Microsoft has released Azure Linux 4.0, its first general-purpose, Fedora-based Linux distribution for Azure customers. The company says the server-focused distro is designed for a consistent cloud-to-local workflow, ships as a virtual machine image, includes
It happens mid-sentence.
At Open Source Summit North America. Brendan Burns—co-founder of Kubernetes and Microsoft’s Corporate VP of Azure Cloud Native and Management Platform—paused during his presentation.. After talking about the arc from open-source to agentic AI. he said Microsoft was ready to announce something the room clearly wasn’t expecting: “Microsoft’s open-source Linux distribution. a supported version of Linux supported by Microsoft. available on Azure. out for anybody to use.”
Behind him, Jim Zemlin, CEO of the Linux Foundation, blinked—then called Burns back onstage to verify he had really just announced a Microsoft Linux distro. Burns confirmed he had.
Zemlin’s reply landed like a verdict on the moment: when Microsoft joined the Linux Foundation, there had been a “big conspiracy theory” that the Linux Foundation was being undermined in partnership with Microsoft. Now, Zemlin said, Microsoft was shipping a Linux distribution. “That’s amazing.”
The surprise wasn’t just what Microsoft announced. It was also when. The news had been intended for Microsoft Techcon in two weeks, but at the last minute, Microsoft decided to release it now.
What Microsoft is shipping—and what it isn’t
Microsoft is calling the new release Azure Linux 4.0. It’s the first full Linux distribution from Microsoft aimed at general-purpose use on Azure, and Microsoft says Azure Linux 4.0 has been split into two products: Azure Container Linux and the new virtual machine edition, Azure Linux 4.0.
The company has built Linux-based software before. Azure Sphere was a Linux-based edge computing device, and Microsoft later rolled out CBL-Marnier, a Linux-based container software platform that was renamed Azure Linux. But until now, Microsoft had not released a general-purpose Linux distribution.
That changes with Azure Linux 4.0.
Lachlan Everson. Microsoft’s Principal Program Manager on Azure’s open-source team. told attendees that Microsoft is turning Azure Linux into a full-fledged general-purpose cloud distribution with Azure Linux 4.0.. At the same time. Microsoft is productizing Flatcar Container Linux as a hardened. immutable container host called Azure Container Linux (ACL).
Azure Linux 4.0 is positioned as a general-purpose virtual machine image for all Azure customers. not just Azure Kubernetes Service (AKS) users.. Everson said that previously Azure Linux was only available to third-party customers through AKS—specifically Azure Linux 3.0.. In the new setup, ACL becomes the container-focused option.
Everson described Azure Linux 4.0 as the culmination of years of internal use and evolution from the earlier Mariner distribution.. Microsoft, he said, had already been running Azure Linux for many years internally, reaching 3.0.. Before this release, it was limited to use as a container host on AKS.. Now, Everson said, the distribution is being made general-purpose—taking “all the learnings” from the Mariner heritage.
Fedora under the hood, GitHub in public
Azure Linux 4.0 is Fedora-based and delivered as an open distribution on GitHub.. Microsoft curates packages and the supply chain to fit Azure’s cloud platform.. Everson said Microsoft decided to use Fedora as an upstream, and that it uses RPMs in the Fedora ecosystem.. He also said Microsoft curates packages and the supply chain to meet Azure needs. and that the distribution is purpose-built for Azure. integrating vertically into Microsoft infrastructure to deliver a “best Azure Linux experience on Azure.”
The code is available now.
Microsoft also signaled it wants developers to get to it more easily from familiar environments.. Everson said developers will be able to run Azure Linux locally through Windows Subsystem for Linux (WSL) images. and that Microsoft is “going to announce WSL images as well.” He also said Azure Linux is already available as a VM image for a VM host on Azure.
But despite the Windows angle, Microsoft is not positioning this as a traditional desktop Linux.. Everson said there are “no plans” for a graphical environment.. Even on a developer machine, he said users should expect a lean environment.. The aim. Everson explained. is a consistent development experience: minimal packages. use VS Code to develop. take workloads from a machine into Azure. and run them with the same platform environment as the cloud.
Kernel stability, security cadence, and a defined runway
For existing Azure Linux 3.x users, Microsoft says the path forward won’t be disruptive. Everson said users can “just upgrade” from “Azure Linux free,” with “Yes” as his response.
Microsoft is promising a defined support window: two years of support.. Everson said the company tries to pick specific kernel versions for the lifetime of that two-year support period. and then offers an upgrade pathway so it remains fully supported and upgradable throughout the two years.. He paired that with a predictable monthly security update rhythm.
Security is central. and Microsoft framed it around owning the supply chain end to end—controlling what goes into the distribution so the surface area stays small.. Everson said that because Microsoft takes care of the supply chain of the pieces used to build the distribution. it has minimal surface area of packages. a curated kernel. and customizations for running on Azure hardware. along with “best in class security.”
Microsoft’s patching approach is monthly for Azure Linux. Everson urged people not to call it “Patch Tuesday.” If a serious Common Vulnerabilities and Exposures (CVE) appears, Microsoft says it will deliver a patched image “as soon as those patches come out.”
Automation is baked in, but optional
Azure Linux is also designed to let customers offload update management—if they want to. Everson said Microsoft can opt into automatic upgrades based on security, whether users are running VMs or AKS. For larger, scaled-out deployments, Everson said updates are handled gradually to avoid disruption.
For customers with fragile or highly customized applications, opting out is still possible. Everson said if customers opt in, they will “always be up to date and secure with the latest versions,” and acknowledged there is a second camp: “You can definitely opt out of it.”
He tied the update philosophy back to keeping up with public disclosures, patches, and the rate of change: updates are built into the operating system so customers aren’t forced to wait.
On the container side, Flatcar becomes “immutable” in practice
While Flatcar remains the upstream project, Microsoft is packaging it for Azure customers. Everson described Flatcar as “purpose-built, immutable, secure by default, production-ready operating system,” and said Azure Container Linux is the productization of that for container workloads in AKS.
He also underscored what immutable means in operational terms.. “Everything’s baked in,” Everson said.. “There is no package manager.” Microsoft bakes the bits into the immutable system; changes happen in customer workloads running in containers rather than by modifying system or application packages inside the host.
Partners remain part of the plan
Even with Microsoft shipping its own distribution, Everson said Azure Linux is meant to complement other distributions rather than replace them.
Everson confirmed Red Hat knows about the new offering.. He then said Microsoft still has an ecosystem of partners and that nothing changes with those relationships.. If customers want to run Red Hat or Ubuntu. he said. that’s “absolutely okay.” Microsoft’s angle is that Azure customers get a “battery-included” experience on Azure. and Everson said Microsoft has eight endorsed distros on its platform and will continue working with them.
The backdrop for all of this is Linux’s role in the modern stack.. In a Microsoft blog post. Microsoft said more than two-thirds of customer cores in Azure run Linux. and that Microsoft 365. GitHub. and OpenAI’s ChatGPT all sit on Linux foundations.. Microsoft added that ChatGPT scales across more than 10 million compute cores worldwide and serves a billion queries a day—made possible. it said. by Linux and Kubernetes.
By the time Burns left the conference, there was a clearer story than the moment of surprise.. Microsoft had effectively moved from building Linux pieces to offering a full server-focused Linux distribution—while keeping the rest of the Linux ecosystem intact.. In other words, outside the desktop, Microsoft is no longer just a company running Linux.. It’s now shipping it.
Microsoft Azure Linux 4.0 Linux distribution Fedora-based GitHub Azure Container Linux Flatcar Kubernetes AKS security updates CVE WSL Fedora Linux Foundation
So Microsoft is Linux now? lol
I saw the headline and thought it was just a marketing thing. Like “Fedora-based” doesn’t mean it’s actually better or anything. But if it works on Azure then sure I guess, people will use it.
Wait, is this like the Linux Foundation got hacked or something? Because I remember people saying Microsoft was trying to undermine Linux and now they’re dropping a distro like it’s normal. Also “It happens mid-sentence”?? what even is that part of the article, that’s confusing.
Microsoft making a Linux distro feels either genius or kinda sus. Fedora-based server distro sounds like it’s just another way to lock people into Azure, which is what they always do. But then the Linux Foundation CEO was like “conspiracy theory” and I’m like so which is it, undermining or helping? Either way, I’m not downloading anything, I’m just watching the drama.