A File Upload Vulnerability occurs when an application allows users to upload files to the server without proper validation and sanitization. This can lead to various security issues, such as malware infections, unauthorized access, and data breaches.

Backend Process of File Upload Vulnerability

1. User Input: The attacker uploads a malicious file to the server.
2. Server Processing: The server processes the uploaded file and stores it on the server.
3. File Execution: The server processes or executes the uploaded file, which can lead to malicious activities.

Example of Bad Code

Consider a vulnerable file upload endpoint in a Flask application:

from flask import Flask, request, send_from_directory
import os

app = Flask(__name__)
UPLOAD_FOLDER = '/var/www/uploads'

@app.route('/upload', methods=['POST'])
def upload_file():
    if 'file' not in request.files:
        return 'No file part', 400
    file = request.files['file']
    if file.filename == '':
        return 'No selected file', 400
    if file:
        file.save(os.path.join(UPLOAD_FOLDER, file.filename))
        return 'File uploaded successfully', 200

if __name__ == '__main__':
    app.run(debug=True)

In this example, the server saves the uploaded file directly to the /var/www/uploads directory without any validation or sanitization, making it vulnerable to malicious file uploads.

Prevention Measures

1. Validate File Types

Ensure that only allowed file types can be uploaded. This can be done by checking the file extension and MIME type.

from flask import Flask, request, send_from_directory
import os

app = Flask(__name__)
UPLOAD_FOLDER = '/var/www/uploads'
ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'gif'}

def allowed_file(filename):
    return '.' in filename and \\
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

@app.route('/upload', methods=['POST'])
def upload_file():
    if 'file' not in request.files:
        return 'No file part', 400
    file = request.files['file']
    if file.filename == '':
        return 'No selected file', 400
    if file and allowed_file(file.filename):
        file.save(os.path.join(UPLOAD_FOLDER, file.filename))
        return 'File uploaded successfully', 200
    return 'File type not allowed', 400

if __name__ == '__main__':
    app.run(debug=True)

2. Sanitize File Names

Sanitize file names to prevent directory traversal attacks and other malicious activities.

import re

def sanitize_filename(filename):
    return re.sub(r'[^\\w\\.-]', '', filename)# Remove any non-alphanumeric characters

@app.route('/upload', methods=['POST'])
def upload_file():
    if 'file' not in request.files:
        return 'No file part', 400
    file = request.files['file']
    if file.filename == '':
        return 'No selected file', 400
    if file and allowed_file(file.filename):
        sanitized_filename = sanitize_filename(file.filename)
        file.save(os.path.join(UPLOAD_FOLDER, sanitized_filename))
        return 'File uploaded successfully', 200
    return 'File type not allowed', 400

3. Store Files in Secure Directories

Store uploaded files in a directory outside the web root to prevent direct access via### Prevention Measures (Continued)

4. Antivirus Scanning

Integrate antivirus scanning to check uploaded files for malicious content.