/usr/share/php/Icinga/Authentication/User/ is in php-icinga 2.1.0-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/php/Icinga/Authentication/User/DbUserBackend.php
/usr/share/php/Icinga/Authentication/User/ExternalBackend.php
/usr/share/php/Icinga/Authentication/User/LdapUserBackend.php
/usr/share/php/Icinga/Authentication/User/UserBackend.php
/usr/share/php/Icinga/Authentication/User/UserBackendInterface.php
/usr/share/php/Icinga/Authentication/UserGroup/DbUserGroupBackend.php
/usr/share/php/Icinga/Authentication/UserGroup/IniUserGroupBackend.php
/usr/share/php/Icinga/Authentication/UserGroup/LdapUserGroupBackend.php
/usr/share/php/Icinga/Authentication/UserGroup/UserGroupBackend.php
/usr/share/php/Icinga/Authentication/UserGroup/UserGroupBackendInterface.php