Optimizely and .NET 8 end of support: 12 questions answered in plain English

On 10 November 2026, Microsoft stopped supporting .NET 8, the framework that sits underneath many Optimizely websites. 

This article will help you understand if you're affected and if so, what does that means for you. I've written this for the people who own the website budget rather than the code. Plain English, as little jargon as possible. 

Firstly, don't panic (at least not yet). Your website hasn't stopped working. But if it still runs on .NET 8, this is a decision you should be making now, not next year.

1. What actually changed on 10 November 2026? 

Microsoft ended support for .NET 8 and .NET 9 on 10 November 2026. That means no more security fixes, servicing updates or technical support from Microsoft for either version. 

Optimizely CMS 12 and Commerce 14 sites run on Microsoft’s .NET platform, and many of them run on .NET 8 today. Recent versions of both platforms also support the replacement, .NET 10, a long-term support release that Microsoft will support until November 2028.(Microsoft)

2. Will my Optimizely website stop working? 

No. Microsoft is explicit that applications built on .NET 8 will continue to run. Nothing switches off. 

What changes is this: any new security weakness found in .NET 8 from now on won't be fixed. Think of it like a car whose manufacturer has stopped issuing safety recalls. It still drives perfectly well today. The question is how comfortable you are driving it in two years' time.

3. How do I know if my site is affected? 

Ask your development team or agency one question: "Which version of .NET is our site running on?" They can check in under a minute. 

If the answer is .NET 8, or anything older, you're affected. As a rough guide, if your site runs on Optimizely CMS 12 or Commerce 14 and hasn't had a framework upgrade in the last year, there's a good chance it's on .NET 8. If you're on CMS 11, skip to question 7.

4. What's the real risk of staying on .NET 8? 

The risk grows over time rather than arriving all at once. Today, a .NET 8 site is as secure as it was last week. But with each passing month from now, that changes. 

In practice, staying put means three things: 

• Security exposure that grows. New vulnerabilities are found in software all the time. From now on, Microsoft won't fix the ones found in .NET 8. AI is surfacing new ways to expose security flaws more quickly than ever before. 
• No safety net. If something goes wrong at framework level, Microsoft technical support is no longer available for .NET 8. 
• Awkward questions. Security audits, IT governance reviews and customer questionnaires increasingly ask whether you run supported software.  

I'd treat this as a governance question as much as a technical one.

5. Is Optimizely CMS 12 compatible with .NET 10? 

Yes. Optimizely confirmed in May 2026 that CMS 12 is fully compatible with .NET 10, after testing it with popular add-ons including Optimizely Forms and Language Manager. (Optimizely) 

For your development team: Optimizely recommends EPiServer.CMS.Core 12.24.0 and EPiServer.CMS.UI 12.34.5 or later. Older CMS 12 versions also run on .NET 10, but show dependency warnings during the build. (Optimizely release notes)

6. What about Optimizely Commerce 14? 

Yes, Commerce 14 has supported .NET 10 since August 2026, from version 14.46.0. (Optimizely) 

Commerce is where upgrades tend to get fiddlier, though. Sites that use Optimizely Search & Navigation alongside Commerce also need EPiServer.Find.Commerce 12.3.0 or later, and that brings its own chain of dependencies. What looks like a small change can take longer than expected to build and test cleanly.

Sources 

Primary sources: 

• .NET 8 and .NET 9 will reach End of Support on November 10, 2026, Rahul Bhandari, Senior Program Manager, Microsoft (.NET Blog, June 2026) 
• Optimizely CMS 12 Now Fully Supports .NET 10, Bien Nguyen, Optimizely (Optimizely World, May 2026) 
• Optimizely Commerce 14 Now Supports .NET 10, Bien Nguyen, Optimizely (Optimizely World, August 2026) 
• 2026 Optimizely CMS 12 release notes, including the 10 April 2026 CMS major version policy (Optimizely documentation)

7. I'm still on CMS 11. Does this affect me? 

Yes, and your position is more urgent than anyone else's on this page. 

CMS 11 doesn't run on .NET 8. It runs on the older .NET Framework. But on 10 April 2026, Optimizely announced that it now actively maintains only CMS 13 and CMS 12. For CMS 11 and earlier, there are no more ongoing bug fixes, security monitoring or patch releases, except for severe security vulnerabilities raised by customers or partners. (Optimizely release notes) 

Your site will keep running, and Optimizely's support team will still help you investigate issues. But you're now two major versions behind, and moving off CMS 11 is a bigger job than a .NET upgrade. It means moving your site's code from .NET Framework onto modern .NET, which involves real rework. 

If you're on CMS 11, I'd start planning now. Not next quarter. Now. 

8. What are my options?

For CMS 12 and Commerce 14 sites, there are three realistic routes: 

These aren't mutually exclusive. Plenty of organisations will do the first now and plan the second or third for later. 

9. How do I choose between the options? 

Start with three questions: 

1. How long do you expect to keep your current website? 
2. Is there anything on your roadmap that CMS 12 can't do? 
3. How much budget and appetite for change do you have this year? 

Our rule of thumb: if a redesign or replatform is likely within 18 months, moving to .NET 10 buys you time cheaply. If your site will be around for longer, CMS 13 deserves a serious look, and it unlocks the latest AI capabilities of the platform, that can transform your business. If you're tired of upgrade projects altogether, SaaS is worth exploring. 

10. How long will .NET 10 keep me covered? 

Until November 2028. .NET 10 is a long-term support release, which Microsoft supports for three years from its launch in November 2025. 

Or to put it another way: this isn't a one-off. Expect to have a version of this conversation again in 2028. The organisations that find it easiest are the ones that treat framework upgrades as routine maintenance rather than an emergency. 

11. What's involved, and can my team do it themselves? 

For a well-maintained CMS 12 site, the framework switch itself is small. Microsoft describes it as changing one setting in the project file, then updating your development and hosting environments. 

Most of the work sits elsewhere: updating Optimizely packages, checking every add-on and integration still works, then testing that nothing has broken. If your team knows Optimizely well and has the time, they can do it. The risk usually hides in the things nobody remembers. A custom integration built in 2021. An add-on whose developer has gone quiet. Commerce sites and heavily customised sites carry more of that risk.

12. How can Niteco help? 

We offer a free .NET 10 readiness check. Within 2 working days, you get a full audit of your solution and a fixed-price quote for the upgrade. 

Niteco is Optimizely's largest global agency partner, and we've completed over 500 Optimizely upgrades. Our AI-powered Upgrade Machine means a full upgrade takes days, not months. If staying on CMS 12 is the right answer for you, we'll say so. If CMS 13 makes more sense, we'll show you why. 

Book your free .NET 10 readiness check

And if you do nothing else this week, ask your team that one question from question 3. It takes a minute, and the answer tells you whether you need to read this article again.