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Introduction:
Most bug bounty hunters and pentesters instinctively skip “protected” endpoints, assuming they are locked down. However, as demonstrated by Offensive Security Engineer Faiyaz Ahmad, digging deeper into these seemingly secure paths can uncover complex broken access control (BAC) flaws—leading from a simple observation to a full account or system compromise. This article extracts the technical essence from his free playlist on underrated vulnerabilities and provides a hands-on methodology to discover and exploit these hidden weaknesses.
Learning Objectives:
- Identify and systematically test protected API and web endpoints for subtle access control bypasses.
- Exploit horizontal and vertical privilege escalation flaws using request tampering, method swapping, and parameter pollution.
- Build a repeatable, tool-agnostic workflow to uncover non-obvious vulnerabilities beyond standard IDOR and XSS.
You Should Know:
- Mapping the Attack Surface: From “Protected” to Exposed
A protected endpoint often returns a 403/401 or a generic error. But “protected” does not mean “unbreakable.” Start by mapping all application roles (anonymous, authenticated user, admin, support) and the resources they can access. Use browser dev tools (Network tab) or a proxy like Burp Suite to capture every request while navigating the app. Pay special attention to endpoints with UUIDs, sequential IDs, or encoded parameters. The playlist’s first lesson: never ignore a 403—intercept it, modify it, resend it.
Step‑by‑step guide:
- Step 1: Log in as a low‑privilege user (e.g., “user1”). Browse the application and note every API call that fetches or modifies data.
- Step 2: Log in as another low‑privilege user (“user2”) and capture the same endpoints.
- Step 3: Replace user2’s identifiers (session cookie, JWT, or user ID parameter) with those of user1 and resend the request. If the server returns user1’s data, you’ve found a horizontal BAC.
- Step 4: For vertical escalation, replace role‑related parameters (e.g., `role=user` →
role=admin) or add privileged headers (X-User-Role: admin).
2. Directory Busting for Hidden Protected Endpoints
Developers often hide administrative or debug endpoints behind non‑standard paths (e.g., /internal/v2/api/users, /admin/backup/export). Use automated directory busting to discover them. Even if they return 403, they are worth testing.
Linux commands (using ffuf and gobuster):
Ffuf with common API wordlist ffuf -u https://target.com/FUZZ -w /usr/share/wordlists/dirb/common.txt -fc 404 Gobuster for API endpoints gobuster dir -u https://target.com/api/ -w /usr/share/wordlists/dirb/api.txt -t 50
Windows commands (PowerShell + dirb alternative):
Using Invoke-WebRequest with a custom wordlist
Get-Content .\wordlist.txt | ForEach-Object {
try { Invoke-WebRequest -Uri "https://target.com/$_" -Method Head -ErrorAction Stop }
catch { if($<em>.Exception.Response.StatusCode -eq 403) { Write-Host "Found protected: $</em>" } }
}
- Testing for Complex BAC via Method and Parameter Tampering
A classic mistake: developers protect `GET` requests but forgetPOST,PUT,DELETE, orPATCH. Always replay the same request with different HTTP methods. Also test parameter pollution (e.g.,user_id=123&user_id=456) and JSON nesting (e.g., `{“user”:{“id”:123}}` →{"user":{"id":456}}).
Step‑by‑step with Burp Suite:
- Step 1: Send the original request to Repeater.
- Step 2: Change the method (GET → POST). If the endpoint accepts the alternative method and bypasses access control, you have a misconfiguration.
- Step 3: Add duplicate parameters (
id=123&id=456). Some servers use the last occurrence, others the first—test both. - Step 4: Modify Content-Type headers (e.g., from `application/json` to
application/x-www-form-urlencoded). This can trick flawed access control middleware that only parses JSON.
Example using curl:
Original GET curl -X GET "https://target.com/api/user/123" -H "Cookie: session=user2" Tampered to POST with different ID curl -X POST "https://target.com/api/user/456" -H "Cookie: session=user2" -d "id=456"
4. Exploiting JWT and Session‑Based Access Control Flaws
Many “protected” endpoints rely on JWTs or custom session tokens. If the token lacks a proper audience (aud) or issuer (iss) validation, an attacker can reuse tokens across roles. Also test for `alg=none` or weak signing keys.
Linux commands to test JWT weaknesses:
Decode JWT (base64) echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoidXNlciJ9" | base64 -d Use jwt_tool to test alg=none and brute‑force python3 jwt_tool.py <JWT> -X a
Step‑by‑step:
- Step 1: Capture the JWT of a low‑privileged user.
- Step 2: Decode the payload and change `role` to
admin. - Step 3: Re‑encode without signing (set
alg: none) or sign using a known weak secret (e.g.,secret). - Step 4: Send the tampered JWT to a protected endpoint. If the server accepts it, you have a critical BAC.
- Automating Access Control Testing with a Simple Bash Script
Manually testing every endpoint for BAC is tedious. Automate the process by comparing responses from two different user contexts.
Bash script (Linux/macOS with `jq`):
!/bin/bash
test_bac.sh - Compare user1 and user2 responses
USER1_COOKIE="session=abc123"
USER2_COOKIE="session=xyz789"
ENDPOINTS=("/api/profile" "/api/orders" "/api/admin/users")
for endpoint in "${ENDPOINTS[@]}"; do
echo "Testing $endpoint"
response1=$(curl -s -H "Cookie: $USER1_COOKIE" "https://target.com$endpoint")
response2=$(curl -s -H "Cookie: $USER2_COOKIE" "https://target.com$endpoint")
if [ "$response1" != "$response2" ]; then
echo "Potential BAC: responses differ for $endpoint"
echo "User1: $response1"
echo "User2: $response2"
fi
done
Run with chmod +x test_bac.sh && ./test_bac.sh. Extend it to test role‑based access by swapping cookies with an admin session.
6. Real‑World Mitigation: Hardening Your Own APIs
If you are a developer, enforce access control on the server‑side for every request—never rely on client‑side hiding. Use a centralized middleware that checks the user’s role and resource ownership before processing.
Example Node.js/Express middleware:
function checkOwnership(req, res, next) {
const userId = req.params.id;
if (req.user.role === 'admin' || req.user.id === userId) {
next();
} else {
res.status(403).json({ error: 'Forbidden' });
}
}
Linux/Windows hardening command: Regularly scan your API endpoints with automated BAC tools like `Authz` (Burp extension) or `RESTler` fuzzer. For cloud environments (AWS, Azure), enforce IAM policies that deny access by default.
7. Building a Mindset for Underrated Vulnerabilities
Faiyaz Ahmad’s key message: “Bugs that don’t look obvious at first” require a shift from checklist testing to exploratory testing. Start with a protected endpoint, then ask: “What if I change the HTTP method? What if I add an admin header? What if I access this from a different subdomain?” Document every deviation and note unexpected status codes (200 OK, 302 redirect, 500 error). Over time, this methodical curiosity turns into a repeatable, high‑impact discovery process.
What Undercode Say:
- Key Takeaway 1: Protected endpoints are often the most rewarding targets because developers assume they are safe, leading to lazy access control implementations.
- Key Takeaway 2: Combining method tampering, parameter pollution, and JWT manipulation can turn a single 403 into a full compromise—without any complex exploit chains.
The post by Faiyaz Ahmad underscores a crucial reality: the bug bounty industry’s obsession with “common” vulnerabilities blinds testers to the low‑hanging fruit of broken access control. Real‑world impact (HoF at Google, UN, NASA) comes not from running automated scanners, but from understanding how developers think and where they cut corners. The free playlist referenced (https://lnkd.in/dAwKfZyM) provides a rare, step‑by‑step look into that thought process. For any aspiring pentester, mastering these techniques will immediately elevate your findings from low/medium to critical severity.
Prediction:
As AI‑assisted code generation becomes mainstream, we will see a surge in subtle access control flaws—because LLMs often copy insecure patterns from training data and fail to enforce context‑aware permissions. Automated BAC scanners will lag behind, forcing human testers to adopt hybrid workflows: using AI to generate test cases for protected endpoints, then manually chaining small misconfigurations into privilege escalation. The next wave of high‑impact bug bounty reports will come from those who combine Faiyaz’s “underrated vulnerability” mindset with AI‑powered fuzzing of role‑based access controls.
▶️ Related Video (78% Match):
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IT/Security Reporter URL:
Reported By: Faiyaz Ahmad – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
