A validating server receives from a client device a first request that does not include a cookie for a validating domain that resolves to the validating sever. The first request is received at the validating server as a result of a proxy server redirecting the client device to the validating domain upon a determination that a visitor belonging to the client device is a potential threat based on an IP (Internet Protocol) address assigned to the client device used for a second request to perform an action on an identified resource hosted on an origin server for an origin domain. The validating server sets a cookie for the client device, determines a set of characteristics associated with the first client device, and transmits the cookie and a block page to the client device that has been customized based on the set of characteristics, the block page indicating that the second request has been blocked.