Oqtane Framework: authenticated users can make server fetch internal URLs

Oqtane Framework before 10.3.0 contains a server-side request forgery vulnerability in the UploadFile endpoint of FileController.cs that allows authenticated registered users to make the server fetch arbitrary URLs by supplying a caller-controlled url parameter to HttpClient.GetStreamAsync without validating the scheme, host, or IP address. Attackers can request internal resources such as cloud instance metadata services or localhost-bound management interfaces, save the fetched response as a file in their own user folder, and retrieve it via the download endpoint to exfiltrate sensitive internal data or enumerate internal network hosts and ports.