How can I get the X509 client certificate from an HTTPS request like what I can do with the servlet framework using HttpServletRequest.getAttribute(“javax.servlet.request.X509Certificate”)?

Unfortunately this is not supported yet. Could you please open an issue https://github.com/ktorio/ktor/issues/new and describe your usecase?

I ran into this problem today, and created an issue for it: https://github.com/ktorio/ktor/issues/1767