It's also not the first dashboard with this situation.
phpMyAdmin or what it's called, back in the days of LAMP stacks that was deployed almost everywhere without much security around. Not sure if it also had sql injection bugs etc, but just the low amount of security considerations most people gave this direct access to the database was probably enough to hack into most servers of that time.
Either don't expose the dashboard, or explicitly give it a service account with zero access. I think it should be well-known by now that anything you run in-cluster gets a builtin service account token, defaulting to the default service account for that namespace.
I ran my k8s dashboard behind a ClusterIP service - You could get to it through the kube-proxy, but you already had to be auth'd to the cluster, first. This was fine for a cluster small enough that any user already had admin, though we did worry a little bit that someone would compromise a pod, scan for a bit, and escalate through the dashboard.
I am not surprised, in the general sense that someone has found a security bug in a large and complex piece of software.
This is basically another good example of why your control plane should be only accessible through a vpn/bastion.