> One of our interesting findings was the iptables rules, since when you enable Private IP access (Which cannot be disabled afterwards), access to the MySQL port is not only added for the IP addresses of the specified VPC network, but instead added for the full 10.0.0.0/8 IP range, which includes other Cloud SQL instances.

> Therefore, if a customer ever enabled Private IP access to their instance, they could be targeted by an attacker-controlled Cloud SQL instance. This could go wrong very quickly if the customer solely relied on the instance being isolated from the external world, and didn’t protect it with a proper password.

I'm not convinced by this, I'm not sure it is vulnerable in the way the author is suggesting "they could be targeted by an attacker-controlled Cloud SQL instance".

First of all, GCE has firewall rules outside of iptables. But the main thing is that the way Cloud SQL does Private IP is via VPC peering. Google creates a VPC on their side, runs MySQL in it, and peers that VPC with your VPC. You actually tell Google what CIDR range to use in the their VPC (the Cloud SQL VPC).

I don't think is it fair to assume that all customers are in the same VPC, and same subnets, with routes between them, and no GCE firewall rules blocking them.