Single-Sign-out not working with Login/Logout button widget on a frontend page when using Default authentication protocol in Sitefinity 14 and above
Azure AD B2C Single-Sign-Out not working when logging out using Login/Logout button widget
User not logged out of external STS when logging out of Sitefinity using login status widget

using Microsoft.Owin.Security;
using System.Web;
using System.Web.Mvc;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Configuration;
using Telerik.Sitefinity.Frontend.Identity.Mvc.Controllers;
using Telerik.Sitefinity.Frontend.Identity.Mvc.Models.LoginStatus;
using Telerik.Sitefinity.Frontend.Mvc.Infrastructure.Controllers;
using Telerik.Sitefinity.Security;
using Telerik.Sitefinity.Security.Claims;
using Telerik.Sitefinity.Security.Configuration;

namespace SitefinityWebApp
{
    public class FixSingleSignOutAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            var controller = filterContext.Controller as LoginStatusController;
            if (controller != null && filterContext.ActionDescriptor.ActionName == "SignOut")
            {
                var modelProperty = controller.GetType().GetProperty("Model");
                if (modelProperty == null)
                {
                    return;
                }

                var model = modelProperty.GetValue(controller, null) as ILoginStatusModel;
                if (model == null)
                {
                    return;
                }

                var logoutUrl = model.GetLogoutPageUrl() ?? controller.GetCurrentPageUrl();
                var validator = ObjectFactory.Resolve<IRedirectUriValidator>();

                if (!validator.IsValid(logoutUrl))
                {
                    logoutUrl = "~/";
                }

                if (Config.Get<SecurityConfig>().AuthenticationMode == Telerik.Sitefinity.Security.Configuration.AuthenticationMode.Claims)
                {
                    var request = filterContext.RequestContext.HttpContext.Request;
                    var owinContext = request.GetOwinContext();
                    var authenticationTypes = ClaimsManager.CurrentAuthenticationModule.GetSignOutAuthenticationTypes();

                    var externalProviderName = owinContext.Authentication.User.FindFirst(c => c.Type == SitefinityClaimTypes.ExternalProviderName);
                    if (externalProviderName != null)
                    {
                        authenticationTypes.Add(externalProviderName.Value);
                    }

                    owinContext.Authentication.SignOut(new AuthenticationProperties
                    {
                        RedirectUri = logoutUrl
                    },
                    authenticationTypes.ToArray());
                }
                else
                {
                    SecurityManager.Logout();
                }

                filterContext.Result = new RedirectResult(logoutUrl);
            }
        }
    }
}

protected void Application_Start(object sender, EventArgs e)
{
    Bootstrapper.Bootstrapped += this.Bootstrapper_Bootstrapped;
}

private void Bootstrapper_Bootstrapped(object sender, EventArgs e)
{
    GlobalFilters.Filters.Add(new FixSingleSignOutAttribute());
}