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Introduction:
Insecure Direct Object References (IDOR) remain one of the most prevalent and critical API security flaws in modern web applications. This vulnerability arises when an application exposes a direct reference to an internal object—such as a file, database record, or user identifier—without proper authorization checks. Recently, a security researcher demonstrated a sophisticated IDOR chain that escalated a simple information disclosure vulnerability into a full-blown payment redirection attack, highlighting the devastating impact of combining broken object-level authorization with insecure HTTP method handling.
Learning Objectives:
- Understand how to identify hidden API endpoints through JavaScript file analysis during reconnaissance.
- Learn how to chain a simple GET request IDOR with a PUT method tampering attack to modify critical user data.
- Master the use of cURL commands and browser developer tools to test for authorization flaws in RESTful APIs.
You Should Know:
1. Initial Reconnaissance: Identifying the Vulnerable GET Request
The first step in this attack chain involved identifying a standard API endpoint that leaked sensitive data. The researcher likely intercepted traffic using a proxy like Burp Suite or simply monitored the Network tab in Chrome Developer Tools while logged into the application.
While browsing the payment settings page, the tool captured a request similar to:
`GET /api/user/payment-methods HTTP/1.1`
When the researcher modified a parameter in this request—for instance, changing the user identifier from `user_id=123` to user_id=456—the server responded with the victim’s payment methods. This is the classic IDOR pattern: the server failed to verify that the authenticated user was authorized to access the data belonging to user 456.
Step-by-step guide to replicating this discovery:
- Log in to the target application with your test account.
- Open Developer Tools (F12) and navigate to the Network tab.
- Interact with features related to user profiles, invoices, or payments.
- Find a request containing identifiers (like numbers, UUIDs, or emails) in the URL or POST body.
- Right-click the request and select Copy > Copy as cURL.
- Open a terminal and paste the cURL command. Modify the identifier to a value belonging to another user.
Original Request (Your Account) curl -i -X GET "https://target.com/api/user/123/payment-methods" -H "Cookie: session=YOUR_SESSION" Attempted IDOR (Victim's Account) curl -i -X GET "https://target.com/api/user/456/payment-methods" -H "Cookie: session=YOUR_SESSION"
If the response returns data for user 456, you have confirmed an IDOR vulnerability.
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Deep Dive: JavaScript File Analysis for Hidden Endpoints
After finding the initial information leak, the researcher did not stop there. A crucial part of modern web application penetration testing involves analyzing JavaScript (JS) files. These files often contain client-side logic that reveals API routes, parameters, and application structures not visible in the standard UI.
Using the “Sources” tab in Developer Tools, or tools like `LinkFinder` or JS-Link, the researcher scanned the JS bundles. Inside a chunk file, they discovered a reference to a hidden, undocumented endpoint: /details.
Step-by-step guide to JS endpoint discovery:
- In Chrome DevTools, go to the Sources tab.
- Expand the top-level domain and look for JavaScript files (ending in .js).
- Press Ctrl+Shift+F to open the search across all files.
- Search for keywords like
api/,endpoint,v1/,internal/, oradmin/. - Alternatively, use a command-line tool like `grep` to extract URLs from downloaded JS files:
Download the JS file wget https://target.com/static/js/main.abcd1234.js Grep for API patterns grep -oP '"/api/v[0-9]/[^"]"' main.abcd1234.js grep -oP 'https?://[^"]' main.abcd1234.js | grep 'api'
This process reveals the hidden endpoint. The researcher likely found something like:
`”/api/user/details”`
3. Testing the Hidden Endpoint
Once the `/details` endpoint was discovered, the researcher tested it using the same IDOR technique. By appending the victim’s identifier or simply accessing it while impersonating the victim via the initial IDOR, they confirmed that this endpoint returned even more sensitive data—likely including full PII, address, and phone number.
Command to test:
curl -i -X GET "https://target.com/api/user/456/details" -H "Cookie: session=YOUR_SESSION"
If successful, this confirms that the hidden endpoint is also vulnerable to IDOR, providing a secondary data source for the attack.
4. HTTP Method Tampering: From GET to PUT
The most critical finding was the ability to change the HTTP method. RESTful APIs often use different HTTP methods to signify different actions: GET for reading, POST for creating, PUT/PATCH for updating, and DELETE for removal. The vulnerability here lies in the server’s access control logic. While the `GET` request might have been protected (however poorly), the `PUT` request on the same endpoint was not adequately secured for cross-user actions.
The researcher simply changed the method from `GET` to `PUT` for the payment methods endpoint.
Step-by-step guide to method tampering:
- Capture the original `GET` request to the vulnerable endpoint.
- In Burp Suite (or by editing the cURL command), change the method to
PUT. - Examine the response body of a typical `GET` request. It likely contains a JSON object representing the payment method.
- Construct a `PUT` request body that mimics this structure, but with the attacker’s payment details (e.g., their PayPal email or credit card token).
{ "userId": "456", "defaultPayment": { "method": "paypal", "email": "[email protected]" } }
5. Send the request.
curl -i -X PUT "https://target.com/api/user/456/payment-methods" \
-H "Cookie: session=YOUR_SESSION" \
-H "Content-Type: application/json" \
-d '{"userId":"456","defaultPayment":{"method":"paypal","email":"[email protected]"}}'
5. Exploitation: The Business Impact Chain
The combination of these vulnerabilities creates a severe business impact. The attack flow is as follows:
1. Information Disclosure (GET /payment-methods): The attacker learns what payment method the victim uses.
2. Information Disclosure (GET /details): The attacker confirms the victim’s identity and other details.
3. Authorization Bypass (PUT /payment-methods): The attacker overwrites the victim’s payment details with their own.
4. Financial Theft: When the victim makes a purchase, or when a recurring subscription bill is processed, the payment is routed to the attacker’s account.
This chain exploits the lack of object-level authorization. The server likely uses a middleware that checks if a user is authenticated (logged in) but fails to check if that user (user 123) is authorized to modify user 456’s payment methods.
6. Defensive Mitigation Strategies
To prevent this type of attack, developers and security teams must implement robust controls:
- Implement Proper Object-Level Authorization: Never trust user-supplied IDs. Always verify that the authenticated user has permission to access the requested object. Use a consistent authorization layer.
Example (Conceptual Python/Flask) @app.route('/api/user/<int:user_id>/payment-methods', methods=['GET', 'PUT']) def payment_methods(user_id): current_user = get_current_user() Critical check: Is the current user allowed to access this user_id? if current_user.id != user_id and not current_user.is_admin: return jsonify({"error": "Unauthorized"}), 403 ... proceed with request - Avoid Exposing Internal Object References: Use indirect references (like random UUIDs) instead of sequential integers (e.g., `/api/user/`a1b2c3 instead of
/api/user/123). - Disable Unused HTTP Methods: If an endpoint should only be read-only, ensure the server blocks
PUT,POST, and `DELETE` requests to that endpoint, returning a `405 Method Not Allowed` status. - Regular JS File Auditing: Ensure no sensitive API endpoints or hardcoded secrets are exposed in client-side JavaScript files. Use tools to scan for unintentional leaks during the CI/CD pipeline.
What Undercode Say:
- Key Takeaway 1: An IDOR is rarely an isolated issue; it often serves as a gateway to deeper vulnerabilities. Always probe further by checking for hidden endpoints in JavaScript and testing alternate HTTP methods.
- Key Takeaway 2: The shift from information disclosure to data manipulation (via PUT) exponentially increases the severity of a bug. In this case, a simple configuration oversight in authorization middleware allowed an attacker to turn a read-only flaw into a direct financial theft mechanism.
This case is a perfect example of why API security must be treated as a distinct discipline from traditional web application security. It underscores the necessity of comprehensive testing that goes beyond standard user flows. For bug bounty hunters and penetration testers, this methodology—recon, JS analysis, and method tampering—remains a goldmine for uncovering critical vulnerabilities.
Prediction:
As more companies adopt microservices and RESTful APIs, we will see a significant rise in “method-based” privilege escalation attacks. Attackers will increasingly automate the discovery of endpoints in JS files and use fuzzing techniques to test all HTTP methods (GET, POST, PUT, DELETE, PATCH) on every discovered endpoint. This will force the industry to adopt “zero-trust” authorization models where every request, regardless of method, is verified against the user’s identity and the target object’s ownership, rather than relying on the assumption that “if you can GET it, you can’t PUT to it.”
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IT/Security Reporter URL:
Reported By: Mostafa Zaki55 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
