggering the vulnerable path, with no server-code change required. For builders shipping or installing MCP servers: treat tool descriptions as a real security surface and audit for taint-style sinks rather than assuming the protocol is safe by default.