Common Flaws in File Upload Validation
Many web frameworks rely solely on client-supplied Content-Type headers or file extensions. Attackers easily bypass these by renaming shells to avatar.php.png or prepending GIF89a headers.
Robust defenses require reading server-side magic bytes, re-encoding the image stream into a fresh buffer, and serving uploads from isolated, cookie-less storage subdomains.
Key Investigative Takeaway
Re-encoding an image into WebP or JPEG on the server destroys all appended web shells and script injection vectors.