FastMCP Windows cmd.exe Injection via Server Name Metacharacters (CVE-2025-64340) - ATR-2026-00537 (089ca0aa-e430-5ede-9c01-0c3afd8a144f)
Detects CVE-2025-64340 (CVSS 7.8 HIGH, CWE-78): FastMCP < 3.2.0 on Windows passes the MCP server name field unsanitized into cmd.exe when installing via fastmcp install claude-code or fastmcp install gemini-cli. The target CLIs resolve to .cmd wrapper scripts; cmd.exe interprets the flattened argument string, executing any metacharacters embedded in the name. PoC: FastMCP(name="test&calc") opens Calculator. Confirmed injection characters: &, |, >, ^, (, ). The fix in 3.2.0 restricts names to [A-Za-z0-9\-_.\ ] only.
Detection covers: (a) MCP server name fields containing cmd.exe metacharacters in JSON/YAML
configuration blobs that would be passed to a shell-backed installer;
(b) FastMCP install invocations with server names containing these characters; (c) Content describing exploitation of this specific vector.
Scan target: MCP exchange (tool_response, content) and user_input for configuration payloads. False-positive rate expected low because legitimate server names do not require & | > ^ ( ).