Business logic flaws are some of the most dangerous yet overlooked vulnerabilities in modern web applications. They don’t rely on complex payloads or advanced tools—just understanding how the application is supposed to work and finding where it doesn’t.

🎯 Target Overview

The application was an e-commerce platform selling premium clothing items. One particular product caught my attention:

Product: Lightweight l33t leather jacket

Price: ~$1400

User account: Standard customer account

The platform also offered multiple promotional features:

• New customer discount coupons
• Newsletter signup rewards

At first glance, everything looked normal.

🧠 Initial Testing

After logging in, I added the leather jacket to my cart and moved to checkout.

Coupon #1 – New Customer Offer

The site advertised a coupon for new customers:

NEWCUST5

Applying this coupon reduced the price by $5, as expected.

I tried basic abuse techniques:

• Logging out and back in
• Changing the email address
• Reapplying the coupon

➡️ Result: Coupon reuse was blocked. So far, so good.

🔍 Finding Another Discount Path

While browsing the site further, I noticed a newsletter signup option at the bottom of the home page.

It promised a discount for signing up.

After subscribing, I received a new coupon:

SIGNUP30

When applied at checkout, this coupon reduced the cart value by $401, effectively covering the entire jacket cost.

This immediately raised a red flag 🚩.

💥 Exploitation: Coupon Stacking Logic Flaw

Here’s where things got interesting.

Step-by-step Exploitation

1. Added the leather jacket to the cart
2. Applied NEWCUST5
3. Applied SIGNUP30 afterward

What Should Have Happened?

The application should reject multiple coupons

Or validate whether discounts exceed product value

Or restrict coupon combinations

What Actually Happened?

The application only validated the last coupon. It did not track previously applied discounts. The total price was recalculated incorrectly.

➡️ Final cart value: $0

No errors. No warnings.

🛒 Order Placement

I proceeded to place the order.

✔️ Order confirmed
✔️ No payment required
✔️ Premium item successfully purchased for free

Impact achieved.

> Business Logic Flaw – Coupon Reuse / Coupon Stacking

📉 Impact

This issue could lead to:

• Complete revenue loss on high-value products
• Abuse by automated scripts
• Loss of trust in promotional systems

🏁 Final Thoughts

This bug wasn’t about bypassing authentication or injecting payloads—it was about thinking logically and questioning assumptions.

If you’re starting out in bug bounty hunting, business logic bugs are gold:

• Less competition
• High impact
• Hard to detect automatically