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An “Arbitrary File Overwrite” vulnerability in an npm package refers to a security flaw where an attacker can manipulate the application to overwrite files on the filesystem with arbitrary content. This can be particularly dangerous, as it might allow the attacker to:

  1. Overwrite Critical Files: An attacker could overwrite configuration files, binaries, or scripts that are crucial to the application or system, potentially leading to a denial of service (DoS), privilege escalation, or remote code execution (RCE).
  2. Inject Malicious Code: If the attacker can control the content that overwrites files, they might be able to inject malicious code that the application or system later executes.
  3. Bypass Security Controls: Overwriting certain files might allow an attacker to bypass security controls, such as access restrictions, or to create backdoors.

How the Vulnerability Typically Works

he vulnerability often arises due to insufficient validation of user input that specifies file paths. For instance, if an application allows users to specify a filename or path for file creation or update, and that input is not properly sanitized, an attacker could use path traversal techniques (e.g., using ../ to navigate to parent directories) to write files outside the intended directory.

Common Causes

Improper Input Validation: The application does not properly validate or sanitize user input that influences file paths.

Directory Traversal: The application allows attackers to traverse the directory structure to access unintended files.

Insecure File Permissions: The application writes files with permissions that allow unauthorized users to modify them.

Example Scenario

Consider an npm package that allows users to upload files to a server. If the package allows users to specify the file path and does not validate it properly, an attacker might craft a request that specifies a path like ../../../../../etc/passwd, resulting in the overwriting of critical system files.

Mitigation

Input Validation: Ensure that all user-supplied input is properly validated and sanitized.

Use Secure APIs: Avoid using APIs that allow arbitrary file paths unless necessary. Prefer APIs that enforce constraints on file locations.

Limit Permissions: Ensure that the application only has the minimum necessary permissions to write to specific directories or files.

Vulnerable code

const express = require('express');
const multer = require('multer');
const path = require('path');
const fs = require('fs');

const app = express();
const port = 3000;

// Configure multer to store uploaded files in the 'uploads' directory
const storage = multer.diskStorage({
    destination: (req, file, cb) => {
        cb(null, 'uploads/');
    },
    filename: (req, file, cb) => {
        // Vulnerability: No proper validation of the filename, allowing path traversal
        const filename = req.body.filename || file.originalname;
        cb(null, filename);
    }
});

const upload = multer({ storage: storage });

// Route to handle file uploads
app.post('/upload', upload.single('file'), (req, res) => {
    res.send('File uploaded successfully');
});

// Serve the form for uploading files
app.get('/', (req, res) => {
    res.sendFile(path.join(__dirname, 'index.html'));
});

// Ensure the uploads directory exists
if (!fs.existsSync('uploads')) {
    fs.mkdirSync('uploads');
}

app.listen(port, () => {
    console.log(`Server running on http://localhost:${port}`);
});

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>File Upload Demo</title>
</head>
<body>
    <h1>Upload a File</h1>
    <form action="/upload" method="post" enctype="multipart/form-data">
        <input type="file" name="file" required><br><br>
        <label for="filename">Save as filename (optional):</label>
        <input type="text" name="filename" id="filename"><br><br>
        <button type="submit">Upload</button>
    </form>
</body>
</html>

Go to http://localhost:3000 in your browser, and you’ll see a file upload form.

Exploit the Vulnerability

  • Upload a File with Malicious Filename: In the file upload form, choose a file to upload, and in the “Save as filename” field, enter a malicious path like ../../server.js. This path is an attempt to overwrite the server’s main script file (index.js).
  • Check the Result: After uploading the file, check the index.js file. If the vulnerability is successfully exploited, the file will be overwritten with the content of the uploaded file.

Mitigation

To mitigate this vulnerability, you should validate and sanitize the filename input to prevent path traversal. For example, you could use path.basename() to ensure that only the filename (without any directory traversal) is used:

const safeFilename = path.basename(req.body.filename || file.originalname);
cb(null, safeFilename);

This change would prevent an attacker from overwriting arbitrary files by restricting file writes to the uploads directory only.

By understanding the mechanics of this vulnerability, developers can better safeguard their applications against such threats.

Categorized in:

Secure Code Review