# RuntimeError: CORS headers missing for preflight request

- **ID:** `python/starlette-cors-headers-missing`
- **Domain:** python
- **Category:** network_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Starlette应用未配置CORS中间件，导致跨域请求被拒绝

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **添加CORSMiddleware到应用** (95% success)
   ```
   from starlette.middleware.cors import CORSMiddleware
app.add_middleware(CORSMiddleware, allow_origins=['*'])
   ```
2. **配置允许的特定域名** (90% success)
   ```
   app.add_middleware(CORSMiddleware, allow_origins=['https://example.com'])
   ```

## Dead Ends

- **在响应中手动添加Access-Control-Allow-Origin头** — 预检请求（OPTIONS）需要特殊处理，手动添加可能遗漏 (60% fail)
- **使用Flask-CORS但项目是Starlette** — Flask-CORS不兼容Starlette (50% fail)