Azure Linux 4.0: Microsoft Ships Its First Server Linux Distro

Azure Linux 4.0 caught everyone off guard. Standing on stage at the Open Source Summit North America in Minneapolis on May 18, 2026, Brendan Burns, Microsoft's Corporate VP of Azure Cloud Native and Management Platform, casually announced what nobody expected: Microsoft is shipping its own general-purpose server Linux distribution. Jim Zemlin, CEO of the Linux Foundation, was backstage and he blinked. So did the Linux-savvy crowd. The news was originally slated for Microsoft Techcon in two weeks. At the last minute, the team decided to let it out early.

An Announcement Nobody Saw Coming

Burns was mid-presentation, discussing the evolution from open source to agentic AI, when he dropped the line that changed everything. "When I started in Azure 10 years ago, it was not the majority operating system running on the Azure cloud. It has become the majority operating system running on the Azure cloud in the past 10 years," he said. Then the pivot. "Today, I think we're really excited to announce that we're going to be having Microsoft's open-source Linux distribution, a supported version of Linux supported by Microsoft, available on Azure, out for anybody to use."

That was it. No fanfare. No elaborate slide deck. Just a few sentences buried in a broader talk. Zemlin called him back on stage. He asked if Burns had really just announced a Microsoft Linux distro. Burns said yes. Zemlin, still processing, said, "When Microsoft joined the Linux Foundation, there was this big conspiracy theory that somehow the Linux Foundation was undermining open source in partnership with Microsoft, and now you announce that you're shipping a Linux distribution. That's amazing."

"It's been a really great journey, and it's been awesome to see everybody within the company rally around it." ; Brendan Burns

From Internal Tool to Public Distro

Microsoft has dabbled in Linux before. There was Azure Sphere for edge computing, then CBL-Mariner, a Linux-based container platform later renamed Azure Linux. But those were narrow tools, not general-purpose distributions. Azure Linux 4.0 changes that calculus entirely. Lachlan Everson, Microsoft's Principal Program Manager on Azure's open-source team, laid out the details after Burns left the conference. The company is splitting its Linux efforts into two tracks. The first is Azure Linux 4.0 itself, a full virtual machine image available to all Azure customers. The second is Azure Container Linux, a hardened, immutable container host built on Flatcar Container Linux.

Until now, Azure Linux was only accessible through the Azure Kubernetes Service as version 3.0. Everson confirmed that going forward, the old AKS-only model becomes Azure Container Linux. The new general-purpose VM image is Azure Linux 4.0. "We've been running Azure Linux for many years internally, and we got through to 3.0, and we only allowed it on as a container host on AKS. What we've done is make it a general-purpose," Everson said. All those years of internal learning from the Mariner heritage now ship to customers directly.

Fedora Under the Hood

Here is the part the press release skipped. Azure Linux 4.0 is built on Fedora Linux. It uses RPMs from the Fedora ecosystem, with Microsoft curating the packages and supply chain specifically for Azure's infrastructure. The code is open and available on GitHub right now. Red Hat knows. Everson confirmed it directly. "Actually, yes," he said when asked if Red Hat was aware.

The distribution will ship as a VM image on Azure, and Microsoft is also preparing WSL images so developers can run it locally on Windows 11. But do not expect a graphical desktop. Everson was blunt. There are "no plans" for a desktop environment. "It's optimized for server-side in the cloud," he said. The developer experience is intentionally lean. "Minimal packages, yeah. The idea is that we offer you a consistent experience to do your development on your machine, and that you can take your workloads as you develop them on your machine and run them with VS Code."

Azure Container Linux Explained

The other half of the story is Azure Container Linux, the immutable container host. Flatcar remains the upstream project, but Microsoft is productizing it for Azure customers while continuing to invest in the upstream ecosystem. "Everything's baked in, so there is no package manager," Everson explained. "We bake the bits into the immutable, and they're in the immutable version." If you need to change anything, those changes run in containers as customer workloads. The host itself stays locked down.

This split makes the lineup clearer than it has ever been:

• Azure Linux 4.0 is the general-purpose VM image for any Azure workload
• Azure Container Linux is the immutable container host for AKS environments
• Both inherit years of Microsoft's internal Linux operational experience

Two Years, Monthly Patches

For existing users of Azure Linux 3.x, Microsoft promises a straightforward upgrade path rather than a disruptive migration. Everson confirmed users can simply upgrade. The support window spans two years, during which Microsoft locks in specific kernel versions and provides an upgrade pathway to newer releases. Security patches arrive on a monthly cadence. Do not call it Patch Tuesday, but the rhythm is familiar. For critical CVEs, patches ship "as soon as those patches come out."

Customers can opt into automatic upgrades based on security. In large deployments, updates roll out gradually to avoid disruption. For fragile or highly customized applications, opting out remains available. "If you opt into that, you will always be up to date and secure with the latest versions," Everson said. "You can definitely opt out of it."

AI Runs on Linux

And this is where it gets interesting. Everson framed Azure Linux 4.0 as a direct response to the AI-native era. "All AI applications are running on the Linux stack," he said. Microsoft has learned to build hard on Linux images through its own internal services, and it now wants to hand that expertise to customers. The blog post Microsoft published alongside the announcement drives the point home. More than two-thirds of customer cores in Azure now run Linux. Microsoft 365, GitHub, and OpenAI's ChatGPT all sit on Linux foundations. When ChatGPT scales across more than 10 million compute cores and serves a billion queries a day, Linux and Kubernetes make it possible.

But that framing misses something. This is not just about AI. It is about Microsoft finally acknowledging something that has been true for years. Outside the desktop, Microsoft is a Linux company. Steve Ballmer once called Linux a cancer. Today, Brendan Burns stands on stage and announces a Microsoft Linux distribution while the Linux Foundation's CEO cheers him on.

Partners Are Not Going Anywhere

Lest anyone think Microsoft is replacing Red Hat or Ubuntu, Everson was explicit. "We still have a great ecosystem of partners, right? This changes nothing with those relationships. If you want to run Red Hat, if you want to run Ubuntu, that's absolutely okay." Microsoft lists eight endorsed distributions on its platform and has no plans to reduce that number. Azure Linux 4.0 is an additional option, not a replacement strategy. "What we saw was an opportunity to give you a battery-included experience on Azure," Everson said.

The code is live now on GitHub. The VM image is available on Azure. WSL images are coming. For anyone running workloads in Microsoft's cloud, the operating system that powers Microsoft's own infrastructure is now open for business.

Frequently Asked Questions

What is Microsoft Azure Linux 4.0?

Azure Linux 4.0 is Microsoft's first server Linux distribution, designed for cloud workloads on Azure.

What are the key features of Azure Linux 4.0?

It offers enhanced security, performance optimizations for Azure, and long-term support.

Is Azure Linux 4.0 free to use?

Yes, it is free to use and available as an open-source project on GitHub.

Can I run Azure Linux 4.0 on-premises?

Azure Linux 4.0 is optimized for Azure but can be deployed on-premises or other clouds.

How does Azure Linux 4.0 differ from other Linux distributions?

It is tailored for Azure integration, with built-in support for Azure services and a minimal footprint.