The art of hunting logical bugs: Exploiting business logic in modern apps

Introduction

When we think of web security, we often focus on XSS, SQLi, CSRF, or SSRF. These are well-known vulnerabilities, and modern applications have security mechanisms to handle them. But business logic vulnerabilities? That’s where real opportunities lie for bug bounty hunters.

Business logic bugs don’t rely on breaking security mechanisms. Instead, they exploit flaws in the application’s logic — how it handles transactions, privileges, discounts, or workflows. No WAF, AI tool, or automated scanner can detect them because these bugs are about misusing features in ways developers didn’t anticipate.

This paper explores how business logic vulnerabilities occur, with realistic attack scenarios, practical exploitation techniques, and prevention methods.

What are business logic bugs?

A business logic vulnerability occurs when an application allows users to perform actions that should not be possible under normal conditions. These bugs arise because:

• Developers assume users will behave correctly.
• Security mechanisms are only focused on known attack patterns.
• The application trusts client-side input too much.

Some classic examples include:

• Purchasing a ₹10,000 item for ₹1 due to a logic flaw in discount calculation.
• Canceling an order and receiving a refund without actually returning the item.
• Exploiting race conditions to claim the same reward multiple times.
• Accessing another user’s account by changing a user ID in the API request.

Let’s analyze these attacks with detailed examples.

Common logical bugs and their exploits

1. Price manipulation: Bypassing discount validations

E-commerce platforms often allow discount coupons, but a common mistake is trusting client-side validation.

Attack Scenario