The default hub image includes some useful debugging tools. You can use the slim version of image to minimise your exposure to vulnerabilities in those optional tools.
hub:
image:
# The slim variant excludes a few non-essential packages that are useful
# when debugging something from the hub pod. To use it, apply this
# configuration.
#
name: quay.io/jupyterhub/k8s-hub-slim
z2jh's security documentation recommends using the slim version of the image for the hub pod. https://z2jh.jupyter.org/en/latest/administrator/security.html#minimal-hub-image