Secure File Upload: Validation, Storage, Malicious Files
How to validate file uploads safely: MIME type checking, magic bytes, size limits, virus scanning, and never executing uploaded files.
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Tags: security, web, file-upload
Secure File Upload: Validation, Storage, and Execution Prevention File upload functionality is one of the highest-risk features in web applications. A malicious file uploaded to your server can lead to cross-site scripting (serving HTML/JS as a "file"), remote code execution (uploading a PHP/Python script), path traversal attacks (overwriting system files), or stored malware distribution. This guide covers validation, storage, and the architecture that prevents uploaded files from ever executing. The Threat Model An attacker who can upload arbitrary files to your application wants to: Execute code: Upload , then access Serve malicious content: Upload an HTML file with JavaScript, get XSS Overwrite files: Use path traversal in filename: Host malware: Use your server to distribute malicious…
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