# Improper Verification of Cryptographic Signature (CWE-347)

The product does not verify, or incorrectly verifies, the cryptographic signature for data.

**Stack:** Python

- Prevalence: 높음 자주 악용됨
- Impact: 치명적 1개의 치명적 심각도 규칙
- Prevention: 문서화됨 4개의 수정 예시

**OWASP:** Cryptographic Failures (A02:2021-Cryptographic Failures) - #2

## Description

Cryptographic signatures are used to verify the authenticity and integrity of data. When signature verification is missing or incorrectly implemented, attackers can forge or tamper with data.

## Prevention

2개의 Shoulder 탐지 규칙을 기반으로 한 Improper Signature Verification 예방 전략.

### Python

Load JWT secret from environment and explicitly specify allowed algorithms

Always specify allowed algorithms explicitly when decoding JWT tokens

## Warning Signs

- [HIGH] JWT implementation has security vulnerabilities
- [HIGH] JWT security issues in FastAPI applications including:
- Weak or hardcoded secrets
- Missing algorit
- [CRITICAL] JWT tokens decoded without algorithm verification or accepting the 'none' algorithm, allowing token 

## Consequences

- 보호 메커니즘 우회
- 승인되지 않은 코드 실행
- 애플리케이션 데이터 수정

## Mitigations

- 데이터를 신뢰하기 전에 항상 서명을 검증하세요
- 서명 검증에는 충분히 검증된 암호 라이브러리를 사용하세요
- JWT는 서명을 항상 검증하고 알고리즘도 검증하세요

## Detection

- Total rules: 4
- Critical: 1
- Languages: python, go, javascript, typescript

## Rules by Language

### Python (2 rules)

- **FastAPI JWT Security Issues** [HIGH]: Detects JWT security issues in FastAPI applications including:
- Weak or hardcoded secrets
- Missing algorithm verification
- Insufficient token validation
- Insecure token storage patterns
  - Remediation: Load JWT secret from environment and explicitly specify the algorithm.

```python
from pydantic_settings import BaseSettings
from jose import jwt

class Settings(BaseSettings):
    SECRET_KEY: str
    ALGORITHM: str = "HS256"

settings = Settings()

def decode_token(token: str):
    return jwt.decode(token, settings.SECRET_KEY, algorithms=[settings.ALGORITHM])
```

Learn more: https://shoulder.dev/learn/python/cwe-347/jwt-security
- **JWT Algorithm Confusion Attack** [CRITICAL]: Detects JWT tokens decoded without algorithm verification or accepting the 'none' algorithm, allowing token forgery.
  - Remediation: Always specify allowed algorithms explicitly when decoding JWT tokens.

```python
payload = jwt.decode(token, SECRET_KEY, algorithms=['HS256'])
```

Learn more: https://shoulder.dev/learn/python/cwe-347/jwt-algorithm-confusion