Apart from setting requiressl there, you could also try to set it on authentication mode too. You should have something like this. 

<authentication mode="Forms">
<forms name=".EPiServerLogin" loginUrl="Util/login.aspx" timeout="120" defaultUrl="~/" requireSSL="true" />
</authentication>

Ah, that's for .EpiserverLogin cookie.

I have that configuration set as well. 

Hi Shella, session is default ASP.NET stuff, so have a look at this SO post: https://stackoverflow.com/a/6190050

Hi Antti: I'm asking about "EPiSessionId" not "ASP.NET_SessionId" which seems to be created when using Profile Store or tracking. It's not even listed on the Epi documentations on Cookies. 

Hi Shella,

sorry as there was no mention about profile store or tracking I just made the assumption you have renamed the ASP.NET session cookie in your solution (and not just go with the default asp.net cookie name).

Anyways if you haven't already looked / found that cookie is coming from the Episerver NuGet package EPiServer.Session. That package contains the class  EPiServer.Session.Services.Internal.DefaultSessionStoreService which writes the cookie like this:

HttpContext.Current.Response.Headers.Add("Set-Cookie", string.Format("{0}={1}; Max-Age={2}; Path=/", "EPiSessionId", sessionId, duration));

And that is done in the Application_BeginRequest event.

As you can see it is directly setting the Set-Cookie header and not using the response cookies collection.

Antti Alasvuo:

Could you please describe how you would implement that the asp.net session cookie is returned with a secure flag?

FYI, adding this to Global.asax will make the EPiSessionId cookie HttpOnly 😃

protected void Application_Start()
{
     var sessionIdCookie = System.Web.HttpContext.Current.Request.Cookies["EPiSessionId"];
     if (sessionIdCookie != null && !sessionIdCookie.HttpOnly)
     {
        sessionIdCookie.HttpOnly = true;
        System.Web.HttpContext.Current.Response.Cookies.Add(sessionIdCookie);
     }
}

Thanks for this information!

If you want to make the EPiSessionId cookie secure and HttpOnly, use this:

protected void Application_BeginRequest()
{
	var sessionIdCookie = System.Web.HttpContext.Current.Request.Cookies["EPiSessionId"];
	if (sessionIdCookie != null)
	{
		sessionIdCookie.HttpOnly = true;
		sessionIdCookie.Secure = true;
		System.Web.HttpContext.Current.Response.Cookies.Add(sessionIdCookie);
	}
}

This addresses the SameSite=None requiring Secure issue detailed below:

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie/SameSite