# Change: Enhance Security Validation

## Why

QA review identified several security gaps that could be exploited:
1. JWT secret keys lack entropy validation, allowing weak secrets
2. File uploads only check extensions, not actual MIME types (content spoofing risk)
3. Missing CSRF protection on sensitive state-changing operations

## What Changes

- **user-auth**: Add JWT secret key strength validation (minimum length, entropy check)
- **user-auth**: Add CSRF token validation for sensitive operations
- **document-management**: Add file MIME type validation using magic bytes detection

## Impact

- Affected specs: `user-auth`, `document-management`
- Affected code:
  - `backend/app/core/security.py` - JWT validation
  - `backend/app/api/v1/endpoints/` - CSRF middleware
  - `backend/app/services/file_service.py` - MIME validation