How to Securely Implement File Uploads in a Flask Application Without Exposing Sensitive Data?
I'm currently working on a Flask application where users can upload files, but I'm concerned about the security implications. I've read that allowing file uploads can lead to vulnerabilities such as arbitrary file execution or denial of service if not handled correctly. I've implemented a basic file upload feature using Flask and Flask-WTF, but I'm not sure if I’m taking the necessary precautions. Currently, my code looks like this: ```python from flask import Flask, request, redirect, url_for, flash from flask_wtf import FlaskForm from wtforms import FileField, SubmitField from werkzeug.utils import secure_filename import os app = Flask(__name__) app.config['SECRET_KEY'] = 'mysecret' app.config['UPLOAD_FOLDER'] = 'uploads/' app.config['ALLOWED_EXTENSIONS'] = {'txt', 'pdf', 'png', 'jpg', 'jpeg', 'gif'} class UploadForm(FlaskForm): file = FileField('File') submit = SubmitField('Upload') def allowed_file(filename): return '.' in filename and \ filename.rsplit('.', 1)[1].lower() in app.config['ALLOWED_EXTENSIONS'] @app.route('/upload', methods=['GET', 'POST']) def upload_file(): form = UploadForm() if form.validate_on_submit(): file = form.file.data if file and allowed_file(file.filename): filename = secure_filename(file.filename) file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename)) flash('File successfully uploaded') return redirect(url_for('upload_file')) return render_template('upload.html', form=form) ``` While this seems to be working, I haven't implemented any file type validation beyond the extension checks. Additionally, I don't have any measures in place to prevent users from uploading excessively large files, which could lead to denial of service. I tried adding a limit on the size of uploads in Flask, but I’m not sure if that’s sufficient. I also read about scanning files for malware before saving them, which sounds like a good idea, but I'm unsure how to integrate that into my workflow. My current Flask version is 2.1.1. Can anyone provide guidance on how to securely handle file uploads in Flask, including the best practices for size limits, file type validation, and possibly malware scanning? Are there any pitfalls I should be aware of, or libraries that can help with these tasks?