Setting folder_id through update() is the same privileged reparent as
move() and bulkUpdate(), but only those two verified the target folder
was inside the caller's library (StaffLibraryScope::folders). update()
validated it only for existence, so a client-scoped staff member could
reparent an in-scope file into a folder shared with a client they are
not assigned to -- which File::scopeVisibleToClient then exposes to that
client, sidestepping the boundary the sharing endpoints enforce
(guardAssignable), and likewise into a public folder without
upload_public.
Apply the same scope->folders()->findOrFail() guard on both the web and
API update(), but only when folder_id actually changes, so re-saving a
file that already sits in an out-of-scope folder (reachable via a direct
client share) still works.
denkfabrik-li