Ubuntu

Ubuntu is developed by Canonical. A Long Term Support (LTS) release comes out every two years in April, with interim releases every six months in between. An LTS release gets five years of standard security maintenance, and Canonical's paid Ubuntu Pro subscription extends that with Expanded Security Maintenance (ESM) for another five years. Interim releases are supported for nine months and aren't tracked by OS Checker.

How security updates work on Ubuntu

Canonical's Ubuntu Security Team fixes vulnerabilities in supported releases and announces them as Ubuntu Security Notices (USN). One notice can cover several CVEs and several releases, and each CVE is given a priority from negligible to critical. Like Debian, Ubuntu backports fixes to the version already in the release rather than moving to a new upstream version.

During standard support the security team covers the main repository, while packages in universe are community-maintained. Ubuntu Pro extends security coverage to universe and, after standard support ends, delivers fixes for the release through its ESM repositories. The Ubuntu CVE tracker shows the status of every CVE per release.

How OS Checker tracks Ubuntu

OS Checker compares each machine's installed packages against Ubuntu's public archive and cross-references Ubuntu Security Notices to flag the updates that fix known CVEs, using Ubuntu's own priority. Updates delivered only through Ubuntu Pro's ESM repositories aren't part of the public archive, so they don't show up as available.

See the full list of supported releases.