I managed to get Shibboleth with SecureAuth working on an IIS server hosting a simple index.html and later a simple ASP.net MVC 4 app, both of which work great.
Today, though, I tried simply making my app point to a new ASP.net CORE app. All I did was change the directory in IIS, nothing special. I installed .NET Core hosting on the server and it works (the app runs and loads just fine) until I need to re-authenticate with SecureAuth. In that case, the https://mywebsite.com/Shibboleth.sso/SAML2/POST at the end results in a 404 instead of a 200. My first thought is the WebConfig, which I know very little about. Here is the old one, and here is the new one, both unchanged from the default templates in Visual Studio 2015.
Is there any additional server configuration that needs to go on, or is there a weird handler in that WebConfig that is triggering undefined behavior? Shibboleth is new to me, so go easy!
Edit: Some extra details... On IIS version 8 I configured the ISAPI and CGI Restrictions, ISAPI Filter, and Handler Mappings (unchecked "Invoke handler only if request is mapped to...") to point to the Shibboleth DLL. All my Shibboleth configuration files worked fine with the old apps as well, so I don't see how it could be those.