MCP-HARDENED

A deliberately small MCP server built to demonstrate security hardening against the OWASP MCP Top 10, with a test suite that proves the defenses hold.

Warning

This repository contains intentionally vulnerable code. Commits at and before the naive-baseline tag implement path traversal, SQL injection, and SSRF on purpose — they are the "before" half of a security demonstration. The build order is deliberate: write the naive version, write attack tests that prove the vulnerability is reachable, then harden until the tests pass. Do not use any code from this repository as a reference implementation, and do not deploy it. This is a teaching artifact, not a library.

Ongoing project. Things may change.

Related MCP server: Vulnerable MCP Server

What this demonstrates

Three read-only tools, each hosting one vulnerability class (plus ping, a smoke test):

Full breakdown of each tool in docs/TOOLS.md.

Audit logging

Every tool invocation is written to logs/audit.jsonl as one JSON object per line:

{"timestamp": "...", "tool": "search_files", "arguments": {"query": "../OUTSIDE_SANDBOX.txt"}, "outcome": "rejected", "error": "path escapes the sandbox root"}

Fields: timestamp (ISO 8601 UTC), tool, arguments as received, outcome (ok or rejected), error on rejection, result_len on success.

Return values are never logged — only their length. Logging content would reintroduce the over-sharing problem MCP10 addresses.

JSON Lines is not just convenient. json.dumps escapes newlines, so an argument containing \n cannot forge a second log entry. A plaintext format would have been forgeable. tests/test_audit.py enforces this.

The log contains attacker-controlled text by design — that is the point of an audit log — so anything consuming it must treat it as untrusted input.

Writes fail open: if the log cannot be written the tool call still succeeds, with the failure reported on stderr. A compliance context would invert this. See residual risk in the coverage doc.

The log is gitignored — it is generated data, like data/*.db.

Before and after

The same test suite, run against the naive implementation and against the hardened one:

• docs/attack-suite-before.txt — attack tests failing, functional tests passing

• docs/attack-suite-after.txt — everything passing

The hardening itself, as a diff: naive-baseline...master

Explicitly out of scope

• Prompt injection — unsolved industry-wide, not attempting

• Tool poisoning — a consuming-side threat; a server cannot prevent a client from connecting to a poisoned server

• DNS rebinding — the host allowlist checks the name, not the resolved IP

• Multi-user authentication, rate limiting

OWASP MCP Top 10 coverage

Full reasoning per item in docs/OWASP-MCP-COVERAGE.md

Stack

Python 3.12 · FastMCP 3.3.1 · pytest · SQLite · stdio transport

Running it

uv sync
uv run scripts/seed_db.py     # creates data/records.db, needed by query_records
uv run pytest -v

To poke at the tools by hand:

npx @modelcontextprotocol/inspector uv run src/server.py

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "hardened-demo": {
      "command": "wsl.exe",
      "args": [
        "-d", "Ubuntu",
        "--",
        "/home/nullstep/.local/bin/uv",
        "run",
        "--directory", "/home/nullstep/mcp-hardened",
        "src/server.py"
      ]
    }
  }
}

Config location: %APPDATA%\Claude\claude_desktop_config.json on Windows, ~/Library/Application Support/Claude/ on macOS. Microsoft Store installs redirect this into the package container under %LOCALAPPDATA%\Packages\Claude_*\LocalCache\Roaming\Claude\.

Two non-obvious requirements when the server runs in WSL:

• uv needs an absolute path. wsl.exe -- <cmd> does not run a login shell, so ~/.local/bin is never added to PATH. A bare uv fails with exit 127.

• --directory is required. wsl.exe inherits and translates the Windows working directory, so the process starts in /mnt/c/Windows/System32 and relative paths do not resolve.

Restart Claude Desktop fully after editing — quit from the system tray, not just closing the window.

Note: ROOT = Path(__file__).resolve().parent.parent in src/server.py means the sandbox, database, and log paths resolve correctly regardless of working directory. --directory and ROOT are independent layers — the config could be wrong and containment would still hold.

Attribution

Built with Claude Code. The implementation, threat model, and documentation in this repository were AI-generated.

My contribution was direction and review: setting the scope, deciding what stays out of scope, approving or rejecting each step, and verifying behavior in the MCP Inspector at each stage.