A Microsoft engineer has confirmed that the default implementation of TLS 1.3 in Windows 11 is leading to compatibility issues with IIS Express when handling client certificates. This problem also occasionally affects the full IIS. The core issue arises from TLS 1.3’s lack of support for a feature called renegotiation, which was present in versions 1.2 and earlier. Renegotiation allowed a server to request a client certificate after the encrypted session had begun. Without this feature, IIS Express is unable to validate client certificates unless they are requested during the initial TLS handshake.
This change poses a significant problem for developers testing projects that depend on mutual TLS (mTLS). In older Windows 11 builds and Server 2022, the browser would reset the connection. However, on version 24H2 and Server 2025, the response from IIS is a 500.0 Internal Server Error along with error code 0x80070032, indicating that the feature is not supported.
As of now, Microsoft has not provided an official fix for IIS Express, leaving developers in a bind. Developer Matt Hamrick has suggested three potential workarounds:
Hamrick notes that some of these methods may require administrative privileges and might get reset after Visual Studio updates. Additionally, full IIS on Windows Server 2025 offers a Negotiate Client Certificate option, which provides more control for administrators.
In contrast, IIS Express is limited by preconfigured bindings set by Visual Studio, restricting the ability for direct management. Furthermore, most web browsers currently do not support the TLS 1.3 extension for post-handshake authentication, meaning that the challenges faced are not solely based on server configuration, but also on client support.
At this point, it remains unclear whether IIS Express will receive a definitive solution, as Hamrick expressed uncertainty by stating he is unsure if a fix will be developed or what it might entail. Consequently, developers utilizing IIS Express must depend on the aforementioned workarounds or make necessary adjustments to their configurations.
More details can be found in the original Microsoft discussion about the issue here.
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