Secure file uploads in web applications

The upload button is only the beginning

Files may be stored, renamed, scanned, extracted, previewed and shared. Each step can introduce risk. An apparently harmless extension does not guarantee the content is safe, and a browser-side file picker restriction can be bypassed.

Accept only what the workflow needs

Define permitted file types and enforce size limits on the server or storage service. Check extensions and content using appropriate validation rather than trusting the supplied Content-Type alone. Generate storage names instead of using unchecked filenames as paths.

Keep confidential uploads private

A report or client document should not become public simply because someone uploaded it. Separate permission to upload from permission to publish or share.

• Check ownership before upload and download
• Use storage outside executable application paths
• Limit upload URLs to the intended object and purpose
• Use short-lived download access only after permission checks
• Define retention and deletion rules

Assess previews and processing

Image converters, document parsers and archive extraction need their own limits and patch process. Consider malware scanning or content disarm and reconstruction where suitable. Decide how suspicious files are quarantined and avoid treating a scan result as a guarantee of safety.

Safe assessment examples

With written permission, use harmless test files to check rejected types, oversized uploads and access from another account. Avoid destructive payloads on a production service. Verify how a user sees processing failures and whether rejected files remain accessible.