Microsoft's Azure Linux 4.0 Goes General-Purpose, Pivots to Fedora Base

Microsoft used Build 2026 to push Azure Linux 4.0 into public preview, marking the first time its in-house distribution can run on any Azure VM rather than serving as a hidden host for AKS nodes and first-party services. The lineage traces back to CBL-Mariner, an internal RPM distro that went public in 2020, became the AKS container host in 2023, and was rebranded Azure Linux in 2024. The 4.0 release is the inflection point where it stops being plumbing and becomes something customers actively choose.

The biggest engineering shift is architectural: rather than hand-maintaining every spec file, Microsoft now tracks Fedora 43 upstream and applies declarative overlays, with each deviation documented and rendered spec files committed to the repo. That brings kernel 6.18 LTS with Azure tuning, dnf5 replacing the lean tdnf package manager, glibc 2.42, systemd 258, OpenSSL 3.5 with post-quantum support, and Python 3.14. FIPS 140-3 certification is in flight, SELinux ships on every image, and SBOMs are published for the full supply chain.

The distro now spans VMs, scale sets, containers (including distroless variants), AKS, and WSL, with no extra OS licensing on Azure Marketplace. Databricks already migrated over 100,000 VMs and LinkedIn moved its infrastructure onto it. The strategic significance is that a major vendor is maintaining a Fedora-derived distribution upstream-first with auditable supply chain practices — a notable evolution from a company that a decade ago was still at arm’s length from Linux.