Treat every uploaded file as hostile until proven otherwise. For creator and gaming platforms that means allowlisting file types, checking the real content rather than the claimed type, renaming files on the server, capping sizes, storing files away from your application, and re-encoding images and video before anyone else sees them.
Why uploads are the soft spot
Creators upload thumbnails, banners, clips, audio, maps and add ons. Each upload path takes bytes from an untrusted person and later serves them to other people. A mistake can mean stored scripts in a profile image, a file that executes on the server, or a flood of large files that fills storage.
The OWASP File Upload Cheat Sheet is a good baseline, and its main message is defense in depth: no single check is enough.
The checklist
1. Allowlist the extensions you need. If creators upload PNG, JPEG and MP4, accept those three. Validate input before looking at the extension.
2. Do not trust the Content-Type header. A client can send any value. Inspect the file to confirm what it really is.
3. Generate filenames on the server. User chosen names can carry path tricks and odd characters. If you must keep the display name, store it as data and limit length and characters.
4. Limit sizes. Cap upload size, and for archives consider the size after decompression. A small zip that expands enormously is a classic way to fill disks.
5. Require authentication and permission. Only let accounts that are allowed to upload do so, and rate limit them.
6. Isolate storage. Keep files on a separate host or outside the webroot, and serve them through a handler that maps IDs to stored objects. Serving from a different domain than your main site keeps stored content from running with your site's privileges.
7. Scan and sanitize. Use malware scanning or sandboxing where it makes sense. Re-encode images so metadata and appended payloads are stripped.
8. Set least privilege permissions on the storage location, and disable per directory configuration overrides on upload directories.
9. Keep media libraries current. Image and video processing code parses complicated formats and has a history of vulnerabilities, so update it regularly.
10. Protect the upload endpoint from CSRF.
Mods and executable uploads
If your platform lets creators upload things that run, such as scripts, plugins or maps with logic, the rules are stricter. Treat the content as code from a stranger. Run it in a sandbox with limited permissions, review before publishing where possible, and give players a clear way to report problems. Another post in this series covers mod safety in more depth.
Costs and abuse
Uploads also cost money. Large files, thumbnail generation and transcoding use memory, CPU and storage. OWASP's resource consumption guidance recommends maximum sizes for all inputs and limits on how often a client can repeat an operation, plus spending limits or billing alerts on paid services.
Where to start
- Pick the top upload path by volume and run the checklist against it.
- Move served uploads to their own domain if they share one with your app.
- Add size and rate limits.
- Add a takedown process and a log of who uploaded what.
Safeguard's dependency scanning can flag known vulnerable versions of the image and media libraries in your upload pipeline. The free scan is at https://gold.safeguard.sh/scan.
Quick check
Is checking the file extension enough?
No. Extensions and Content-Type headers can be faked. Verify the real file type and re-encode media where you can.
Sources
Last reviewed: 19 October 2026.
