"Default web page" vulnerability is useful to detect unused Web server that are active on a server. Very often, stopping the Web server solves a lot of other vulnerabilities, related to the (useless) Web site.
But very often, there's a necessary Web site, running properly, whose "default web page" is either a redirection or an authentication page.
The flaw is due to misconfiguration of Server, which allows to
access default pages when the server is not used.
Successful exploitation will allow remote attackers to obtain sensitive
information that could aid in further attacks.