mcp-fence

Local-first security scanner, MCP protocol inspector, dynamic fuzzer,
Docker sandbox, and report generator for Model Context Protocol
servers.

mcp-fence is a developer tool. Point it at your mcp.json (or any
running MCP server you wrote) and it will:

1. Statically scan the config and source for tool poisoning,
   dangerous startup commands, plaintext secrets, schema gaps, and
   unauthenticated HTTP transports.
2. Inspect the server live over stdio — runs initialize,
   tools/list, gathers schemas/annotations.
3. Fuzz every tool with schema-aware payloads (path traversal,
   command injection, SSRF, prompt injection, oversize input, type
   confusion). Safe by default; opt in to runtime payloads via
   --toy-mode or --allow-unsafe.
4. Sandbox any stdio server in Docker with sensible profiles
   (strict, filesystem-readonly, network-deny, dev).
5. Report findings as a plain-text table, JSON, SARIF 2.1.0 (for
   GitHub code scanning), or a fully offline HTML page.

It is non-destructive, local-first, and uses no cloud LLM by
default. An optional local-LLM judge can talk to a local Ollama or
OpenAI-compatible endpoint.

Install

From PyPI:

pip install mcp-fence

From source (for contributors):

git clone https://github.com/DaoyuanLi2816/mcp-fence
cd mcp-fence
python -m venv .venv
source .venv/bin/activate          # Windows: .venv\Scripts\activate
python -m pip install -e ".[dev]"

Requires Python 3.11+.

30-second quickstart

# 1. Bring up the bundled examples in your working directory.
mcp-fence init-example ./mcp-fence-examples

# 2. Static scan the intentionally poisoned metadata server.
mcp-fence scan examples/vulnerable_metadata_server/mcp.json

# 3. Live-inspect the safe baseline.
mcp-fence inspect examples/safe_server/mcp.json

# 4. Fuzz the arbitrary-file-read server.
mcp-fence fuzz examples/vulnerable_filesystem_server/mcp.json

# 5. Generate a `docker run` command that sandboxes any of the above.
mcp-fence sandbox examples/vulnerable_filesystem_server/mcp.json \
    --profile strict --dry-run

# 6. Turn a saved JSON result into an offline HTML report.
mcp-fence scan examples/vulnerable_metadata_server/mcp.json \
    --format json --output /tmp/scan.json
mcp-fence report /tmp/scan.json --format html --output /tmp/scan.html

Example output

mcp-fence 0.1.0 :: scan :: target=examples/vulnerable_metadata_server/mcp.json
  summary: total=1 score=7/100 verdict=FAIL
  by_severity: high=1
  by_category: secrets=1

# Findings
  SEV   RULE     TITLE                    CATEGORY  WHERE                    DETAIL
  HIGH  MCPG006  Plaintext secret in env  secrets   param=OPENAI_API_KEY     Environment variable OPENAI_API_KEY appears to be a plaintext secret.

Supported transports

Supported risk classes

Full catalog in docs/rule_catalog.md. Highlights:

• Tool poisoning — prompt-injection phrases in tool descriptions,
  hidden HTML comments, zero-width characters, confusable tool names
  (Unicode + ASCII visual confusables like Iist_files vs
  list_files).
• Dangerous startup commands — shell=True, curl | sh, sudo,
  /var/run/docker.sock, --privileged, references to ~/.ssh,
  ~/.aws.
• Transport binding — 0.0.0.0 + no auth, HTTP without bearer.
• Schema risks — unbounded strings, missing additionalProperties,
  high-risk param names (command, path, url, webhook, …)
  without pattern/enum/maxLength.
• Dynamic — path traversal hitting a planted fake secret, command
  injection marker echoed back, SSRF accepting metadata IPs, malformed
  input passing schema validation, sensitive patterns in tool output.
• Protocol — server hangs, schema mismatches, non-JSON output on
  stdio.

Why MCP servers need their own scanner

MCP server output flows directly into an LLM's context. Anything in a
tool's description, name, or response can be interpreted by the
assistant as instructions:

• Tool metadata is attacker-controllable once you install someone
  else's server.
• Tool inputs are LLM-controllable and untrusted by default.
• Tool outputs end up in the LLM's prompt and can carry injection
  payloads from external content.

General-purpose SAST / npm-audit / pip-audit don't model any of this.
mcp-fence has rules and fuzzers built specifically for it.

Safety boundaries

• Non-destructive payloads. The command-injection marker is
  echo MCPG_FUZZ_MARKER_8f2a. No rm, mv, chmod, or destructive
  primitives are ever emitted.
• Local-first. No code, configuration, or scan result is uploaded
  anywhere.
• No cloud LLM by default. Optional --llm-judge talks to a local
  endpoint only.
• No public-network scanning. SSRF payloads test the tool's
  validation behaviour; we don't initiate outbound requests for you.
• Safe path probing. Path-traversal payloads target a planted
  fake_secret.txt inside the bundled examples or any explicit
  --traversal-target. They never aim at /etc/shadow or ~/.ssh/.

--allow-unsafe is an explicit safety hatch for use inside the
mcp-fence sandbox Docker profile. See SECURITY.md and
docs/sandboxing.md.

Optional local-LLM judge (Ollama / vLLM)

For semantic suspiciousness scoring on tool descriptions, enable the
optional judge:

ollama pull qwen3:8b
mcp-fence scan examples/vulnerable_metadata_server/mcp.json \
    --inspect --llm-judge ollama --llm-model qwen3:8b

Sized for a 16 GB GPU (e.g. RTX 4080). Failures are silent: the core
scan completes either way. See docs/local_llm.md.

GitHub Action

Drop .github/workflows/mcp-fence.yml
into any repo with an mcp.json. It scans every config and uploads
SARIF to GitHub's code scanning dashboard.

- name: install mcp-fence
  run: pip install mcp-fence
- name: scan
  run: mcp-fence scan path/to/mcp.json --format sarif --output mcp-fence.sarif
- uses: github/codeql-action/upload-sarif@v3
  with:
    sarif_file: mcp-fence.sarif
    category: mcp-fence

Five most useful commands

mcp-fence scan examples/vulnerable_metadata_server/mcp.json
mcp-fence inspect examples/safe_server/mcp.json
mcp-fence fuzz examples/vulnerable_filesystem_server/mcp.json
mcp-fence sandbox examples/vulnerable_shell_server/mcp.json --profile strict --dry-run
mcp-fence report /tmp/scan.json --format html --output /tmp/scan.html

Roadmap

See docs/roadmap.md. v0.2 adds AST-based source
scanning, HTTP/SSE live inspector, a local SSRF capture server, and a
pre-trained tool-poisoning classifier.

License

Apache-2.0. See LICENSE.