Securing File Uploads in AWS Workflows

A candidate interviewing for L5 @ Google was asked to break down the design of Google Drive. Another candidate who was interviewing for the role of SDE-III @ Amazon, was asked another with a file upload system question. I’ve faced these too. System design rounds love “simple” file upload questions until you add one layer of complexity: – Add virus scanning? Whole new security headache. –  Add multi-region storage? Now you’re fighting replication and consistency. –  Add instant previews or image compression? Welcome to async pipelines and job queues. Here’s my personal checklist of 15 things you must get right when building file upload systems: 1. Never upload directly to your backend → Use presigned URLs so files go straight to S3, GCS, or your object storage, offloading bandwidth and freeing up backend resources. 2. Validate file type by content, not extension → Don't trust “.jpg” or “.pdf”, read the file’s magic bytes or headers to catch disguised executables or corrupted files. 3. Set strict file size limits (early) → Prevent memory blowups, denial of service, and accidental 50GB “cat video” uploads. 4. Multipart/chunked uploads for large files → Upload big files in chunks, so you can retry failed pieces, resume partial uploads, and never lose user progress. 5. Resumable uploads matter → User’s WiFi dies? Don’t make them start from scratch. Store upload progress, support resume tokens. 6. Async virus scanning before “marking ready” → Queue the scan; don’t let user-access or public-sharing happen until the result is clean. 7. Never trust user-supplied metadata → Recompute MIME types, image dimensions, video duration, etc., server-side, attackers will fake everything. 8. Expire unused presigned URLs fast → Every upload/download link should expire in minutes, not days. Stops replay attacks and stale-link leaks. 9. Background post-processing → Thumbnails, transcoding, compression, indexing, all should be async jobs, not blocking the upload. 10. Signed download URLs only  → Never expose raw S3 or GCS paths. Every download link should be time-bound and permission-checked. 11. Enforce per-user and per-IP rate limits → Throttle abusive clients, prevent brute force, and stop sudden spikes from melting your backend. 12. Encrypt files at rest (and in transit)  → Use server-side encryption (SSE) on S3 or GCS, plus HTTPS/TLS for every file transfer. 13. Version every upload → Store new files with unique IDs or version suffixes, never overwrite by default. Enables “undo” and rollback, and prevents race conditions. Continued ↓ – P.S: Say Hi on Twitter: https://lnkd.in/g9H82Q98 — P.P.S: Feel free to reach out to me if you're preparing for a switch, want to chat about interview preparation, or how to move to the next level in your career: https://lnkd.in/guttEuU7

File uploads sound easy - until you actually build them. Here are 10 things you must get right 👇 1 Never upload directly to your backend - use presigned URLs so files go straight to S3/GCS. 2 Validate file type by content, not just extension - .exe can easily be renamed to .jpg. 3. Set file size limits early - avoid memory blowups and bandwidth abuse. 4. Use multipart/chunked uploads for large files - retry failed chunks, not the whole file. 5. Add resumable uploads - let users pick up after a failed connection. 6. Always virus-scan uploads asynchronously before marking them “ready.” 7. Don’t trust user metadata - extract real MIME/type/dimensions server-side. 8. Expire unused presigned URLs - prevents old links from being reused maliciously. 9. Post-process asynchronously - thumbnails, compression, and indexing should happen in background jobs. 10. Secure access with signed download URLs - never expose raw S3 paths publicly.

Four years ago, I built a product for a client that almost crashed from its own success. We launched fast. MVP was live in three weeks. Users loved it until they started uploading massive PDFs, images, and videos, despite having a max upload size of 100MB per file Suddenly: ❌ Uploads started timing out ❌ Server CPU spiked ❌ Storage filled up ❌ And complaints rolled in daily It wasn’t a code bug though, it was a scaling problem. And it taught me a painful but crucial lesson: 👉 If your SaaS involves file uploads, and you don’t architect for scale early on, you’re building a ticking time bomb. That aside, here’s how I scale file upload systems to handle millions of uploads today: ✅ 1. Object Storage First Never store files on your app server. Ever. I go straight to Amazon S3, Cloudflare R2 or Backblaze B2 Reason for that is - Virtually infinite storage - Built-in redundancy - Compatibility with CDNs Easy lifecycle & permission management ✅ 2. Use Resumable Uploads Big files + spotty connections = user frustration. That’s why I implement chunked + resumable uploads using Tus.io. There are more options but DYOR This means if your internet drops, you don’t have to start over. ✅ 3. Presigned URLs for Direct Uploads Let the client talk to the storage directly, not your backend. Typical flow: 1. Client: “I want to upload.” 2. Server: “Here’s a secure presigned URL.” 3. Client uploads directly to storage. This results in less backend load, faster upload speeds and a much cleaner architecture ✅ 4. Process in the Background Once uploaded, files usually need some love: 🪏 Compress images 🪏 Transcode video 🪏 Analyze or extract metadata I use: - Background queues (Inngest, RabbitMQ) - Workers (Node, Python, AWS Lambda) - ffmpeg / Exif tools N.B: Don’t block the user, process it async and notify when done. ✅ 5. Secure Your Pipeline - Limit file types & extensions - Enforce file size limits - Use virus/malware scanning (I use ClamAV) - Validate uploads on the backend As for that client project I rebuilt it using this system. It now processes hundreds of uploads a day with zero downtime. The takeaway? You don’t need a massive DevOps team to scale smart. You need architecture that makes sense for what you’re building. Most SaaS founders and CTOs are so busy shipping that they don’t think about this until it’s too late. If you’re building or rebuilding a SaaS and plan to handle user uploads at any scale, build like you already have 10,000 users. That’s how we build at Sqaleup Inc. Let’s chat if you want this kind of bulletproof upload architecture in your product 🚀