Icinga DB Web hidden/protected custom variables are prone to filter enumeration

Icinga DB Web provides a graphical interface for Icinga monitoring. Before 1.1.4 and 1.2.3, an authorized user with access to Icinga DB Web, can use a custom variable in a filter that is either protected by icingadb/protect/variables or hidden by icingadb/denylist/variables, to guess values assigned to it. Versions 1.1.4 and 1.2.3 respond with an error if such a custom variable is used.