Secure File Uploads and Handling Malicious Files

Uploading files is common in web apps, whether for profile pictures, documents, or media. But if not handled properly, file uploads can create serious security risks.

Without the right safeguards, attackers can upload malicious scripts, overwrite critical system files, or exploit file handling weaknesses to gain unauthorized access, potentially compromising your entire application.

In this lesson, we’ll learn how to securely handle file uploads in Express.js using Multer. We’ll see how to enforce file type validation, size limits, and path traversal protection to protect our app from common threats.

Understanding file upload security risks

Allowing file uploads without proper validation and security controls makes your application vulnerable to exploits. Here are some of the most common risks:

• Arbitrary file uploads: Attackers may upload executable scripts (e.g., .exe, .sh, .php) that could run on the server.

• Path traversal attacks: Malicious users might manipulate file paths to overwrite or access restricted system files.

• Denial of service (DoS) attacks: Uploading excessively large files can overwhelm server resources, leading to crashes or downtime.

Fortunately, these risks are manageable. To keep our application safe, let’s look at ways to enforce strict validation rules, limit file sizes, and secure file storage.

Setting up file uploads in Express

Express doesn’t handle file uploads out of the box, so we’ll use Multer, a middleware that simplifies file processing.