SkillAudit report — punkpeye/fastmcp

Scanned 2026-04-30 by SkillAudit v0.3 (surface-tiered static checks + LLM-assisted prompt-injection red-team).

Commit: 72c1166 · Stars: 3055 · Days since last push: 4

LLM prompt-injection probe: no-tool-surface

Overall grade: F (35/100)

Security findings

Production sources:

• HIGH src/DiscoveryDocumentCache.ts:101 — HTTP client call with user-controlled argument 'url' — no URL allowlist / validation found in file

const res = await fetch(url);

• WARN src/FastMCP.ts:82 — HTTP client call with user-controlled argument 'input' (validation markers present but not verified against this call-site)

const response = await fetch(input.url);

• WARN src/FastMCP.ts:151 — HTTP client call with user-controlled argument 'input' (validation markers present but not verified against this call-site)

const response = await fetch(input.url);

Test source (low-weight) — 3 total, deduct 5/0 per high/warn:

• HIGH src/FastMCP.https.test.ts:93 — Template-string URL with interpolation — no validation possible on composed string

const response = await fetch(\http://localhost:${port}/ping\);

• HIGH src/FastMCP.oauth-proxy.test.ts:45 — Template-string URL with interpolation — no validation possible on composed string

const dcrResponse = await fetch(

• HIGH src/FastMCP.oauth-proxy.test.ts:62 — Template-string URL with interpolation — no validation possible on composed string

const metadataResponse = await fetch(

Permissions

_No findings on this axis._

Credentials

Examples / samples (low-weight) — 1 total, deduct 5/0 per high/warn:

• HIGH src/examples/session-context.ts:247 — console.* of process.env — entire env leaks to stdout/stderr and LLM context

console.log(\`

Maintenance

_No findings on this axis._

Compatibility

Production sources:

• WARN (meta) — No engines (Node) or python_requires declared — cross-client compatibility unverified

Documentation

Production sources:

• WARN (meta) — No SECURITY.md — no disclosure channel for vulnerabilities

missing

Methodology

SkillAudit v0.3 clones the repo at the provided ref (default: default branch, HEAD) into an ephemeral sandbox, runs six static checks over .js/.ts/.py sources, queries the GitHub API for maintenance signals, and runs an LLM-assisted prompt-injection red-team over the MCP tool surface. Each axis is scored against the published rubric — surface tiers, per-(axis, surface) caps, grade buckets, and worked examples are all documented there.

The v0.3 calibration update introduces surface tiering: every finding is tagged with the code path it lives in (production / installer / examples / benchmarks / scripts / test). Production findings deduct at full weight (-30 high, -10 warn); installer findings deduct at half (-15 / -5); examples, benchmarks, top-level scripts, and tests deduct at low weight (-5 / 0). This stops a chatty benchmarks/ or samples/ directory from dominating an otherwise-clean MCP server's grade.

The prompt-injection axis extracts each server.tool(...) / @app.tool registration + the first ~60 lines of handler body, hands them to Claude Haiku 4.5 with a red-team system prompt, and asks for structured findings on untrusted-content flow into tool responses. One API call per scan, bounded at ~15K input tokens.

How to improve this grade

• Security — static: validate tool-input URLs against an allowlist before fetch/axios calls; use execFile with argv arrays instead of exec with template strings; never pass untrusted strings to subprocess with shell=True.
• Security — prompt injection: never return fetched web-page / file / email content verbatim in a tool response. Wrap with a framing marker (e.g., <untrusted-content>...</untrusted-content>), summarize rather than inline, and never let untrusted content share a turn with credentials or other tool output.
• Credentials findings: redact env-var reads before log lines and error messages; treat any string that ends up in a tool response as public.
• Maintenance: if the repo is inactive, document the maintenance model — "MCP tool, no breaking changes expected" is a legitimate signal.
• Docs: add a README install + usage section with a copy-pasteable command; add a SECURITY.md with a disclosure channel.

_Report generated by skillaudit.dev_