Ubuntu 18.04.6 LTS is now released with critical security fix. Ubuntu 18.04 LTS is supported until 2023.
Ubuntu 18.04.6 LTS release announcement:
The Ubuntu team is pleased to announce the release of Ubuntu 18.04.6 LTS
(Long-Term Support) for its Desktop and Server products.
Unlike previous point releases, 18.04.6 is a refresh of the amd64 and
arm64 installer media after the key revocation related to the BootHole
vulnerability, re-enabling their usage on Secure Boot enabled systems.
More detailed information can be found here:
https://ubuntu.com/blog/grub2-secure-boot-bypass-2021
Many other security updates for additional high-impact bug fixes are also
included, with a focus on maintaining stability and compatibility with
Ubuntu 18.04 LTS.
Maintenance updates will be provided for 5 years for Ubuntu Desktop,
Ubuntu Server, Ubuntu Cloud, and Ubuntu Base. 
