CVE-2026-41235: Froxlor has an authorization bypass in FTP shell assignment via missing server-side `available_shells` enforcement

Froxlor 2.3.6 lets administrators configure system.available_shells as the approved shell list that customers may assign to FTP users. However, the server-side FTP account handlers do not enforce that whitelist when processing add or edit requests.

As a result, an authenticated customer with shell delegation enabled can submit an arbitrary shell such as /bin/bash even when the panel UI only offers more restricted choices. In deployments that use the default nssextrausers integration, the attacker-controlled shell is then propagated into the system account database, leading to real host shell access.