SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/html-sanitization/

HTMLSanitizationModify_Global.asax.cs

using System;
using Telerik.Microsoft.Practices.Unity;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Data;
using Telerik.Sitefinity.Security.Sanitizers;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            ObjectFactory.Initialized += RegisterCommonTypesEventHandler;
        }

        private void RegisterCommonTypesEventHandler(object sender, ExecutedEventArgs e)
        {
            if (e.CommandName == "RegisterCommonTypes")
            {
                ObjectFactory.Container.RegisterType<IHtmlSanitizer, SitefinityExtendedHtmlSanitizer>(new ContainerControlledLifetimeManager());
            }
        }
    }

    public class SitefinityExtendedHtmlSanitizer : HtmlSanitizer
    {
        public SitefinityExtendedHtmlSanitizer()
        {
            base.AllowedAttributes.Add("custom_attribute");
        }
    }
}