# Business Logic Errors (CWE-840)

The product does not properly implement the business logic rules, which may allow users to manipulate the system in unintended ways.

- Prevalence: Medium 3 languages covered
- Impact: High 3 high-severity rules
- Prevention: Documented 3 fix examples

**OWASP:** Broken Access Control (A01:2021-Broken Access Control) - #1

## Description

Business logic errors occur when the application's implementation doesn't correctly enforce the intended business rules. Unlike technical vulnerabilities, these are flaws in the application's design or logic.

## Prevention

Prevention strategies for Business Logic Errors based on 3 Shoulder detection rules.

### Go

Calculate financial values server-side from trusted data sources instead of accepting client-submitted totals

### Node.js

Calculate totals and prices server-side using database values instead of client-submitted data

### Python

Calculate totals server-side using database prices instead of client-submitted values

## Warning Signs

- [HIGH] client-controlled prices or amounts flowing to payment operations without server-side validation
- [HIGH] client-controlled business-critical values (price, quantity, discount)
flowing to payment or busines

## Consequences

- Bypass Protection Mechanism
- Gain Privileges
- Modify Application Data

## Mitigations

- Clearly document business rules and their security implications
- Test edge cases and unusual workflows
- Implement server-side validation of all business rules

## Detection

- Total rules: 3
- Languages: go, javascript, typescript, python

## Rules by Language

### Go (1 rules)

- **Business Logic Bypass** [HIGH]: Client-controlled financial values flow to payment operations without server-side calculation.
  - Remediation: Fetch prices from the database instead of trusting client values.

```go
func checkout(c *gin.Context) {
    productID := c.PostForm("product_id")
    var product Product
    db.First(&product, productID)
    total := product.Price * float64(quantity)
    processPayment(total)
}
```

Learn more: https://shoulder.dev/learn/go/cwe-840/business-logic-bypass

### Javascript (1 rules)

- **Business Logic Bypass** [HIGH]: Detects client-controlled prices or amounts flowing to payment operations without server-side validation.
  - Remediation: Calculate totals server-side using database prices.

```javascript
const product = await Product.findById(productId);
const total = product.price * quantity;
await stripe.charges.create({ amount: total, currency: 'usd' });
```

Learn more: https://shoulder.dev/learn/javascript/cwe-840/business-logic-bypass

### Typescript (1 rules)

- **Business Logic Bypass** [HIGH]: Detects client-controlled prices or amounts flowing to payment operations without server-side validation.
  - Remediation: Calculate totals server-side using database prices.

```javascript
const product = await Product.findById(productId);
const total = product.price * quantity;
await stripe.charges.create({ amount: total, currency: 'usd' });
```

Learn more: https://shoulder.dev/learn/javascript/cwe-840/business-logic-bypass

### Python (1 rules)

- **Business Logic Bypass** [HIGH]: Detects client-controlled business-critical values (price, quantity, discount)
flowing to payment or business operations without server-side validation.
  - Remediation: Calculate totals server-side using database prices instead of client values.

```python
@app.post('/checkout')
async def checkout(item_id: int, quantity: int):
    product = Product.query.get(item_id)
    total = product.price * quantity
    stripe.Charge.create(amount=total)
```

Learn more: https://shoulder.dev/learn/python/cwe-840/business-logic-bypass