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Introduction:
In the competitive arena of bug bounty hunting, a singular focus on common vulnerability classes can yield extraordinary rewards. As demonstrated by a hunter who earned $28,000 from just two programs, Access Control vulnerabilities—specifically Broken Access Control (BAC)—remain a goldmine for ethical hackers. This article deconstructs the methodology, tools, and mindset required to systematically find and exploit these flaws that allow unauthorized access to sensitive data or privileged functions.
Learning Objectives:
- Understand the core concepts and real-world impact of Broken Access Control (IDOR, Privilege Escalation, etc.).
- Master a professional reconnaissance and testing methodology for uncovering access control flaws.
- Learn to craft effective proof-of-concept exploits and articulate findings for maximum bounty rewards.
You Should Know:
1. Reconnaissance and Endpoint Mapping: The Foundation
Before testing a single parameter, you must map the application’s attack surface. This involves discovering all endpoints, understanding user roles, and identifying object referencing patterns.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Spidering & Proxy Logging. Use Burp Suite or OWASP ZAP to crawl the target application while logged into a low-privilege account. This populates your sitemap with authenticated endpoints.
` Using Burp Suite’s built-in spider: Right-click on the site map -> ‘Spider this host’`
Step 2: Endpoint Enumeration with ffuf. Complement proxy logs with fuzzing to discover hidden directories, API routes, and parameterized URLs.
`ffuf -w /usr/share/wordlists/seclists/Discovery/Web-Content/common.txt -u https://target.com/FUZZ -mc 200,301,302,403`
Step 3: Role Definition. Clearly document all user roles (e.g., user, editor, admin) and their intended permissions within the app. This is your test matrix.
2. Testing for Insecure Direct Object References (IDOR)
IDOR occurs when an application uses user-supplied input (like an ID in a URL) to access an object without proper authorization checks.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Identify Direct References. Look for parameters like ?id=123, ?invoice=456, /api/v1/user/789/profile.
Step 2: Manipulate Values. Using two authenticated accounts (Acct A and Acct B), try to access Acct B’s object by substituting its ID while using Acct A’s session.
Scenario: You are logged in as user_id=100. You see a request GET /api/orders?user_id=100. Change it to GET /api/orders?user_id=101.
Step 3: Automated Testing with Scripts. For API testing, write a simple Python script to iterate through ID ranges.
import requests
cookies = {'session': 'your_low_priv_session_cookie'}
for id in range(100, 150):
resp = requests.get(f'https://target.com/api/document/{id}', cookies=cookies)
if resp.status_code == 200 and "Unauthorized" not in resp.text:
print(f"[!] Potential IDOR at ID: {id}")
3. Horizontal to Vertical Privilege Escalation
Horizontal escalation is accessing another user’s data (IDOR). Vertical escalation is gaining higher privileges (e.g., user to admin).
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Locate Administrative Endpoints. During recon, you may find paths like /admin/, /api/admin/users. Access them as a normal user.
Step 2: Test for Missing Role/Permission Checks. If a `GET /admin/panel` returns 403, try a POST /admin/panel/addUser. The check might be missing for non-GET methods.
Step 3: Parameter Tampering for Role Claims. If a JWT token or a hidden form field contains a `”role”:”user”` parameter, try changing it to `”role”:”admin”` or `”isAdmin”:false` to true.
Windows Command for JWT inspection: `certutil -decode [bash] temp.txt & type temp.txt` (Base64Url decode the payload).
4. Bypassing Path-Based Access Control
Applications sometimes control access by hiding or restricting UI links, not by securing the backend endpoint.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Forced Browsing. Attempt to directly access directories or files common to higher-privilege users.
`ffuf -w wordlists/admin-paths.txt -u https://target.com/FUZZ -mc 200,301`
Step 2: Analyze Source Code (Client-Side). Use browser dev tools (Ctrl+Shift+I) to examine JavaScript files for hardcoded API keys or administrative endpoints commented out or accessible only via conditional logic that can be bypassed.
5. Testing Mass Assignment and Function-Based Access Control
Mass assignment exploits endpoints that automatically bind user input to object properties, allowing attackers to update privileged fields. Function-Based Access Control (FBAC) flaws occur when the server fails to verify if a user is authorized to execute a specific function.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Identify Object Updates. Find `POST/PUT /api/users/profile` or similar update endpoints. Capture the legitimate request.
Step 2: Add Privileged Parameters. Add parameters like "role":"admin", "credit_balance":10000, or `”email_verified”:true` to the JSON/XML payload and observe if they are accepted.
Step 3: Test Function-Level Checks. If a normal user can invoke an action via POST /api/invoices/create, can they also invoke POST /api/invoices/deleteAll? Test all HTTP methods on discovered endpoints.
6. Cloud-Native Access Control Pitfalls (AWS S3, IAM)
Misconfigured cloud storage and permissions are a major source of BAC in modern apps.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Discover Cloud Assets. Look for URLs pointing to `.s3.amazonaws.com` or `.cloudfront.net` in source code and JS files.
Step 2: Test for S3 Bucket Misconfigurations. Check for publicly listable or writable buckets.
`aws s3 ls s3://target-bucket/ –no-sign-request` (Lists if public)
`aws s3 cp test.txt s3://target-bucket/ –no-sign-request` (Tests write)
Step 3: Exploit Over-Permissive IAM Roles. If you compromise an AWS key, check its permissions: aws iam list-attached-user-policies --user-name <username>. Overly broad policies like `”Action”: “s3:”` are critical finds.
7. Mitigation and Secure Coding Practices
The ultimate goal is to help developers fix these issues.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Implement Server-Side Checks. Every single request must be authorized server-side. Use a central access control routine.
Pseudo-Code: `if (!req.user.can(‘delete’, invoice)) return res.status(403);`
Step 2: Use Indirect References. Map a random UUID or a hashed value to internal IDs to prevent easy guessing.
Step 3: Adopt a Zero-Trust Model. Never trust the client. Validate permissions per function/endpoint, deny by default, and conduct regular automated security testing.
What Undercode Say:
- Mindset Over Tools: The hunter’s tip, “Don’t change the program, change your approach,” underscores that persistence and a deep, creative understanding of application logic trump constantly switching targets or relying solely on automated scanners. Systematic manual testing within a defined scope yields consistent results.
- The Business Impact of BAC: Access control flaws directly translate to data breaches, fraud, and system compromise, explaining their high value in bounty programs. They are business logic flaws that automated tools often miss, making skilled hunters indispensable.
Prediction:
The prevalence of Broken Access Control will intensify with the proliferation of microservices and complex APIs, where enforcing consistent authorization across distributed systems is challenging. Future impactful finds will increasingly involve chaining low-severity IDORs or misconfigurations across different services (cloud, API, database) to achieve full system compromise. Bug bounty hunters who master graph-based API analysis and cloud security testing will dominate the leaderboards, as programs increasingly reward hunters who can demonstrate systemic risk rather than isolated bugs.
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Reported By: Vaishnav Pardhi – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
