Transport Layer Security

When the add-in connects to a REST endpoint using HTTPS, the add-in relies on the system default behavior for Transport Layer Security (TLS) to determine which TLS protocol is to be used.

Because the add-in runs within the Excel process, it cannot rely entirely on the .NET Framework 4.8 default setting to do this. To ensure that the system default behavior is in effect, the add-in sets the AppContext.DontEnableSystemDefaultTlsVersions property to false for the current app domain.

See the following Microsoft documentation:

• If your app targets .NET Framework 4.7 or later versions
• Configuring security via AppContext switches (for .NET Framework 4.6 or later versions)