It was found that a non privileged tenant can add a router interface to a shared / external network's subnet with an IP address outside the subnet's allocation pool. This can result in a Denial of Service.
Upstream issue:
https://bugs.launchpad.net/neutron/+bug/1757482
Upstream patch:
https://git.openstack.org/cgit/openstack/neutron/commit/?id=54aa6e81cb17b33ce4d5d469cc11dec2869c762d