# Unrestricted Upload of File with Dangerous Type (CWE-434)

The product allows the upload of files without properly validating the file type, which can lead to execution of malicious code.

**Stack:** JavaScript

- Prevalence: 높음 자주 악용됨
- Impact: 높음 3개의 높은 심각도 규칙
- Prevention: 문서화됨 3개의 수정 예시

**OWASP:** Broken Access Control (A01:2021-Broken Access Control) - #1

## Description

When users can upload files without restriction, attackers may upload executable files, scripts, or other dangerous content that can be executed by the server or other users.

## Prevention

1개의 Shoulder 탐지 규칙을 기반으로 한 Unrestricted File Upload 예방 전략.

### JavaScript

Add fileFilter to multer to validate uploaded file types

## Warning Signs

- [HIGH] Multer middleware at ... lacks fileFilter validation
- [HIGH] multer file upload middleware used without proper fileFilter validation

## Consequences

- 승인되지 않은 코드 실행
- 애플리케이션 데이터 읽기
- 애플리케이션 데이터 수정

## Mitigations

- 파일 유형은 확장자만이 아니라 서버에서 검증하세요
- 업로드된 파일은 웹 루트 외부에 저장하세요
- 허용되는 파일 유형에 대해 허용 목록을 사용하세요
- 업로드된 파일은 실행을 방지하기 위해 이름을 바꾸세요

## Detection

- Total rules: 3
- Languages: go, javascript, typescript, python

## Rules by Language

### Javascript (1 rules)

- **Unrestricted File Upload** [HIGH]: Detects multer file upload middleware used without proper fileFilter validation.
Without fileFilter, attackers can upload any file type including executables,
web shells, and other malicious files.
  - Remediation: Add fileFilter to validate uploaded file types:

const upload = multer({
  fileFilter: (req, file, cb) => {
    const allowed = ['image/jpeg', 'image/png'];
    if (allowed.includes(file.mimetype)) {
      cb(null, true);
    } else {
      cb(new Error('Invalid file type'), false);
    }
  }
});

### Typescript (1 rules)

- **Unrestricted File Upload** [HIGH]: Detects multer file upload middleware used without proper fileFilter validation.
Without fileFilter, attackers can upload any file type including executables,
web shells, and other malicious files.
  - Remediation: Add fileFilter to validate uploaded file types:

const upload = multer({
  fileFilter: (req, file, cb) => {
    const allowed = ['image/jpeg', 'image/png'];
    if (allowed.includes(file.mimetype)) {
      cb(null, true);
    } else {
      cb(new Error('Invalid file type'), false);
    }
  }
});