Puts back UmbracoAntiForgeryAdditionalDataProvider for backwards compat reasons but it is not used

src/Umbraco.Web/Security/UmbracoAntiForgeryAdditionalDataProvider.cs

@@ -0,0 +1,91 @@
+using System;
+using Umbraco.Web.Mvc;
+using Umbraco.Core;
+using System.Web.Helpers;
+using System.Web;
+using Newtonsoft.Json;
+using System.ComponentModel;
+
+namespace Umbraco.Web.Security
+{
+    [Obsolete("This is no longer used and will be removed from the codebase in future versions")]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class UmbracoAntiForgeryAdditionalDataProvider : IAntiForgeryAdditionalDataProvider
+    {
+        private readonly IAntiForgeryAdditionalDataProvider _defaultProvider;
+
+        /// <summary>
+        /// Constructor, allows wrapping a default provider
+        /// </summary>
+        /// <param name="defaultProvider"></param>
+        public UmbracoAntiForgeryAdditionalDataProvider(IAntiForgeryAdditionalDataProvider defaultProvider)
+        {
+            _defaultProvider = defaultProvider;
+        }
+
+        public string GetAdditionalData(HttpContextBase context)
+        {
+            return JsonConvert.SerializeObject(new AdditionalData
+            {
+                Stamp = DateTime.UtcNow.Ticks,
+                //this value will be here if this is a BeginUmbracoForms form
+                Ufprt = context.Items["ufprt"]?.ToString(),
+                //if there was a wrapped provider, add it's value to the json, else just a static value
+                WrappedValue = _defaultProvider?.GetAdditionalData(context) ?? "default"
+            });
+        }
+
+        public bool ValidateAdditionalData(HttpContextBase context, string additionalData)
+        {
+            if (!additionalData.DetectIsJson())
+                return false; //must be json
+
+            AdditionalData json;
+            try
+            {
+                json = JsonConvert.DeserializeObject<AdditionalData>(additionalData);
+            }
+            catch
+            {
+                return false; //couldn't parse
+            }
+
+            if (json.Stamp == default) return false;
+
+            //if there was a wrapped provider, validate it, else validate the static value
+            var validateWrapped = _defaultProvider?.ValidateAdditionalData(context, json.WrappedValue) ?? json.WrappedValue == "default";
+            if (!validateWrapped)
+                return false;
+
+            var ufprtRequest = context.Request["ufprt"]?.ToString();
+
+            //if the custom BeginUmbracoForms route value is not there, then it's nothing more to validate
+            if (ufprtRequest.IsNullOrWhiteSpace() && json.Ufprt.IsNullOrWhiteSpace())
+                return true;
+
+            //if one or the other is null then something is wrong
+            if (!ufprtRequest.IsNullOrWhiteSpace() && json.Ufprt.IsNullOrWhiteSpace()) return false;
+            if (ufprtRequest.IsNullOrWhiteSpace() && !json.Ufprt.IsNullOrWhiteSpace()) return false;
+
+            if (!UmbracoHelper.DecryptAndValidateEncryptedRouteString(json.Ufprt, out var additionalDataParts))
+                return false;
+
+            if (!UmbracoHelper.DecryptAndValidateEncryptedRouteString(ufprtRequest, out var requestParts))
+                return false;
+
+            //ensure they all match
+            return additionalDataParts.Count == requestParts.Count
+                && additionalDataParts[RenderRouteHandler.ReservedAdditionalKeys.Controller] == requestParts[RenderRouteHandler.ReservedAdditionalKeys.Controller]
+                && additionalDataParts[RenderRouteHandler.ReservedAdditionalKeys.Action] == requestParts[RenderRouteHandler.ReservedAdditionalKeys.Action]
+                && additionalDataParts[RenderRouteHandler.ReservedAdditionalKeys.Area] == requestParts[RenderRouteHandler.ReservedAdditionalKeys.Area];
+        }
+
+        internal class AdditionalData
+        {
+            public string Ufprt { get; set; }
+            public long Stamp { get; set; }
+            public string WrappedValue { get; set; }
+        }
+
+    }
+}