Is IIS Express TLS 1.3 ASP.NET Error 500 on Windows 11 24H2 / Server 2025 Breaking App?

Why Is Windows Server 2025 Causing a Baffling ASP.NET Error? Get Your Ultimate Guide to a Quick Resolution.

Developers using the latest Windows operating systems may encounter a specific error that stops their web applications from working in a local test environment. An Error 500 can appear when running an ASP.NET application with IIS Express on Windows 11 24H2 or Windows Server 2025, which is caused by compatibility issues with the TLS 1.3 security protocol. Microsoft has indicated that it will not be releasing an official fix, but there are clear workarounds you can use to solve the problem.

Understanding the Error

When you try to test a web application on your local computer using IIS Express, the program might fail to start correctly. On older systems like Windows 10, you might see a vague message like “can’t reach the page.” However, on the newer Windows 11 24H2 and Windows Server 2025 operating systems, the error is more specific. You will likely see an Error 500, which is a generic server error, but it may also be accompanied by the error code 0x80070032. This code means that a specific request is not supported.

This issue specifically arises when you are testing functionalities that involve client certificates. The problem has always been present in Windows 11, but it is becoming more noticeable as more developers upgrade from Windows 10, especially with its support ending. Since Windows Server 2025 is also affected, system administrators and developers working in server environments will face this challenge as well.

The Root Cause of the Conflict

The problem is not a bug in your code or a fault in your project setup. Instead, it most likely comes from a manual change made in the past to a configuration file for IIS Express. This file, named applicationhost.config, controls how the IIS Express web server behaves on your machine.

At some point, this file was likely edited to include one of the following settings within the site’s binding information:

• sslFlags=”SslNegotiateCert”
• sslFlags=”SslRequireCert”

These settings instruct IIS Express to perform an action related to client certificates at the very beginning of a connection. A client certificate is like a digital ID that a user’s browser presents to the server to prove their identity. The SslRequireCert flag forces the server to demand a certificate, while SslNegotiateCert allows the server to request one without making it mandatory.

Technical Details of the TLS 1.3 Issue

The reason these settings cause an error on new Windows versions is due to an important update in web security protocols. Windows 11 and Windows Server 2025 use TLS 1.3 by default for all network traffic. TLS, or Transport Layer Security, is the standard technology used to encrypt information sent over the internet, ensuring your data is private and secure.

An older feature of TLS, known as renegotiation, allowed the client and server to change the parameters of a secure connection after it was already established. The SslNegotiateCert and SslRequireCert flags rely on this renegotiation process to request the client certificate. However, the TLS 1.3 protocol was designed for better security and performance, and one of the changes was the complete removal of support for renegotiation.

Therefore, when IIS Express, guided by the old setting in your configuration file, tries to initiate renegotiation on a system that strictly uses TLS 1.3, the request fails. The operating system’s security layer does not support the action, which results in the 0x80070032 error, and IIS Express returns the generic Error 500 to the browser.

The Recommended Workaround

Microsoft has outlined a primary workaround to resolve this issue. The solution involves disabling TLS 1.3 for incoming server connections on your development machine. This change does not affect the security of your regular web browsing, as it only applies to server applications running on your computer, like IIS Express. Outgoing connections from your browser and other clients will continue to use the modern TLS 1.3 protocol without any changes.

To apply this fix, you need to make a small change in the Windows Registry.

1. Open the Registry Editor by pressing the Windows key, typing “regedit,” and pressing Enter. You will need administrator permissions.
2. Navigate to the following key in the registry tree: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server
3. You may need to create the TLS 1.3 and Server keys if they do not exist.
4. Inside the Server key, create a new DWORD (32-bit) Value named Enabled and set its value to 0.
5. Create another new DWORD (32-bit) Value named DisabledByDefault and set its value to 1.
6. After making these changes, you must restart your computer for them to take effect.

This forces incoming server connections on your machine to use an older protocol like TLS 1.2, which still supports renegotiation, allowing IIS Express to handle client certificates as it did before. For a developer’s workstation, this change is considered safe and has minimal impact.

Alternative Solutions

While disabling incoming TLS 1.3 is the officially recommended method, there are other approaches you could consider.

Edit the Configuration File

The most direct solution is to remove the setting that causes the problem. You can open your applicationhost.config file, locate the site binding, and delete the sslFlags=”SslNegotiateCert” or sslFlags=”SslRequireCert” attribute. If your local development process does not strictly depend on early negotiation of client certificates, this is the simplest fix and requires no system-wide changes.

Use a Different Development Server

ASP.NET Core applications can be run using the Kestrel web server directly without IIS Express. Kestrel is the default cross-platform server for ASP.NET and may offer different configuration options for handling client certificates that are more compatible with TLS 1.3.