MCP Audit

MCP Audit is an open-source security regression scanner for Model Context Protocol servers. It extracts tool capabilities and permissions, detects risky cross-tool data flows, compares security posture across Git revisions, and emits CI-friendly evidence.

The first version focuses on Python/FastMCP projects and the core product thesis from the spec: show what an MCP server can do, what changed, and whether the change creates a dangerous capability path.

MCP005 is the capability-graph rule: it identifies when one tool returns classified data and another tool in the same MCP registration context can send it to an external destination. Findings include the source, data classification, sink, destination, path, source evidence, impact, and remediation.

Install locally

uv sync

Run project commands through uv run, or activate the virtual environment with source .venv/bin/activate before using bare commands.

The package also supports isolated CLI installation directly from a checkout:

pipx install .
mcp-audit --version
mcp-audit scan .

The product and CLI are named MCP Audit, while the PyPI distribution is named mcp-capdiff:

pipx install mcp-capdiff
mcp-audit --version

Scan a server

uv run mcp-audit scan .

Scans fail closed when source cannot be read or when no MCP tools are discovered. Use --allow-empty only when an empty result is intentional.

Useful output formats:

uv run mcp-audit scan . --format json
uv run mcp-audit scan . --format sarif --output mcp-audit.sarif

Generate a capability manifest

uv run mcp-audit manifest .

Compare against a Git baseline

uv run mcp-audit diff --base origin/main

The diff report focuses on security changes: before/after risk, added and changed tools, expanded capabilities, new findings, resolved findings, and approval removal.

Run tests

uv run pytest

The corpus contains positive, negative, and edge cases under fixtures/, including host allowlisting, path-root validation, approval gating, and cross-tool sensitive-data paths.

GitHub Action

The repository includes a composite action that runs the security diff, uploads SARIF to GitHub Code Scanning, and fails the check when policy thresholds are crossed:

permissions:
  actions: read
  contents: read
  security-events: write

steps:
  - uses: actions/checkout@v4
    with:
      fetch-depth: 0
  - uses: actions/setup-python@v5
    with:
      python-version: "3.12"
  - uses: your-org/mcp-audit@v0.1
    with:
      target: .
      base: origin/main
      policy: mcp-audit.yaml

JSON reports and manifests use the versioned schema documented in docs/report-schema.md.

The copyable vulnerable FastMCP demo includes four before/after pull-request scenarios and its own Action workflow.

V0.1 rules

• MCP001 arbitrary shell execution.
• MCP002 unrestricted filesystem access.
• MCP003 arbitrary URL or SSRF surface.
• MCP004 high-impact side effect without approval.
• MCP005 sensitive read to external write path.
• MCP007 unbounded security-sensitive input.
• MCP010 destructive tool exposed.
• MCP016 capability escalation in a diff.
• MCP017 approval removed in a diff.

Each rule's detection behavior, examples, remediation, and limitations are documented in docs/rules.

Demo fixtures

uv run mcp-audit scan fixtures/safe_server
uv run mcp-audit scan fixtures/vulnerable_server

The vulnerable fixture includes a sensitive file read tool, an unrestricted URL fetcher, shell execution, and a destructive operation. MCP Audit should flag the individual findings and the cross-tool path:

read_customer_file -> agent_context -> fetch_url

Policy

The default policy fails on high and critical findings. A minimal policy file can set a risk threshold and suppress reviewed findings:

policy:
  ci:
    fail_on:
      - critical
      - high
    max_risk_score: 60

suppress:
  - rule: MCP003
    tool: internal_fetch
    reason: "Network egress is enforced by service mesh"
    expires: 2027-01-31

Suppression reasons are required. Expired suppressions no longer hide findings, and mcp-audit policy check reports them. See policy documentation.

Version contracts

CLI, ruleset, report schema, and manifest schema versions evolve independently. See versioning and report schema.

Passing MCP Audit is technical security evidence, not a legal compliance determination.