Hi,
HikaShop doesn't have any known security issue so we can't fix what's not broken. If you're getting hacked, it could come from any extension on your website, or even a hole in Joomla itself. Also, the error message you provided doesn't indicate that there is a security issue. This indicates that there is a DDOS attack on your website (
en.wikipedia.org/wiki/Denial-of-service_attack
). In short, a lot of computers are automatically trying to access your website/server in order to flood it so that it can't function properly. The can happen regardless of use HikaShop or even Joomla. The solutions to such attacks are numerous and can go from setting up firewall rules, or installing a Firewall extension on your website like RS Firewall or AdminTools Pro, to increasing the capacity of the server, or changing server/host/IP address etc, but are not linked to HikaShop, so we cannot help you in that case.