✅ 1) What it is (very simple definition)

Access control vulnerability happens when

A user can access something they should NOT be allowed to access.

Examples include accessing:

  • another user's data
  • admin-only pages
  • restricted APIs
  • hidden features

Even when the app "looks secure" on UI — Theare access control vulnerabilities? backend fails to enforce permissions properly.

This is also called:

  • Authorization flaw
  • Broken access control
  • IDOR (Insecure Direct Object Reference)

🎯 2) Why it exists (what problem access control was meant to solve)

Web apps need to:

  • Show each user only their own data
  • Restrict admin features
  • Separate normal users vs premium vs staff
  • Protect internal systems

So apps implement access control toThe

Ensure privacy => User should only see their account privacy Enforce roles => account Only admins can delete users. Prevent abuse => users Only staff can download reports Protect revenue => reports Premium features behind a subscription.

The problem? Many developers trust the frontend UI

⚙️ 3) How it works (step-by-step — browser → request → server)

Let's take a simple case:

A user opens theinstead

/user/profile?id=101

🧭 Step-by-step flow

1️⃣ Browser sends request

GET /user/profile?id=101
Cookie: session=user1

2️⃣ Server extracts two values

  • session user → user1
  • requested user → id=101

3️⃣ Correct behavior (secure) Serveropens the checks:

does user1 own profile 101?

If NO → deny access

4️⃣ Vulnerable behavior The serverServer only checks:

Is the user logged in?

Then returns data anyway.

Result → user can modify ID manually:

/user/profile?id=102

Now they can view another user's profile.

That is broken access control.

🧩 4) Simple Examples (easy to imagine)

🟢 Example 1 — Change User ID in URL

GET /orders?id=1001   → your order
Change to:
GET /orders?id=1002   → someone else's order

If data loads → That's the server and an IDOR vulnerability (exploitable).

🟢 Example 2 — Hidden Admin Button in UI

The UI hides the "Delete User" button for normal users. But the request The UI hides the still valid:

POST /admin/deleteUser?id=55

If user can send request via Burp…

and server allows it…

The vulnerability is privilege escalation.the request

🌍 5) Real-world example (modern web app/API)privilege escalation.

Imagine a mobile banking API:

GET /api/v1/accounts/92391/transactions
Authorization: Bearer token_user_A

User changes account number in proxy:

/api/v1/accounts/92392/transactions

If transactions return:

  • privacy violation
  • financial exposure
  • regulatory breach

This is high-severity broken access control.

This is extremely common in the following:/API)

  • fintech
  • healthcare
  • SaaS CRMs
  • e-commerce order APIs

⚠️ 6) What can go wrong (misconfigurations)

Here are real root causes seen in production:

❌ Trusting frontend controls

UI hides admin features Butin the following: the APIBut API has no role validation.

❌ Only validating login — not ownership

Developer checks:

  • Is user authenticated? ✅

But does NOT check:

  • Does this user own this object? ❌

❌ Relying on client-side values

Such as:

  • role stored in cookies
  • role stored in JWT without verification
  • user ID passed from UI

❌ Reusing APIs across roles

Same API called by:

  • admin panel
  • normal user dashboard

Backend assumes:

"Only admins will call this API"

Attackers do.

🧨 7) How attackers abuse it (basic → advanced)

Let's move from beginner → professional exploitation logic.

🟡 Basic Attacks

  • Modify ID parameter
  • Change username in the request. the request.
  • Replace account number
  • Remove UI restrictions

Example:

/user/profile?id=101 → 102 → 103

🟠 Intermediate Attacks

  • modify JSON body
{ "user_id": 55 }
  • change object references
  • bypass missing server-side validation

🔴 Advanced Real-World Exploitation

1) Replace your JWT user ID.

The original JWT contains

"uid": 101

Modify to:

"uid": 102

If backend does not validate signature properly → Account Takeover

2) Role escalation via header tampering

Add:

X-User-Role: admin

or

X-Organization-Id: 1

If backend trusts header → Privilege Escalation

3) Accessing internal admin APIs

Normal user accesses:

/internal/api/exportUsers
/internal/api/generateReports

If no authorization checks → full database exposure.

🧪 8) Where & how to test manually in VAPT

Here's how professionals approach this.

🔎 Where to look

Target endpoints like the following:

  • /user/*
  • /account/*
  • /orders/*
  • /profile/*
  • /api/v1/*
  • /admin/*

🧰 Manual Testing Workflow (Burp / Proxy)

  1. Capture request
  2. Identify sensitive identifiers:
  • id
  • account_id
  • user_id
  • email
  • org_id

3. Change values manually

4. Replay request

Observe:

Result & Interpretation Access denied => Secure. Redirect to login => auth enforced Error 403 => secure Data returned => valid exploitable finding

🎯 Validation checklist (avoid false positives)

A finding is valid & exploitable only if

  • Another user's data is exposed. or
  • unauthorized operation succeeds

Just changing the ID and getting

  • error
  • null
  • empty response

is NOT a vulnerability.

🧱 9) Common beginner misunderstandings & false positives

❌ "App returns 404 after ID change—it's vulnerable."

No. That means the record doesn't exist.

❌ "UI blocks the action—so it's secure."

Test API anyway.

UI ≠ Security control.

❌ "I changed ID but got same data."

Many apps ignore unknown IDs.

Not exploitable → not a valid issue.

❌ "Any 200 OK response = vulnerability."

Not always.

You must confirm:

  • cross-user data exposure OR
  • privilege escalation

Otherwise, it's noise.

Useful Links:

Learn more = https://owasp.org/Top10/2021/A01_2021-Broken_Access_Control/

lab = https://portswigger.net/web-security/access-control

tools = https://github.com/encodedguy/bugbounty-cheatsheet/blob/master/AccessControl/WebSecurity.md

scripts = https://hacking-tools-scripts.binaryshield.in/