GoKawiilMicrosoft’s in-house Linux, the distribution that grew out of CBL-Mariner, just hit public preview as a general-purpose cloud OS you can run on any Azure VM. Here is why that is a real step in Microsoft’s Linux journey, not just a version bump.
Microsoft shipped Azure Linux 4.0 into public preview at Build 2026, and for the first time you can run it on any Azure virtual machine, not just as the host underneath Azure Kubernetes Service. That sounds like a small distinction. But, this is the moment Microsoft's in-house Linux stops being a special-purpose appliance distro and becomes a general-purpose Linux distro.
I have been following this distribution since before it had a marketing name. So let me put 4.0 in context...
What I keep on about
Microsoft has built more than one Linux distribution. Back in February 2022 I went looking through Microsoft's package mirrors and found CBL-Delridge, a Debian-based distro that powered Azure Cloud Shell. It was never announced. Mary Jo Foley wrote it up at ZDNet after reading that post. By November 2022, Delridge was 404: its apt repository went dark and Cloud Shell moved to Microsoft's other Linux: CBL-Mariner.
CBL stands for Common Base Linux, a whole family of internal distros named after Seattle geography. Delridge was the Debian one. Mariner was an RPM one, built from scratch with spec files borrowed from Photon OS, Fedora, and Linux From Scratch. Mariner is the one that survived. In March 2024 Microsoft renamed it Azure Linux and renamed the GitHub repository to match.
So when I say Azure Linux, I mean the distribution that started internal development in September 2019, went public on GitHub in November 2020, hit 2.0 in April 2022, and has been the container host for AKS since 2023. None of that history was aimed at you running it on your own VM.
That is what changes now.
What is actually new in 4.0
Azure Linux 4.0 is derived from Fedora, right now a Fedora 43 snapshot, rather than assembled package by package the way 1.0 through 3.0 were. Microsoft no longer maintains every spec file by hand. Instead it tracks Fedora upstream and applies declarative overlays, where every deviation from Fedora carries a written description of why it exists. The rendered spec files are checked into the repository so you can read exactly what Microsoft changed and why.
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