# 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: 高 頻繁に悪用される
- Impact: ハイ 3 件の重大度ハイのルール
- Prevention: 文書化済み 3 件の修正例

**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

1 件の Shoulder 検出ルールに基づく Unrestricted File Upload の予防策。

### 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

- 未承認コードの実行
- アプリケーションデータの読み取り
- アプリケーションデータの変更

## Mitigations

- ファイルタイプは拡張子だけでなくサーバー側で検証する
- アップロードされたファイルは Web ルートの外に保存する
- 許可するファイルタイプには許可リストを使用する
- アップロードされたファイルはリネームして実行を防ぐ

## 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);
    }
  }
});