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Addressing TLS 1.3 Compatibility Issues in IIS Express on Windows 11

MattHamrick's avatar
MattHamrick
Icon for Microsoft rankMicrosoft
Aug 29, 2025

If you are using a Windows 11 workstation and encountering errors using IIS Express with an ASP.NET project (or any project being tested on IIS Express) that utilizes client certificates (also known as "mutual TLS" or "mTLS"), this post is for you.

As Windows 10 approaches its end-of-support, these issues are becoming more prevalent as this problem has always existed on Win11. This guide will help you identify the symptoms, understand the cause...
Updated Aug 29, 2025
Version 1.0
trevorharrris-antech's avatar
trevorharrris-antech
Copper Contributor
Mar 10, 2026

Spent 5 days troubleshooting before I found this blog post, disabled tls 1.3 for server, and issue was resolved. sheesh. Why is this still an issue with IISExpress a year later?