Business Logic Vulnerabilities: The Bugs Scanners Cannot Find

Some of the most damaging vulnerabilities contain no malicious payload at all. Every request is well-formed, every input is valid, and yet the attacker gets something they should not: free products, extra credits, another user's access, or a skipped approval step. These are business logic vulnerabilities.

What makes them different

Injection and XSS are about the application misinterpreting input. Business logic flaws are about the application doing exactly what it was coded to do, when the code does not match the rules the business intended.

Real-world patterns

Pricing and payments

• Changing the price or quantity in the request: "quantity": -1 producing a refund.
• Applying a discount code multiple times, or stacking codes that should be exclusive.
• Modifying the currency while keeping the number the same.
• Completing an order after a payment failure by calling the confirmation endpoint directly.

Workflow bypass

• Skipping a step in a multi-step process, such as verification or approval, by calling the final endpoint.
• Reusing a one-time action, such as a free trial, by creating new accounts or changing an identifier.
• Downgrading a plan while retaining premium features.

Limits and quotas

• Exceeding seat limits by inviting users in parallel.
• Bypassing usage limits by spreading requests across API keys or organisations.
• Unlimited free credits through referral loops.

Trust assumptions

• Trusting client-side calculations, such as totals computed in the browser.
• Assuming a user can only reach a page through the UI.
• Assuming an email address proves ownership without verification.

Why scanners miss them

A scanner does not know that a negative quantity is wrong, that a coupon should work once, or that step 3 requires step 2. Detecting these flaws requires understanding what the application is for.

How to prevent business logic flaws

1. Write down the rules: what must always be true about orders, credits, plans and permissions.
2. Enforce rules on the server, recalculating prices, totals and entitlements from trusted data.
3. Model workflows as state machines, and reject transitions that skip states.
4. Use database constraints and transactions to enforce limits atomically.
5. Write abuse case tests: "a user cannot apply the same coupon twice," "an unpaid order cannot be fulfilled."
6. Monitor for anomalies, such as unusual refunds, credit spikes or rapid account creation.

Testing for logic flaws

Testing requires exploring the application as a user, understanding its rules, then deliberately breaking them: replaying, reordering, skipping and modifying requests. This is where AI agents add value over traditional scanners, as they can read the application, infer intent and try abuse cases across workflows. SecurityInfinity agents test authenticated workflows and report logic issues with evidence. Start free.

Frequently asked questions

Are business logic flaws common?

Yes. They are among the most frequently rewarded findings in bug bounty programmes, especially in e-commerce, fintech and SaaS billing.

How severe are they?

It varies widely, from minor quota bypasses to direct financial loss or full access to other customers' data.

Can threat modelling help?

Yes. Thinking through how features can be abused during design prevents many logic flaws before code is written.