A .NET developer has outlined a practical, layered approach to securing file uploads, warning that simple implementations often fail to prevent malicious uploads. The core strategy involves three sequential checks: extension allowlisting, file size limits, and signature validation using 'magic numbers'. This baseline significantly reduces risk compared to blindly accepting any file.

Extension checks are the first, cheapest defense but are easily bypassed by renaming files. Adding a hard size limit prevents resource abuse and denial-of-service attacks. The most critical layer inspects the file's header bytes to verify its true format, effectively countering extension spoofing where a malicious executable is renamed with a .jpg extension.

The author provides concrete C# code for .NET 10, covering everything from a configurable validator class to handling PDF's unique signature placement. While this layered method isn't bulletproof, it establishes a solid security baseline. For higher assurance, experts recommend adding format conformance checks and malware scanning before files are persisted.

This guidance aligns with broader OWASP recommendations, emphasizing defense-in-depth. Developers are urged to move beyond the 'it works' mentality and treat file uploads as a critical attack surface. The approach offers a clear, implementable path to hardening a common feature against trivial bypasses and common exploits.