mcpdone-audit 0.6.0

Static analyzer for MCP server repos: 4 checks (BadHost / Starlette CVE-2026-48710, FastMCP wrapper-layer asyncio.run bug, loose @mcp.tool() schemas, subprocess command-injection w/ cross-function taint propagation).

mcp-audit

Static security + correctness audit for MCP server repos.

pip install mcpdone-audit     # PyPI distribution (the plain 'mcp-audit' name is squatted; PEP 541 blocks it)
mcp-audit                     # scan the current directory
mcp-audit /path/to/repo       # scan a specific repo
mcp-audit --json              # machine-readable output
mcp-audit --check fastmcp_wrapper_layer  # one check only
mcp-audit --list-checks       # list available checks

Exit codes: 0 clean, 1 at least one finding, 2 usage error.

What it checks

Check ID	Severity	What it finds
starlette_badhost	HIGH / MED	Starlette < 1.0.1 in pyproject.toml, requirements*.txt, uv.lock, poetry.lock, pdm.lock. BadHost (CVE-2026-48710) lets a crafted HTTP Host header bypass path-based authorization. Affects any HTTP/SSE-transport MCP server. Stdio servers are unaffected.
fastmcp_wrapper_layer	HIGH	Sync @mcp.tool() functions that call asyncio.run(...) inside their body. FastMCP invokes tools inside an already-running event loop; asyncio.run() raises RuntimeError. Looks fine in unit tests, dies on the first real protocol call.
tool_input_validation	LOW	@mcp.tool() parameters typed as bare str / bytes / Any / list[Any] / dict[..., Any] or with no annotation at all. The schema FastMCP exposes to the LLM is the substrate prompt-injection-via-tool-description attacks rely on; constraining it (Annotated[str, Field(max_length=N)], Literal[...], Pydantic models) closes the window without losing expressiveness. Hygiene check, not a CVE — expect findings even on well-written servers. Added in v0.2.
command_injection	HIGH	@mcp.tool() functions where a tool parameter (or a local tainted via assignment / .format() / string concat) flows into os.system, os.popen, or subprocess.* with shell=True or a tainted-interpolated command string. v0.4 added same-file cross-function taint propagation: the analyzer now follows local helper calls (positional + keyword binding, recursion-visited guard), so tool -> helper -> sink flows are caught. Cross-file taint remains out of scope. The list-of-args / no-shell pattern is correctly NOT flagged. Added in v0.3, cross-function in v0.4.

More checks are landing — hard-coded secrets, write-API tools missing a FORBIDDEN_NAMES-style guardrail, read-only-by-default violations, path traversal in filesystem-touching servers.

Output format

$ mcp-audit examples/bad/
[HIGH  ] starlette_badhost @ uv.lock
           uv lockfile pins starlette==0.36.3 — vulnerable to BadHost (CVE-2026-48710). Patched in 1.0.1.
           -> Upgrade Starlette to >=1.0.1 (the BadHost patch). If FastAPI pulls Starlette transitively, pin it explicitly. ...

[HIGH  ] fastmcp_wrapper_layer @ server.py:18
           tool 'fetch_url' (def) calls asyncio.run() inside its body. FastMCP invokes tools inside an already-running event loop, and asyncio.run() raises RuntimeError when nested. This will fail at the first real MCP protocol call even if every unit test passes.
           -> Convert the tool to `async def` and replace `asyncio.run(...)` with `await`. ...

mcp-audit: 2 finding(s) — 2 high

--json emits one object: {"root": "...", "finding_count": N, "findings": [...]}. Each finding has check, severity, path, line, message, remediation.

What this is not

• It is not a runtime sandbox. Static analysis only.
• It does not install your venv to introspect it. It reads what's declared (manifests + lockfiles + source).
• It will not detect every vulnerability — only the classes its checks know about. Treat zero findings as "no known issues from this tool," not as a clean bill.

Background

• BadHost write-up: https://mcpdone.com/blog/badhost-mcp-servers
• FastMCP wrapper-layer bug write-up: https://mcpdone.com/blog/fastmcp-wrapper-layer-bug

Development

git clone https://github.com/Alienbushman/mcpdone-samples
cd mcpdone-samples/mcp-audit
pip install -e ".[dev]"
pytest
python smoke_test.py

To add a check: drop src/mcp_audit/checks/<name>.py exposing a module-level CHECK_ID and a check(root: Path) -> list[Finding] callable. Register it in src/mcp_audit/checks/__init__.py. Add fixtures + tests under tests/.

License

MIT.