Unrestricted File Upload via MIME Type Spoofing Leads to Stored XSS

A file upload validation bypass allows any authenticated user to upload arbitrary HTML, SVG, or other executable file types to the server by spoofing the Content-Type header. The uploaded files are then served by nginx with a Content-Type derived from their original extension (text/html, image/svg+xml), enabling Stored Cross-Site Scripting (XSS) in the context of the application's origin. This can lead to session riding, account takeover, and full compromise of other users' accounts.