The vulnerability exists due to insufficient sanitization of user-supplied input passed to the 'g_admin_rep' parameter in 'admin/admin_module.php' script. This can be exploited to include arbitrary files from local resources or remote resources by using a relative path in the 'g_admin_rep' parameter. Additionally, an attacker can exploit this vulnerability to include files with multiple extensions, which can be used to execute arbitrary code on the vulnerable system.