This Is How I Hacked a Login System Using Logic Flaws

(No Brute Force, No Rate Limits Bypassed – Educational Case Study)

Disclaimer
This article is strictly for educational and defensive learning purposes.
The application, flows, endpoints, and responses are fully anonymized.
No real service was harmed, and this content demonstrates how logic mistakes—not hacking tools—break authentication systems.

Why Login Systems Fail (Even With “Security”)

Most developers believe a login system is secure if it has:

• Rate limiting ✅
• CAPTCHA ✅
• Strong passwords ✅
• MFA (sometimes) ✅

What they forget is logic.

🔑 A login system can be cryptographically perfect and still be completely broken.

This post shows how I logged in as another user
➡️ without guessing passwords
➡️ without brute force
➡️ without exploiting the database

Just by abusing how the system thinks.

Target Overview (Anonymized)

• Type: Consumer web application
• Auth flow:
  • Email
  • Password
  • OTP verification (email-based)
• Security controls present:
  • Rate limiting
  • Account lockout
  • CAPTCHA after failures

On paper? Secure.
In reality? Fragile.

Phase 1: Mapping the Login Flow (Critical Step)

Instead of attacking /login directly, I mapped the entire flow:

1. Submit email + password
2. Server responds: “OTP sent to your email”
3. Submit OTP
4. Login success

This is important:

Any multi-step flow is a logic attack surface

Phase 2: Observing State Handling

I opened DevTools → Network and watched each step.

Step 1 Response (after password):

{
  "status": "OTP_REQUIRED",
  "session_id": "a9f3c1e2"
}

🚩 Red Flag #1:
Authentication state is tracked using a client-side session ID.

Phase 3: Testing Assumptions (Always Question “Required”)

The app said OTP was required.

But did the backend enforce it?

I tried something simple.

Phase 4: Skipping the OTP Step Entirely

Instead of submitting OTP, I sent this request manually:

POST /api/auth/complete HTTP/1.1
Content-Type: application/json

{
  "session_id": "a9f3c1e2"
}

Response:

{
  "token": "eyJhbGciOi..."
}

😐
No OTP.
No validation.
No error.

I was logged in.

Why This Worked (Important)

The backend logic was:

IF session_id exists
→ issue token

Instead of:

IF session_id exists AND otp_verified == true
→ issue token

This is not a coding bug.

This is a logic flaw.

Phase 5: Expanding the Attack Surface

Now I tested other variations.

Case 1: Wrong Password + OTP Step

• Submitted wrong password
• Still received session_id
• OTP flow triggered anyway

That means:

Password correctness was not enforced before OTP

Phase 6: Account Takeover Scenario

Here’s the full attack chain:

Step-by-step Attack

1️⃣ Enter victim’s email
2️⃣ Enter any password
3️⃣ Receive session_id
4️⃣ Skip OTP verification
5️⃣ Call /auth/complete
6️⃣ Get valid auth token

Result:

Full account access without credentials

Phase 7: Confirming With Another Account

I repeated the attack using:

• A fresh account
• Different emails
• Different browsers

Same result.

This confirmed it wasn’t:

• Session confusion
• Cache issue
• Race condition

It was systemic.

Why Rate Limiting & CAPTCHA Didn’t Help

Security features mean nothing if logic is broken.

Root Cause Analysis

❌ Developer Mistakes

• Trusted frontend flow enforcement
• Didn’t bind OTP verification to auth state
• Treated steps independently instead of atomically

✅ How This Should Be Built

Authentication must be stateful and atomic.

Correct approach:

if not session.otp_verified:
    deny_login()

Even better:

• Store auth state server-side
• Use short-lived, signed state tokens
• Invalidate session if steps are skipped

How I Hunt Login Logic Bugs (My Exact Method)

This is the playbook:

1️⃣ Break the flow

• Skip steps
• Reorder steps
• Replay steps

2️⃣ Reuse identifiers

• session_id
• temp_token
• state param

3️⃣ Change inputs

• Wrong password
• Correct OTP
• No OTP

4️⃣ Observe assumptions

• “This step always happens”
• “User can’t reach this endpoint”

Attack those assumptions.

Tools Used

Severity & Impact

This vulnerability allows:

• 🔓 Full account takeover
• 🔑 Bypass MFA entirely
• 📩 Access private data
• 💸 Financial abuse

Severity:

Critical (Authentication Bypass)

This is worse than SQLi in many cases.

Why These Bugs Are Everywhere

Because developers:

• Focus on features, not flows
• Test happy paths only
• Assume users behave correctly

Attackers don’t.

Defensive Checklist (For Developers)

• ✅ Enforce step order server-side
• ✅ Bind auth state to verified conditions
• ✅ Expire incomplete sessions
• ✅ Treat login as a single transaction
• ❌ Never trust frontend “required” steps

Final Thoughts

You didn’t need:

• Password cracking
• Exploit kits
• Malware
• Advanced tools

You needed logic.

That’s why logic flaws are the highest-paid bug bounty class.