Enable ASP.NET Core 2 cookies to read old ASP.NET Forms Authentication cookies by implementing a custom ISecureDataFormat.
NuGet: 
Usage:
var section = Configuration.GetSection("FormsAuthentication"); var faOptions = new FormsAuthenticationOptions() { DecryptionKey = section.GetValue<string>("DecryptionKey"), ValidationKey = section.GetValue<string>("ValidationKey"), EncryptionMethod = section.GetValue<EncryptionMethod>("EncryptionMethod"), ValidationMethod = section.GetValue<ValidationMethod>("ValidationMethod"), }; services .AddAuthentication(options => { options.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme; options.DefaultChallengeScheme = CookieAuthenticationDefaults.AuthenticationScheme; options.DefaultSignInScheme = CookieAuthenticationDefaults.AuthenticationScheme; }) .AddCookie(options => { options.Cookie.Name = section.GetValue<string>("CookieName"); options.AccessDeniedPath = "/Login/Upgrade/"; options.LoginPath = "/Login/"; options.ReturnUrlParameter = "returnurl"; options.TicketDataFormat = new FormsAuthenticationDataFormat<AuthenticationTicket>( faOptions, FormsAuthHelper.ConvertCookieToTicket, FormsAuthHelper.ConvertTicketToCookie ); });The FormsAuthHelper.ConvertCookieToTicket and FormsAuthHelper.ConvertTicketToCookie helper methods convert an ASP.NET Core AuthenticationTicket to a FormsAuthenticationCookie and vise versa. This class contains the same data as a old ASPNET Cookie.