Listen to this Post
Introduction:
Broken Access Control (BAC) remains the most critical web application security risk according to OWASP Top 10, yet it is consistently underestimated. When an application fails to enforce proper authorization on user actions, attackers can view, edit, or delete data belonging to other users – often leading to account takeover, data breaches, and lucrative bug bounties. This article walks through a real-world Broken Access Control finding, from reconnaissance to responsible disclosure, including the exact commands and techniques used to bypass validation and earn the bounty.
Learning Objectives:
- Understand how Broken Access Control and Insecure Direct Object References (IDOR) manifest in modern APIs and web apps.
- Learn step-by-step manual and automated techniques to identify, exploit, and remediate BAC vulnerabilities.
- Master practical Linux/Windows commands, Burp Suite configurations, and reporting workflows for bug bounty programs.
You Should Know:
1. Understanding Broken Access Control and IDOR
Broken Access Control occurs when an application allows a user to act outside their intended permissions. The most common variant is IDOR (Insecure Direct Object Reference), where user-supplied input (like ?user_id=123) directly points to an object without proper checks. Attackers simply change the ID to access other users’ data. This flaw affects REST APIs, GraphQL endpoints, file downloads, and even admin panels. For example, a typical vulnerable request:
GET /api/profile?userId=1001 HTTP/1.1 Host: target.com Cookie: session=abc123
If changing `userId=1002` returns another user’s profile, BAC is confirmed.
2. Reconnaissance and Identifying BAC Targets
Start by mapping all endpoints where object identifiers appear. Use browser developer tools (Network tab) or Burp Suite Proxy to capture traffic. Look for:
– Numeric IDs in URLs (/invoice/4567)
– UUIDs or hashes in parameters (/order?id=550e8400)
– API endpoints with user_id, account, doc_id, file, `ref`
– GraphQL queries with ID arguments
– Hidden form fields containing database keys
Linux commands for recon (using curl and grep):
Fetch a page and extract all IDs in URLs
curl -s https://target.com/dashboard | grep -oP '(id|user_id|doc)=\d+' | sort -u
Test IDOR by iterating IDs
for id in {1000..1020}; do
curl -s "https://target.com/api/user/$id" -H "Cookie: session=YOUR_SESSION" | jq '.email'
done
Windows PowerShell equivalent:
1..20 | ForEach-Object {
$response = Invoke-WebRequest -Uri "https://target.com/api/user/$_" -Headers @{Cookie="session=YOUR_SESSION"}
$response.Content | Select-String "email"
}
- Testing for BAC with Burp Suite and Commands
Burp Suite is the industry standard. Follow these steps:
– Set Burp as proxy (127.0.0.1:8080) and install CA certificate.
– Browse the authenticated application – Burp logs every request.
– Send a request containing an object ID to Repeater (Ctrl+R).
– Modify the ID parameter to adjacent values (e.g., 1001 → 1002, then 9999, then 0, then negative numbers).
– Observe response differences: status 200 with different data vs 403/401. A 200 without proper authorization is a finding.
Automated fuzzing with ffuf (Linux):
Fuzz ID parameter from 1 to 5000 ffuf -u "https://target.com/api/user/FUZZ" -w <(seq 1 5000) -H "Cookie: session=YOUR_SESSION" -fc 404,403,401
Using wfuzz for parameter tampering:
wfuzz -c -z range,1-1000 -H "Cookie: session=abc" https://target.com/account?id=FUZZ
4. Exploiting BAC via Parameter Tampering
Once a vulnerable endpoint is found, escalate the impact. Common techniques:
– Horizontal Privilege Escalation: Access another regular user’s data (e.g., change `user_id` to a different user’s ID). If you can view their orders, messages, or personal info, that’s a valid bug.
– Vertical Privilege Escalation: Try to access admin functions by adding parameters like ?admin=true, ?role=admin, or modifying `X-Forwarded-For` headers. Sometimes changing `user_id=1` (often admin) works.
– Method Tampering: If a GET request fails, try POST, PUT, DELETE with the same ID. Many apps block GET but allow POST on the same endpoint.
– Path Traversal in IDs: Use `../` sequences: `/api/user/../admin/profile` or `?file=../../../etc/passwd`
Example exploit chain:
GET /api/v1/orders?userId=1001 HTTP/1.1 Host: vuln-app.com Authorization: Bearer eyJhbG...
Change to userId=1002. If you see another user’s orders, report it. To prove impact, extract sensitive fields like email, phone, address, or payment info.
5. Advanced BAC: Horizontal and Vertical Privilege Escalation
Beyond simple ID swapping, advanced BAC includes:
- JWT tampering: Decode the JWT (jwt.io) and change the `sub` or `user_id` claim. If the server doesn’t verify signature properly, you become any user.
Decode JWT without verifying signature (Linux) echo "eyJhbGc..." | cut -d. -f2 | base64 -d | jq .
- API versioning quirks: Try `/api/v2/user/1001` if `/v1` is fixed. Older versions often lack proper checks.
- GraphQL IDOR: Use introspection to find object types. Query like:
query { user(id: 1002) { email, creditCard } } - Mass assignment: Add extra parameters like `{“user_id”:1002,”role”:”admin”}` in POST/PUT requests. The server might blindly update fields.
Remediation command example (for developers – enforce authorization middleware):
Flask example – proper check
@app.route('/api/user/<int:user_id>')
def get_user(user_id):
if current_user.id != user_id and not current_user.is_admin:
return "Forbidden", 403
return data
6. Mitigation and Secure Coding Practices
To fix BAC, implement defense-in-depth:
- Server-side authorization for every endpoint – never trust client-side checks.
- Use indirect references: Map internal IDs to random tokens (
/profile/token_abc123instead of/profile/1001). - Enforce least privilege with role-based access control (RBAC).
- Log all access violations and alert on anomalous ID enumeration.
- Linux sysadmins can use `auditd` to monitor file access patterns that might indicate BAC attempts:
sudo auditctl -w /var/www/private/ -p r -k private_access
- Windows: Enable advanced audit policies (Local Security Policy → Advanced Audit Configuration → Audit File System).
7. Reporting and Earning Bounties
When you confirm a BAC vulnerability, document with:
- Step-by-step reproduction (URLs, parameters, cookies).
- Screenshots or video proof.
- Impact statement: what data an attacker could access (PII, financial, admin functions).
- Suggested remediation (add authorization check, use RBAC).
Use platforms like HackerOne, Bugcrowd, or company VDP. For the finding shown in the post, the hunter earned a bounty by clearly demonstrating unauthorized access to another user’s account.
Sample report template:
IDOR in /api/profile endpoint allows viewing any user’s personal data
> Steps:
- Login as user A, visit https://target.com/api/profile?id=1001
- Change id=1002 (user B) – full B’s profile returned.
Impact: Attacker can enumerate all user IDs and leak emails, addresses, and payment methods.
What Undercode Say:
- Broken Access Control remains the highest-paid bug bounty category because it directly compromises data confidentiality and integrity. Mastering IDOR and parameter tampering yields consistent results.
- Automation with tools like ffuf, Burp Intruder, and custom scripts significantly speeds up enumeration, but manual logic flaws (e.g., JWT tampering, GraphQL introspection) often lead to critical bounties.
- Secure coding must shift left: every object-level access requires authorization middleware. Relying on frontend obfuscation or non-guessable IDs (UUIDs) is not enough – always verify on the server.
Prediction:
As AI-assisted code generation becomes ubiquitous, Broken Access Control will surge due to auto-generated endpoints lacking proper authorization checks. Attackers will leverage LLMs to fuzz APIs at scale, discovering BAC in minutes. Defenders will adopt runtime authorization enforcement (e.g., Open Policy Agent) and automated API security testing in CI/CD pipelines. The bounty for creative BAC chains will double, and ethical hackers who master both manual logic and AI tooling will dominate the bug bounty leaderboards.
▶️ Related Video (78% Match):
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Tanishsaini299 Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
