/usr/lib/firebird/2.1/security2.fdb is in firebird2.1-server-common 2.1.4.18393-0.ds2-6build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /var/lib/firebird/2.1/system/security.fdb
/var/lib/firebird/2.1/system/security.fdb