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Is there a risk, that CVNs get attacked to interrupt the blockchain

You can not easily distinguish regular full nodes from CVNs because both run the exact same software and do the same thing except that CVNs also create blocks. Thus, there is no easy way to directly target a particular CVN. Proof-of-cooperation only defines what node ID is to create the next block and not what IP address it has or where it is located.

Almost all P2P crypto-currencies are vulnerable to DOS attacks. Because the IP addresses of all nodes is public information. Thus, you can flood all the known nodes and interfere with the network. There are currently about 10000 Bitcoin nodes and about 100 FairCoin nodes active. For a serious attacker it wouldn't make any difference if 10000 or 100 nodes are to be attacked.

Nevertheless, we have been discussing this issue. If we'd face such a situation one day we'd inter-connect CVNs directly and/or use well-connected proxy nodes on some VPS. This would keep the blockchain running. Remember, the worst case is that an attacker could bring the blockchain to a temporary halt, that's it. It simply would continue when the attack is over.