# Improper Input Validation (CWE-20)

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

**Stack:** Python

- Prevalence: High Frequently exploited
- Impact: High 6 high-severity rules
- Prevention: Documented 13 fix examples

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

## Description

Input validation is a frequently-used technique for checking potentially dangerous inputs in order to ensure that the inputs are safe for processing within the code, or when communicating with other components. When software does not validate input properly, an attacker is able to craft the input in a form that is not expected by the rest of the application.

## Prevention

Prevention strategies for Improper Input Validation based on 2 Shoulder detection rules.

### Python

Use Pydantic models with Field validators instead of raw Request objects

Validate business-critical inputs with range constraints using Pydantic or manual checks

## Warning Signs

- [MEDIUM] FastAPI endpoints that accept raw Request objects instead of
Pydantic models
- [MEDIUM] business-critical input values (discount, refund, quantity, price)
that are used in operations witho

## Consequences

- Execute Unauthorized Code
- Modify Application Data
- DoS
- Read Application Data

## Mitigations

- Assume all input is malicious. Use an accept known good input validation strategy
- When performing input validation, consider all potentially relevant properties
- Do not rely exclusively on looking for malicious or malformed inputs

## Detection

- Total rules: 13
- Languages: python, go, javascript, typescript

## Rules by Language

### Python (2 rules)

- **FastAPI Missing Request Validation** [MEDIUM]: Detects FastAPI endpoints that accept raw Request objects instead of
Pydantic models. This bypasses FastAPI's automatic validation and can
lead to type confusion and injection vulnerabilities.
  - Remediation: Use Pydantic models instead of raw Request objects for automatic validation.

```python
from pydantic import BaseModel, EmailStr, Field

class UserCreate(BaseModel):
    username: str = Field(min_length=3, max_length=50)
    email: EmailStr

@app.post("/users")
async def create_user(user: UserCreate):
    return {"user": user}
```

Learn more: https://shoulder.dev/learn/python/cwe-20/missing-validation
- **Business Logic Input Validation** [MEDIUM]: Detects business-critical input values (discount, refund, quantity, price)
that are used in operations without proper validation. Missing validation
can lead to financial fraud, inventory errors, or business logic bypass.
  - Remediation: Validate business-critical inputs with range constraints using Pydantic.

```python
from pydantic import BaseModel, Field

class DiscountRequest(BaseModel):
    discount: float = Field(..., ge=0, le=100)
    quantity: int = Field(..., gt=0)

@app.post('/apply-discount')
async def apply_discount_route(request: DiscountRequest):
    apply_discount(request.discount)
```

Learn more: https://shoulder.dev/learn/python/cwe-20/input-validation