JupyterHub 4.1.0 is available on PyPI and conda-forge and includes fixes for security vulnerabilities, as well as backporting several bugfixes and small enhancements from 5.0 development.
A security advisory will be published next week at http://github.com/jupyterhub/jupyterhub/security/advisories/GHSA-7r3h-4ph8-w38g
All JupyterHub deployments are encouraged to upgrade.
Thanks to everyone who contributed!