Uploads cross a trust boundary
A file-upload button looks like a small feature, but it lets an external user place complex data inside your system. Attackers can disguise executable content, exploit parsers, overwrite paths, exhaust storage, or upload sensitive material and share it incorrectly. The correct mindset is simple: every uploaded file is untrusted, including its name, extension, metadata, and claimed content type.
Validate in layers
Use allowlists for required formats, inspect file signatures instead of trusting extensions, enforce size limits, generate server-side names, and reject path characters. Images and documents may need decoding or conversion in an isolated process because parsers have vulnerabilities too. Malware scanning helps, but it is one control among many and should not be treated as proof that a file is safe.
Store away from execution
Keep uploads outside the application web root and prevent them from being executed. Object storage with short-lived signed URLs can separate application logic from file delivery, but authorization must be checked before a URL is issued. Encrypt sensitive objects, log access, define retention, and avoid public buckets unless public access is the explicit product requirement.
Plan the full lifecycle
Security continues after upload. Files are previewed, transformed, downloaded, shared, archived, and deleted. Each step introduces another permission and processing boundary. A secure design documents the entire lifecycle and tests how the system behaves when validation, scanning, or transformation fails.