Security for File Sharing Apps
File sharing apps accept, store, and serve files from users. Each stage — upload, storage, and download — introduces security risks from malware delivery to unauthorized access.
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Why Security Matters for File Sharing Applications
File uploads are one of the most dangerous features to implement. Malicious files can exploit server-side processing, deliver malware to other users, or be used to escalate privileges. AI-generated code typically accepts any file without validation. Access control on downloads is equally critical — shared links that don't expire, missing authorization checks, and predictable file URLs all lead to unauthorized access.
Security Risks
Malicious file upload
criticalUsers uploading executable files, scripts, or files exploiting processing vulnerabilities.
Mitigation
Validate file types by magic bytes. Reject executables. Re-encode media files. Scan with antivirus where possible.
Unauthorized file access
highFiles accessible to anyone with the URL, or predictable file paths allowing enumeration.
Mitigation
Use signed URLs with expiry for file access. Verify authorization before generating download links. Use random file IDs, not sequential.
Storage exhaustion
mediumUsers uploading massive files to exhaust storage quotas and increase costs.
Mitigation
Enforce per-user and per-file size limits. Monitor total storage usage and implement quotas.
Security Checklist
Check magic bytes, not just extensions. Maintain an allowlist of accepted types.
Verify the requester has permission to access each file before serving it.
Enforce per-file and per-user storage limits.
Generate time-limited signed URLs for file downloads instead of permanent links.
Host uploaded files on a different domain to prevent cookie theft.
Force download for non-image files to prevent browser execution.
Real-World Scenario
A developer builds a document sharing app using Lovable with Supabase Storage. Files are stored in a public bucket for simplicity. An attacker discovers the predictable URL pattern (project-id.supabase.co/storage/v1/object/public/files/user123/document.pdf) and enumerates user IDs to download private documents from hundreds of users.
Frequently Asked Questions
Should I use public or private Supabase Storage buckets?
Always use private buckets for user files. Generate signed URLs server-side after verifying the user has permission to access the file. Public buckets should only be used for truly public assets like logos.
How do I prevent users from uploading malware?
Validate file types by reading magic bytes. Use an allowlist of accepted types. For extra safety, scan files with ClamAV or a cloud antivirus API. Re-encode images and documents when possible to strip embedded payloads.
What's the risk of serving files from the same domain?
If an uploaded HTML file is served from your main domain, it can access your cookies and execute scripts in your origin context. Serving from a separate domain (e.g., files.example.com) isolates uploaded content from your application.
Security for Other Use Cases
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