An Application ASP.Net vulnerability.

It is a cryptographic weakness inherent in the application. Telerik's solutions is either to patch/upgrade or disable certain functionality (a handler) within the application (See: https://www.telerik.com/support/kb/aspnet-ajax/details/cryptographic-weakness).

It seems that neither could be done in your situation from the fact that you were asked to provide a solution on F5.

One way of mitigating the situation is to enforce user authentication for the application on the F5, preferably with the use of APM. That way you would have a way of controlling the access at least.