在ASP.net MVC .netframework 4.8中，使用的身份验证方式是System.Web.Security.FormsAuthentication，而在ASP.net MVC Core 6中使用的是Microsoft.AspNetCore.Authentication。

要实现两个版本之间的用户身份验证共享，可以采用以下步骤：

1. 在ASP.net MVC .netframework 4.8中，使用System.Web.Security.FormsAuthentication将用户身份验证令牌以cookie的方式保存在浏览器中。

2. 在ASP.net MVC Core 6中，使用Microsoft.AspNetCore.Authentication.Cookies将从ASP.net MVC .netframework 4.8中传递过来的用户身份验证令牌解密，并验证身份。

在ASP.net MVC .netframework 4.8中的代码示例：

var ticket = new FormsAuthenticationTicket(
    1,
    user.UserName,
    DateTime.Now,
    DateTime.Now.AddMinutes(30),
    false,
    userData,
    FormsAuthentication.FormsCookiePath);

var encryptedTicket = FormsAuthentication.Encrypt(ticket);

var cookie = new HttpCookie(
    FormsAuthentication.FormsCookieName,
    encryptedTicket);

HttpContext.Response.Cookies.Add(cookie);

在ASP.net MVC Core 6中的代码示例：

services
    .AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
    .AddCookie(options =>
    {
        options.Cookie.Name = FormsAuthentication.FormsCookieName;
        options.Cookie.Path = FormsAuthentication.FormsCookiePath;
        options.TicketDataFormat =
            new FormsAuthenticationTicketDataFormat(
                FormsAuthentication.FormsCookieName);
    });

public class FormsAuthenticationTicketDataFormat :
    ISecureDataFormat
{
    private readonly string _cookieName;
    private readonly FormsAuthenticationTicketSerializer _serializer;

    public FormsAuthenticationTicketDataFormat(string cookieName)
    {
        _cookieName = cookieName;
        _serializer = new FormsAuthenticationTicketSerializer();
    }

    public string Protect(AuthenticationTicket data)
    {
        throw new NotImplementedException();
    }

    public AuthenticationTicket Unprotect(string protectedText)
    {
        var cookie = HttpContext.Request.Cookies[_cookieName];
        var ticket = FormsAuthentication.Decrypt(cookie.Value);

        return _serializer.Deserialize(ticket.UserData);
    }
}