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Introduction:
Broken Access Control (BAC) remains the OWASP Top 10’s number one risk, yet even “well-secured” endpoints can harbor logic flaws that evade standard IDOR testing. Attackers who think beyond parameter tampering—analyzing session contexts, multi-step workflows, and hidden privilege inheritance—can transform a seemingly minor oversight into complete system takeover.
Learning Objectives:
- Identify complex Broken Access Control (BAC) patterns that bypass traditional IDOR detection tools
- Build an exploit chain combining privilege escalation with hidden endpoint enumeration
- Apply both Linux and Windows command-line techniques to test and validate access control misconfigurations
You Should Know:
- Deconstructing the “Secure” Logic – Where BAC Hides in Plain Sight
The post describes an endpoint that looked properly access controlled—likely using role checks, token validation, or resource ownership verification. However, the flaw emerged from a logic assumption: the application trusted that all previous security gates would hold. The exploit chain likely involved:
- Multi-step process abuse: A privileged step (e.g., admin approval) was skipped by directly calling a subsequent API endpoint.
- Hidden parameter inheritance: The application used a `userId` from a JWT but also accepted an override parameter like `target_user` in a later request.
- Race condition between checks: An access decision made at step 1 wasn’t re-validated at step 5.
Step‑by‑step guide to finding this class of bug:
- Map the complete workflow – Use Burp Suite or ZAP to record every request from login to privileged action. Identify state-changing endpoints (POST/PUT/DELETE).
-
Isolate access control checks – For each endpoint, note which headers, cookies, or parameters are used for authorization (e.g.,
Authorization: Bearer,X-User-Id, `role` cookie). -
Attempt step skipping – After completing step 2 (e.g., adding an item to cart), directly call the step 4 URL (e.g.,
/api/order/confirm). Use `curl` to replay without step 3:
Linux / MacOS – replay a POST request with modified session
curl -X POST https://target.com/api/order/confirm \
-H "Authorization: Bearer $TOKEN" \
-H "Content-Type: application/json" \
-d '{"order_id": "12345", "bypass_step": true}'
- Test for parameter pollution – Add
admin=true,isAdmin=1, `role=superuser` to query strings or JSON bodies. On Windows with PowerShell:
Windows PowerShell – test IDOR with integer enumeration
$headers = @{ Authorization = "Bearer $env:TOKEN" }
1..100 | ForEach-Object {
try {
$response = Invoke-RestMethod -Uri "https://target.com/api/user/$_" -Headers $headers -Method Get
if ($response) { Write-Host "Access to user $_" }
} catch {}
}
- Look for “secure” headers that can be removed – Some apps check `X-Requested-With` or
Referer. Remove them and see if access is granted.
Why this works: Developers often implement layered security but fail to re-validate at each step. A missing `X-CSRF-Token` in a later request or a cached privilege decision opens the door.
- Weaponizing the Flaw – From BAC to Full Compromise
Once a broken access control is confirmed, escalation typically involves chaining it with another weakness. In the post’s case, the attacker likely:
- Bypassed object-level authorization (IDOR) to read another user’s private data
- Combined that with privilege escalation (e.g., changing a parameter from `role=user` to `role=admin` in a PUT request)
- Then leveraged admin functions to compromise the entire tenant or application
Step‑by‑step exploit chain construction:
- Confirm IDOR on a read endpoint – Change a numeric ID in `/api/invoice/100` to
/api/invoice/101. If you see another user’s invoice, you have a read primitive. -
Check for write IDOR – Use the same pattern on update endpoints. Example: `PUT /api/profile/100` with `{“email”:”[email protected]”}` – if you can modify user 101’s email, that’s a write primitive.
-
Test for vertical privilege escalation – Look for endpoints that accept a `role` or `group` parameter. Send a PATCH request:
Linux – attempt to promote own account
curl -X PATCH https://target.com/api/user/me \
-H "Authorization: Bearer $TOKEN" \
-H "Content-Type: application/json" \
-d '{"role": "administrator"}'
- If successful, enumerate admin-only APIs – Use Burp Intruder with a wordlist of common admin paths (
/admin,/api/admin/users,/v2/internal/health). On Linux:
Use ffuf to fuzz admin endpoints ffuf -u https://target.com/FUZZ -w /usr/share/wordlists/dirb/common.txt -H "Authorization: Bearer $TOKEN" -fc 403,404
- Exploit the final payload – Once admin access is obtained, extract all users’ data or execute system commands if the admin panel has file upload or command injection flaws.
Mitigation commands for defenders (run on Linux server hosting the app):
Audit file permissions for sensitive configs find /var/www/app -name ".env" -o -name "config" | xargs ls -la Check for hardcoded secrets in code grep -r "API_KEY|SECRET|PASSWORD" /var/www/app --include=.js --include=.py Use OWASP ZAP's automation framework to test BAC zap-cli quick-scan --self-contained --spider -r -s all https://target.com
- Advanced BAC Testing – Beyond Standard IDOR Patterns
Standard IDOR testing changes `?id=1` to ?id=2. Complex BAC involves logic context, time-of-check/time-of-use (TOCTOU), and session merging.
Step‑by‑step advanced techniques:
- Merge sessions – Log in as two users (low privilege and high privilege) in the same browser or using two `curl` sessions. Take a high-privilege request and replay it with the low-privilege session cookie.
-
Test for horizontal privilege escalation via referer header – Some apps trust the `Referer` header to indicate origin. Modify it:
curl -X POST https://target.com/api/delete-note \
-H "Authorization: Bearer $LOW_TOKEN" \
-H "Referer: https://target.com/admin/dashboard" \
-d '{"note_id": "999"}'
- Exploit mass assignment – Send extra JSON fields that the model accepts but the UI hides. Example:
{"username": "attacker", "is_admin": true}. On Windows withcurl.exe:
Windows curl – mass assignment test
curl.exe -X PUT https://target.com/api/user/update `
-H "Authorization: Bearer $env:TOKEN" `
-H "Content-Type: application/json" <code>-d "{\</code>"username`": `"attacker`", `"role`": `"admin`"}"
- Use GraphQL for deeper BAC – Introspect the GraphQL endpoint (
/graphql). Look for mutations that accept `userId` as an argument when you’re not supposed to change others.
Example malicious mutation
mutation {
updateUser(input: { id: "101", email: "[email protected]" }) {
success
}
}
5. Automate with custom Python script:
Python 3 – BAC fuzzer with session reuse
import requests
session = requests.Session()
session.headers.update({"Authorization": "Bearer YOUR_TOKEN"})
for i in range(1, 200):
resp = session.get(f"https://target.com/api/invoice/{i}")
if resp.status_code == 200 and "unauthorized" not in resp.text.lower():
print(f"[!] IDOR found on invoice {i}")
print(resp.text[:200])
4. Cloud and API Hardening Against Complex BAC
Modern apps run on cloud (AWS, Azure, GCP) with API gateways. Complex BAC often exploits misconfigured IAM roles or API Gateway authorizers.
Step‑by‑step cloud BAC testing:
- Examine JWT claims – Decode the JWT using `jwt_tool` on Linux:
Install jwt_tool git clone https://github.com/ticarpi/jwt_tool cd jwt_tool python3 jwt_tool.py <JWT_TOKEN> -d
Look for claims like scope, permissions, groups. Try to change `”user”` to `”admin”` and re-sign with `none` algorithm if the server allows it.
- Test AWS IAM misconfigurations – If the app uses AWS Cognito or IAM roles, attempt to assume a higher role:
List available roles aws sts assume-role --role-arn "arn:aws:iam::123456789012:role/AdminRole" --role-session-name "test"
- API Gateway resource policies – Send requests to internal stages (
/prodvs/dev) – sometimes `/dev` has weaker access control.
curl -X GET https://api.target.com/dev/users/all -H "Authorization: Bearer $TOKEN"
Defender hardening commands (Linux):
Enforce proper CORS and preflight checks Configure nginx to reject unauthorized methods sudo nginx -t && sudo systemctl reload nginx Use OPA (Open Policy Agent) to enforce fine-grained BAC opa eval --data policy.rego 'data.app.allow' --input input.json
What Undercode Say:
- BAC is not just IDOR – Complex logic flaws hide behind multi-step workflows, parameter pollution, and session merging. Standard scanners miss these.
- Think like a state machine – Every request changes the application state. Attackers skip steps, replay sequences, and combine privileges. Validate access at every transition, not just entry.
The post’s insight—“what looked like proper access control turned out to be complex broken access control”—mirrors real-world breaches (e.g., Uber 2016, Instagram 2019). Developers often assume that if an endpoint requires a valid token, it’s safe. But tokens don’t enforce horizontal separation (user A vs user B) or vertical separation (user vs admin) without explicit checks on every resource ID. The most dangerous BAC bugs occur when an application reuses the same API handler for multiple roles, relying on the frontend to hide buttons. Attackers simply call the hidden endpoint directly. To defend, adopt zero-trust API design: each request must prove its authorization, independent of prior requests. Use policy-as-code (e.g., Open Policy Agent) and run automated BAC test suites that simulate multi-step user journeys with role-switching.
Prediction:
As AI-generated code becomes prevalent, BAC flaws will increase—LLMs often generate role-based access control stubs that check only the first step of a workflow. Attackers will leverage AI to automatically map application state machines and identify step-skipping vulnerabilities. In 2026–2027, expect a surge in “BAC chains” where attackers combine a low-privilege read IDOR with a mass assignment to promote themselves, leading to full SaaS tenant compromises. Defenders will shift to runtime authorization enforcement (e.g., eBPF-based API firewalls) and behavioral BAC anomaly detection.
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Reported By: Faiyaz Ahmad – Hackers Feeds
Extra Hub: Undercode MoN
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