If your web vendor just sent a .NET upgrade quote, the reason is a date: Microsoft ends support for .NET 8 and .NET 9 on November 10, 2026. Your site will keep running after that day. Microsoft will stop fixing security problems in the software underneath it.
This piece is for executive directors and IT leads at nonprofits and associations who run Optimizely, Sitefinity, Sitecore or another CMS built on Microsoft’s .NET. It explains what the date means, where each CMS stands as of October 2026, what to ask your vendor and how to read the quote. Our developer guide to .NET 8 end of support and Umbraco 13 covers the technical steps. Other year-end dates are in the Q4 2026 platform calendar.
The short version: the target is .NET 10, which Microsoft supports until November 14, 2028. Optimizely’s own pages disagree about whether CMS 12 is officially supported on .NET 10, so get your vendor’s answer in writing. A quote should name the current version, the target version, what is tested and who patches the site afterward. If the work cannot finish by November 10, record that decision and set a firm finish date.
What .NET 8 end of support means, in Microsoft’s words
.NET is the Microsoft software layer that many CMS platforms run on. Your CMS vendor ships the CMS. Microsoft ships and patches .NET.
Microsoft’s .NET support policy defines end of life as "the date when Microsoft no longer provides fixes, updates, or online technical assistance." It treats end of life and end of support as the same thing.
The .NET team’s June 29 announcement is plain about both sides. Apps built on .NET 8 or .NET 9 "will continue to run." It also says, "Staying on an unsupported version means you are exposed to security vulnerabilities that will not be patched."
So nothing changes on November 11 that a visitor would see. The risk grows with each new .NET flaw published after that date, because no fix will come for .NET 8.
If your site runs on Azure App Service, the hosting answer is the same. Microsoft’s App Service language support policy says apps on an ended runtime "continue to run unchanged," but App Service "can’t provide security patches or related customer support" for that version.
The dates behind every .NET upgrade quote
Microsoft’s support policy, last updated October 25, 2026, lists these dates.
| Version | Release type | End of support |
|---|---|---|
| .NET 8 | Long Term Support (LTS) | November 10, 2026 |
| .NET 9 | Standard Term Support (STS) | November 10, 2026 |
| .NET 10 | Long Term Support (LTS) | November 14, 2028 |
.NET 9 shipped a year after .NET 8 but ends the same day. In September 2025, Microsoft’s .NET blog extended standard-term support from 18 to 24 months, which moved .NET 9’s end to November 10, 2026. A site that moved to .NET 9 faces the same deadline as one on .NET 8.
The .NET team’s announcement recommends .NET 10, "an LTS release supported through November 2028." That is the version a quote should target.
Optimizely CMS 12 and .NET 10: what Optimizely’s pages say
This is where Optimizely customers need care. Optimizely has published three statements, and they do not line up.
A May 25, 2026 post by Bien Nguyen on Optimizely World, the company’s developer site, says CMS 12 "is fully compatible with .NET 10." It recommends these package versions or higher:
- EPiServer.Cms.Core 12.24.0
- EPiServer.Cms.UI 12.34.4
The post says earlier CMS 12 versions also work on .NET 10 but can raise two dependency vulnerability warnings, which these versions resolve. It says Optimizely Forms and Language Manager were tested and work on .NET 10.
Two other Optimizely pages say less. The CMS 12 system requirements page lists only .NET 8 as of October 2026. The CMS 13 FAQ, updated October 23, 2026, says "CMS 12 will not officially support .NET 10." It names CMS 13 as "the required upgrade path" for teams that want .NET 10.
For an executive, the gap matters. "Compatible" and "officially supported" can mean different things when you open a support ticket. Ask your vendor which statement its plan relies on, and ask Optimizely support to confirm it for your account.
Optimizely Commerce and CMS 13
If your site sells memberships, books or events through Optimizely Commerce, there is one more layer. An August 7, 2026 Optimizely World post says Commerce 14 "officially supports .NET 10 starting with version 14.46.0." Sites that use Optimizely Search & Navigation with Commerce also need EPiServer.Find.Commerce 12.3.0 or later.
Some vendors will quote CMS 13 instead. Per Optimizely’s CMS 13 FAQ, CMS 13 reached general availability on March 31, 2026, and "requires .NET 10." That FAQ estimates the upgrade from CMS 12 at "2 to 6 months on average." It also says Commerce 14 does not work with CMS 13, so Commerce sites need Commerce 15.
A CMS 13 quote is a larger project than a runtime update on CMS 12. Know which one you are buying.
Sitefinity, Sitecore and Umbraco
Progress Sitefinity. The Sitefinity 15.4 system requirements list .NET Framework 4.8 for the server. That is a separate Microsoft product. Microsoft’s .NET Framework lifecycle page lists no end date for 4.8. The front-end part changed this year. Progress’s 15.4.8632 release notes, dated July 9, 2026, say "The Sitefinity ASP.NET Core Renderer has been updated to .NET 10." If your site uses that renderer, ask your vendor whether you are on that update or later.
Sitecore. We did not find a Sitecore page that states .NET 10 support for its products as of late October 2026. Ask your vendor or Sitecore directly which .NET version your setup runs and what Sitecore supports.
Umbraco. Umbraco 13 runs on .NET 8 and reaches its own end of life on December 14. Our Umbraco 13 guide covers that path.
Questions to ask before you sign a .NET upgrade quote
Your job is to understand the plan and hold the vendor to its dates. Ask for these answers in writing:
- What runs today? The .NET version and CMS version on production, and on any connected apps such as an AMS sync or a payment handler.
- What is the target? .NET 10, and the exact CMS package versions, such as EPiServer.Cms.Core 12.24.0 or later for Optimizely CMS 12.
- Which vendor statement backs the plan? For Optimizely, the May blog post, the system requirements page or the CMS 13 FAQ.
- What does the quote cover? Runtime, CMS packages, add-ons, custom code, hosting changes and testing.
- When does it go live? A date, and how it compares with November 10.
- Who patches after go-live? Who applies monthly .NET 10 and CMS updates, and whether that is in the support contract.
Microsoft gives the same advice for software you did not build. Its announcement recommends "reaching out to the software developer or vendor" who produced the app.
How to read a .NET upgrade quote
A .NET upgrade quote usually bundles several kinds of work. Separating them makes the price easier to judge.
| Line item | What it is | What to check |
|---|---|---|
| Runtime | Moving the site from .NET 8 or 9 to .NET 10 | Target version named; development and hosting both covered |
| CMS packages | Updating the CMS to versions that support .NET 10 | Exact package versions listed |
| Add-ons and custom code | Forms, search, commerce, integrations | Each add-on named with its .NET 10 status |
| Hosting | Installing .NET 10 on servers or cloud plans | Who does it, and whether hosting costs change |
| Testing | Checking key pages and transactions | Login, join, renew, donate, register and search tested |
Two warning signs are worth a follow-up question. One is a quote with no testing line. The other is a quote that says "upgrade to the latest version" without naming it. If the quote moves you to a new major CMS version, such as Optimizely CMS 13, expect a longer timeline and ask for it as a separate project.
If your .NET upgrade cannot finish by November 10
As of October 30, November 10 is eleven days away. Finishing late is a decision to make on purpose.
A practical plan for a late project:
- Apply the last .NET 8 update. Ask the vendor to install whatever Microsoft ships on or before November 10.
- Set a firm finish date. Put it in writing, with the vendor’s name on it.
- Record the decision. Note that the organization accepts the risk of an unsupported runtime until that date. Our Drupal 10 board brief shows one way to frame this for a board.
- Start with public pages. Upgrade sites that take payments or logins before internal tools.
Budgeting for the next .NET upgrade
This will happen again. Microsoft’s support policy says a new major release ships "every year in November." Even-numbered releases are LTS and get three years of support. Odd-numbered releases are STS and get two.
For budgeting, that means:
- .NET 10 ends November 14, 2028. Your next .NET upgrade needs to land before then.
- The next LTS would be .NET 12. Under Microsoft’s annual cadence, that points to November 2027.
- .NET 11 is a standard-term release. As of late October 2026, Microsoft’s policy page listed it as a release candidate.
A simple rule: budget for a runtime upgrade every two to three years, in the fiscal year before the LTS date ends. Ask your vendor to include it in the support plan, so the next quote is not a surprise. Microsoft products end on fixed dates, as Office 2021 and Windows 11 24H2 did on the same October day.
Frequently Asked Questions
Will our website stop working on November 10?
No. Microsoft says apps built on .NET 8 or .NET 9 will continue to run. What stops is security fixes, updates and technical help for those versions.
Why is our vendor quoting a .NET upgrade now?
Microsoft ends support for .NET 8 and .NET 9 on November 10, 2026. A vendor that keeps your site on a supported runtime has to move it to .NET 10.
Which .NET version should we upgrade to?
.NET 10, the current Long Term Support release. Microsoft supports it until November 14, 2028.
Does Optimizely CMS 12 support .NET 10?
Optimizely’s pages differ. A May 2026 Optimizely World post says CMS 12 is fully compatible with .NET 10 from EPiServer.Cms.Core 12.24.0 and EPiServer.Cms.UI 12.34.4. The CMS 13 FAQ says CMS 12 will not officially support .NET 10. Ask Optimizely to confirm for your account.
Do we have to move to Optimizely CMS 13?
Not to run on .NET 10, according to Optimizely’s May 2026 post. Optimizely’s CMS 13 FAQ calls CMS 13 the required path for .NET 10 and estimates 2 to 6 months for the upgrade. Ask your vendor which path it is quoting.
Is Sitefinity affected?
The Sitefinity server runs on .NET Framework 4.8, which the November 10 date does not cover. Progress updated the Sitefinity ASP.NET Core Renderer to .NET 10 in July 2026. Ask whether your site uses the renderer.
What does a fair .NET upgrade quote include?
The current and target versions, CMS package versions, add-ons, hosting changes, testing of key transactions and who patches the site afterward. Each should be a named line.
What if we cannot finish before November 10?
Apply the last .NET 8 update Microsoft ships, set a firm finish date in writing and record that the organization accepts the risk until then.
When will we need the next .NET upgrade?
Before November 14, 2028, when .NET 10 support ends. Under Microsoft’s yearly cadence, the next Long Term Support release would be .NET 12 in November 2027.
