The SPA default page middleware could not return the default page '/index.html' in production application

So the reason we’re getting this is someone is trying to POST AND OPTIONS to /index.html not GET. Which causes this error message and causes a 500 error.

Of course of Azure/IIS sees too many errors in too short a time it tries and recycles the app. the problem is that it’s doing it over and over again which causes everything in that App Service Plan to crash. :<

How do we properly handle this without the site throwing a 500 error?

Note that this only happens in production, not development and as far as I can tell there is no way to handle this error with your own route. This is a major DOS attack vector in spa services that needs to be fixed ASAP.

Another possible workaround is:

app.UseWhen(context => HttpMethods.IsGet(context.Request.Method), builder =>
{
    builder.UseSpa(spa =>
    {
    });
});

This will only run the SpaMiddleware for Get requests, letting all other request pass to the next middleware.