Security updates are offered for the 3 most recent minor CKAN releases. They are included in patch releases, but not acknowledged in the release announcement or CHANGELOG.rst, hence the advice to always run the latest patch releases.
For more about CKAN releases see: http://docs.ckan.org/en/latest/maintaining/upgrading/index.html#ckan-releases
If you find a potential security vulnerability please email security@ckan.org, rather than creating a public issue on GitHub.