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Introduction:
In the high-stakes world of bug bounty hunting, discovering a critical vulnerability often hinges on understanding how web applications handle user authorization. This article explores a real-world account takeover scenario, breaking down the technical steps required to identify and exploit Insecure Direct Object References (IDOR). We will reconstruct the methodology used to find such bugs, providing a technical roadmap for penetration testers and security researchers looking to secure their own applications or bag their next bounty.
Learning Objectives:
- Understand the mechanics of IDOR (Insecure Direct Object Reference) vulnerabilities and how they lead to privilege escalation.
- Learn to use Burp Suite for intercepting traffic and fuzzing object parameters.
- Master Linux command-line tools for reconnaissance and API endpoint analysis.
- Implement mitigation strategies at the code and server configuration level.
You Should Know:
1. Reconnaissance: Mapping the Attack Surface
Before exploiting a vulnerability, you must understand the target. For a platform like Yandex, this involves enumerating subdomains and hidden API endpoints.
Step‑by‑step guide explaining what this does and how to use it.
Start with subdomain discovery using tools like `assetfinder` and `httpx` on a Linux terminal. This reveals all publicly accessible services belonging to the main domain.
Install tools (if not already installed) go install github.com/tomnomnom/assetfinder@latest go install github.com/projectdiscovery/httpx/cmd/httpx@latest Find subdomains and filter live hosts echo "yandex.com" | assetfinder --subs-only | httpx -silent -o live_subdomains.txt
This command pipes the list of potential subdomains into httpx, which checks for live HTTP/HTTPS services, outputting a clean list of active targets. Often, bug bounty hunters find staging or internal tools (api.internal.yandex.com, admin.test.yandex.com) that are less secure than the main production site.
2. Parameter Fuzzing for Object References
Once you have a live endpoint (e.g., `https://profile.yandex.com/user/dashboard`), the next step is to identify parameters that reference user data. These are often found in API calls made by the web application.
Step‑by‑step guide explaining what this does and how to use it.
Using Burp Suite, navigate to the target site and perform standard actions (view profile, update settings). Look for requests containing identifiers like user_id, account, file, or document. Use Burp Intruder or a command-line fuzzer like `ffuf` to test these parameters.
Using ffuf to fuzz for user IDs in an API endpoint ffuf -u https://api.yandex.com/v1/user/FUZZ/details -w /usr/share/wordlists/seclists/Discovery/Web-Content/burp-parameter-names.txt -fc 403,404
This command replaces `FUZZ` with common parameter names (like id, uid, profile_id). If the endpoint returns a 200 OK for a parameter that isn’t yours (e.g., `?user_id=12345` where 12345 is another user), you have likely found an IDOR.
3. Exploitation: The Account Takeover
In the original post, the researcher likely found a direct reference to another user’s object. Let’s simulate this using a standard API workflow.
Step‑by‑step guide explaining what this does and how to use it.
- Log in to your account and intercept the request when loading your profile picture or personal data.
2. Send the request to Burp Repeater.
- Modify the object identifier (e.g., change `user_id=YOUR_ID` to `user_id=ADMIN_ID` or
user_id=123456).
If the application fails to verify that the object belongs to the authenticated user, the server will return the victim’s data.
GET /api/v1/account/details?user_id=456789 HTTP/1.1 Host: api.yandex.com Cookie: session=YOUR_VALID_SESSION_COOKIE
A successful response would include the victim’s email, phone number, or private documents. In critical cases, changing a `POST` request to `PUT` or `DELETE` could allow modification or deletion of the victim’s data, leading to full account takeover.
4. Windows Alternative: PowerShell for Recon
While Linux is preferred for its tooling, similar recon can be performed on Windows using PowerShell.
Step‑by‑step guide explaining what this does and how to use it.
You can use `curl` (available in PowerShell) to test for basic API access control issues.
Test for horizontal privilege escalation
$headers = @{ "Cookie" = "session=YOUR_SESSION" }
$response = curl -Uri "https://api.yandex.com/v1/user/123456/details" -Headers $headers -Method GET
Write-Output $response.Content
If this returns data for User 123456 while you are logged in as a different user, the API is vulnerable.
5. Exploiting via Mobile API Endpoints
Often, mobile applications use different API versions that are less robust than their web counterparts. Using a proxy like Burp Suite on your mobile device, intercept traffic while using the Yandex app.
Step‑by‑step guide explaining what this does and how to use it.
Configure your phone to route traffic through your PC’s Burp listener. Look for API endpoints prefixed with `/mobile/` or /app/. Test the same IDOR vectors here. Sometimes, the mobile API lacks the authorization checks present in the web API, making it a goldmine for bounty hunters.
POST /mobile-api/v2/change_email HTTP/1.1
Host: m.yandex.com
Content-Type: application/json
Authorization: Bearer {token}
{"user_id":"123456","new_email":"[email protected]"}
A 200 OK response without a proper CSRF token or password confirmation confirms a critical vulnerability.
6. Mitigation: Implementing Secure Direct Object References
For developers and security engineers, preventing IDORs is about moving away from trusting user input.
Step‑by‑step guide explaining what this does and how to use it.
Server-Side Check (Node.js/Express Example):
Instead of using the ID from the request body, fetch the user ID from the session.
// Vulnerable Code
app.get('/api/user/:id', (req, res) => {
let userId = req.params.id; // Attacker controls this
db.getUserData(userId).then(data => res.json(data));
});
// Secure Code
app.get('/api/user/:id', (req, res) => {
let sessionUserId = req.session.user.id; // From authenticated session
let requestedId = req.params.id;
// Verify the requested object belongs to the session user
if (sessionUserId !== requestedId && !req.session.isAdmin) {
return res.status(403).json({ error: "Forbidden" });
}
db.getUserData(requestedId).then(data => res.json(data));
});
Implementing these access control checks on the backend is the only reliable defense.
What Undercode Say:
- Never Trust User Input: The core lesson from this Yandex bounty is that any identifier passed from the client (URL, POST body, headers) must be treated as hostile and validated against the server-side session.
- Depth in Recon Wins: The difference between finding a bug and missing it often lies in the recon phase. Utilizing tools like `ffuf` and analyzing mobile traffic expands the attack surface significantly.
The discovery of this IDOR highlights a persistent issue in modern web development: the rush to build features often outpaces the implementation of robust access controls. While OAuth and JWTs handle authentication, authorization is frequently left to client-side hints. As applications grow increasingly complex with microservices and serverless architectures, the attack surface for broken object-level authorization expands. This bounty serves as a reminder that basic web application flaws remain lucrative because they are consistently found in even the most sophisticated tech stacks. Security must shift left, integrating automated access control tests into the CI/CD pipeline to catch these flaws before they reach production.
Prediction:
As AI begins to auto-generate API endpoints and boilerplate code, we will see a rise in “mass assignment” and IDOR vulnerabilities. Attackers will leverage AI to rapidly fuzz millions of parameter combinations across thousands of discovered endpoints, making automated discovery of these bugs faster and more frequent. Consequently, the industry will see a push towards “Zero-Trust Architectures” at the API level, where every request must be explicitly verified regardless of the session token presented.
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IT/Security Reporter URL:
Reported By: Aditya Singh4180 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
