# Improper Verification of Cryptographic Signature (CWE-347)

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

**Stack:** Python

- Prevalence: High Frequently exploited
- Impact: Critical 1 critical-severity rules
- Prevention: Documented 4 fix examples

**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

Prevention strategies for Improper Signature Verification based on 2 Shoulder detection rules.

### 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

- Bypass Protection Mechanism
- Execute Unauthorized Code
- Modify Application Data

## Mitigations

- Always verify signatures before trusting data
- Use well-tested cryptographic libraries for signature verification
- For JWT, always verify the signature and validate the algorithm

## 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