# 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: 高 頻繁に悪用される
- Impact: ハイ 6 件の重大度ハイのルール
- Prevention: 文書化済み 13 件の修正例

**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

2 件の Shoulder 検出ルールに基づく Improper Input Validation の予防策。

### 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

- 未承認コードの実行
- アプリケーションデータの変更
- DoS
- アプリケーションデータの読み取り

## Mitigations

- すべての入力を悪意あるものと想定する。既知の安全な入力のみを受け入れる検証戦略を使用する
- 入力検証を行う際は、関連する可能性のあるすべての属性を考慮する
- 悪意のある、または不正な形式の入力を探すことだけに依存しない

## 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