Comment 0 for bug 909207

The EC2 API was updated in response to bug 802046 to return the security groups associated with an instance. Unfortunately, the Nova API didn't get that change, and so it is currently not possible to retrieve just the security groups associated with an instance.

https://bugs.launchpad.net/nova/+bug/802046
https://github.com/openstack/nova/blob/master/nova/api/ec2/cloud.py#L1101

The Nova API should return relevant security groups in the details view.
https://github.com/openstack/nova/blob/master/nova/api/openstack/v2/views/servers.py#L85