Is my site is under attack ?

Hi,
I am getting following error in my nop 3.20 log from last 3 days
Please someone let me know what is this.But up-till now i think someone was trying to use cross site scripting on my registration page. please correct me if i am wrong .Thanks
Following is the ERROR

A potentially dangerous Request.Form value was detected from the client (recaptcha_challenge_field="...ebster. <a href="http://www....").

The details for the log entry.Full message:   System.Web.HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (recaptcha_challenge_field="...ebster. <a href="http://www...."). at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection) at System.Web.HttpRequest.<>c__DisplayClass5.<ValidateHttpValueCollection>b__3(String key, String value) at System.Web.HttpValueCollection.EnsureKeyValidated(String key) at System.Web.HttpValueCollection.Get(String name) at Nop.Web.Framework.UI.Captcha.CaptchaValidatorAttribute.OnActionExecuting(ActionExecutingContext filterContext) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.InvokeActionMethodFilterAsynchronouslyRecursive(Int32 filterIndex) at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass30.<BeginInvokeActionMethodWithFilters>b__2e(AsyncCallback asyncCallback, Object asyncState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters, AsyncCallback callback, Object state) at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass1e.<BeginInvokeAction>b__16(AsyncCallback asyncCallback, Object asyncState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object state) at System.Web.Mvc.Controller.<BeginExecuteCore>b__1c(AsyncCallback asyncCallback, Object asyncState, ExecuteCoreState innerState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) at System.Web.Mvc.Controller.<BeginExecute>b__14(AsyncCallback asyncCallback, Object callbackState, Controller controller) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) at System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) at System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__3(AsyncCallback asyncCallback, Object asyncState, ProcessRequestState innerState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Whether your website is under an intentional attack, I don't think anyone will be able to answer to this with question.
But this error surely means that HTML is being inserted into your form fields.

You might want to consider using a CDN service like Cloudflare or Incapsula. We have been using these for our own website and are quite happy with the added security they provide. I believe Incapsula are better in terms of security but are a bit more costly.

I hope this is useful!

Thank you for your kind support.

Best Regards,
Ahsan Muneer