There is an updated Samba package for bionic in the security team PPA here:
It contains fixes for CVE-2016-2124, CVE-2020-25717, CVE-2020-25722 and CVE-2021-3671 which appear to be the most severe issues from the latest round of updates.
If someone is running Samba as an AD-DC, could you please test them and comment in the following bug:
Thanks!