In standard framework, we can context.user.Request.ServerVariables.Get("AUTH_USER");but how to access similar variable in dot net core
You need to use Features, it seems you need IServerVariablesFeature
Your code will look like this:
var svf = httpContext.Features.Get<IServerVariablesFeature>();
var authUser = svf == null ? "No feature :( Do you have IIS?" : svf["AUTH_USER"];