Port-Knocking vulnerabilities?

[Kappa Alpha Pi Eta <kappaalphapieta () hotmail com>]

Hi listers.

so I read this thread about port-knocking (altough called "reflexsive firewalls"). I'd never heard of that and found 
that to be an very interesting mechanism. Now I just keep wondering, what an attacker could possibly do to intrude 
system secured in such a way. So there are no open ports at all, also, there's no way the attacker could access the 
computer physically or via social engineering. The attacker knows that a knock-server is running and that there's some 
daemon waiting to become accessible (what ever that may be).
What could a attacker do to somehow get access to that machine? And how can I secure that machine from that kind of 
attacks.

Thanks in advance,
Kajin
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