Found a minor bug that caused the Authentiction HTTP Module to crash when HTTP_USER_AGENT header is not present in the request.

AuthenticationModule.vb - OnAuthenticateRequest, line 6.

The current code in repository:

'ACD-8846, WorkItems 6416,4766, and 4077
            Dim strRequest As String = request.ServerVariables("HTTP_USER_AGENT").ToString
            If strRequest Is Nothing Then
                Exit Sub
            End If

Removing the ToString in bold fixed the error. (The If clause below will never be true. Nothing.ToString will throw before the check.)

After fixing this I am now able to log in to DotNetNuke using current windows credentials from a .NET WPF Client just by issuing a HttpWebRequest GET to WindowsSignin.aspx and keeping the cookies that are returned. :)