Layered file upload security in .NET: Extension allowlisting, size limits, and magic number validation
By
Christian Haase
Summary
A practical guide to implementing layered file upload security in .NET applications. The article covers three essential validation layers: extension allowlisting (whitelisting approved extensions rather than blocking dangerous ones), size limits to prevent denial-of-service attacks, and signature/magic number validation to verify file types by inspecting binary headers rather than trusting file extensions. It also discusses extension spoofing attacks where malicious executables are renamed to look like image files, and provides a baseline approach with suggestions for additional security measures like antivirus scanning and content-disposition validation for stronger assurance.
Source
bskyLayered file upload security in .NET: Extension allowlisting, size limits, and magic number validationdev.toKey quotes
· 3 pulledI thought file uploads were simple… until I saw how often they aren't.
At some point, most of us build 'a simple file upload'. And most of the time, it works.
Until you realize that in a lot of applications, you can rename malicious.exe to holiday-photo.jpg and upload it without so much as a raised eyebrow.
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