fix: resolve RCE vulnerability via unrestricted file upload (#1850)

[atul-upadhyay-7]

Summary

This PR fixes the Remote Code Execution (RCE) via Unrestricted File Upload vulnerability described in #1850.

Problem

The profile photo upload endpoint stored files in a local server directory served via express.static. Attackers could upload malicious scripts leading to full server compromise.

Changes

Profile Photo Upload (backend/routes/users.js)

• Moved uploads from local storage to Supabase Storage (sandboxed bucket)
• Added strict MIME type allow-listing (JPEG, PNG, WebP, GIF only)
• Added magic byte validation using file-type library
• Added rate limiting (10 uploads per hour per user)
• Server-generated storage paths (no client path injection)
• Reduced max file size from 5MB to 2MB
• Reused getSupabaseAdmin from utils/supabase.js

Testing

• TypeScript compilation: Clean
• Security tests: 5/5 passed
• All unit tests: 37/37 passed

Fixes #1850

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📥 Commits

Reviewing files that changed from the base of the PR and between 3565841 and 2ab0102.

📒 Files selected for processing (1)

• backend/routes/users.js

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